Radiation control device, radiation therapy system, method of operating the radiation control device, program and storage medium

Abstract

To more correctly control irradiation of a radiation beam to a subject from a radiation beam source.SOLUTION: An irradiation control device 20 and an irradiation control method generate a plurality of shift images by shifting a perspective image with a prescribed pitch width within a prescribed range in a head-tail direction, calculate a normalization correlation coefficient between each of the plurality of shift images and a DRR image, and decide a shift amount of the shift image corresponding to the maximum normalization correlation coefficient in the plurality of normalization correlation coefficients as a positional deviation amount.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to an irradiation control device, a radiation therapy system, an irradiation control Device operation method, a program, and a storage medium.

Background Art

[0002] Patent Document 1 discloses a radiation therapy system. The radiation therapy system includes a treatment planning device, a radiation therapy device, a fluoroscopic image generation device, and an irradiation control device.

[0003] The treatment planning device formulates a treatment plan for a subject. The radiation therapy device has a gantry and a radiation beam source. The radiation beam source is provided on the gantry. In radiation therapy, the subject is placed on the rotation axis of the gantry. The radiation beam source irradiates a radiation beam on a treatment target site such as a tumor in the subject when the gantry is rotating around the rotation axis. Thereby, radiation therapy for the tumor is performed. The fluoroscopic image generation device is arranged substantially coaxially with the rotation axis. The fluoroscopic image generation device generates a fluoroscopic image of the subject located on the rotation axis.

[0004] The treatment plan includes a CT image of the subject at a specific respiratory phase and isocenter coordinates for specifying the irradiation position of the radiation beam at the respiratory phase of the subject. Further, when performing radiation therapy, the position of the tumor often varies periodically with the respiration of the subject.

[0005] Therefore, the irradiation control unit controls the irradiation of the radiation beam from the radiation beam source to the subject based on the treatment plan and fluoroscopic images. Specifically, the irradiation control unit generates a DRR (Digitally Reconstructed Radiography) image of the subject at predetermined angles of the gantry based on the CT image and isocenter coordinates. The irradiation control unit also calculates the positional shift between the position of the subject's diaphragm in the DRR image and the position of the subject's diaphragm in the fluoroscopic image for the same rotation angle of the gantry. More specifically, the irradiation control unit generates two new DRR images by shifting the DRR image in the direction of the subject's head and feet, respectively. Next, the irradiation control unit calculates the normalized correlation coefficient between the original DRR image and the two new shifted DRR images and the fluoroscopic image. If the normalized correlation coefficient between the original DRR image and the fluoroscopic image is greater than the normalized correlation coefficients between the two new DRR images and the fluoroscopic image, the irradiation control unit permits the irradiation of the radiation beam from the radiation beam source to the subject. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2022-24401 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] To more accurately control the irradiation of the radiation beam from the radiation beam source to the subject, it is desirable to be able to calculate with greater precision the positional difference between the position of the subject's diaphragm in the DRR image and the position of the subject's diaphragm in the fluoroscopic image.

[0008] The present invention aims to solve the problems described above. [Means for solving the problem]

[0009] A first aspect of the present invention is an irradiation control device that controls the irradiation of a radiation beam from a radiation beam source to a subject based on a treatment plan for the subject and a fluoroscopic image of the subject, wherein the treatment plan includes a CT image of the subject at a specific respiratory phase and isocenter coordinates for specifying the irradiation position of the radiation beam at the respiratory phase of the subject, the radiation beam source is provided in a gantry and is capable of irradiating the subject with the radiation beam when the subject is located on the rotation axis of the gantry, the fluoroscopic image is generated by a fluoroscopic image generation device arranged substantially coaxially with the rotation axis, and the irradiation control device includes a treatment plan acquisition unit that acquires the treatment plan, a DRR image generation unit that generates DRR images of the subject at predetermined angles of the gantry based on the CT image and isocenter coordinates included in the acquired treatment plan, and the fluoroscopic image and the fluoroscopic image The system includes a fluoroscopic image acquisition unit that acquires the rotation angle of the gantry when the image is generated, a positional displacement amount calculation unit that calculates the amount of positional displacement between the position of the subject's diaphragm in the generated DRR image and the position of the subject's diaphragm in the acquired fluoroscopic image for the same rotation angle, and an irradiation permission determination unit that permits irradiation of the subject with the radiation beam from the radiation beam source when the amount of positional displacement is within a predetermined value. The positional displacement amount calculation unit generates a plurality of shifted images by shifting the fluoroscopic image within a predetermined range in the head-to-tail direction of the subject by a predetermined step width along the head-to-tail direction, calculates a normalized correlation coefficient with the DRR image for each of the plurality of generated shifted images, and determines the amount of shift in the head-to-tail direction of the shifted image corresponding to the largest normalized correlation coefficient among the plurality of calculated normalized correlation coefficients as the amount of positional displacement.

[0010] A second aspect of the present invention is a radiotherapy system comprising: a treatment planning device for formulating a treatment plan for radiotherapy for a subject; a gantry; a radiation beam source provided in the gantry, wherein the radiation beam can be irradiated onto the subject from the radiation beam source when the subject is located on the rotation axis of the gantry; a fluoroscopic image generation device arranged substantially coaxially with the rotation axis and generating a fluoroscopic image of the subject; and an irradiation control device that controls the irradiation of the radiation beam from the radiation beam source to the subject based on the treatment plan and the fluoroscopic image, wherein the treatment plan includes a CT image of the subject at a specific respiratory phase and isocenter coordinates for specifying the irradiation position of the radiation beam at the respiratory phase of the subject, and the irradiation control device includes a treatment plan acquisition unit for acquiring the treatment plan from the treatment planning device, and based on the CT image and isocenter coordinates included in the acquired treatment plan, the DRR image of the subject at a predetermined angle of the gantry The system comprises a DRR image generation unit that generates images at intervals, a fluoroscopic image acquisition unit that acquires the fluoroscopic image and the rotation angle of the gantry when the fluoroscopic image was generated from the fluoroscopic image generation device, a positional displacement amount calculation unit that calculates the amount of positional displacement between the position of the subject's diaphragm in the generated DRR image and the position of the subject's diaphragm in the acquired fluoroscopic image for the same rotation angle, and an irradiation permission determination unit that permits irradiation of the subject with the radiation beam from the radiation beam source when the positional displacement amount is within a predetermined value. The positional displacement amount calculation unit generates a plurality of shifted images by shifting the fluoroscopic image within a predetermined range in the head-to-tail direction of the subject by a predetermined step width along the head-to-tail direction, calculates a normalized correlation coefficient with the DRR image for each of the plurality of generated shifted images, and determines the amount of shift in the head-to-tail direction relative to the fluoroscopic image of the shifted image corresponding to the largest normalized correlation coefficient among the plurality of calculated normalized correlation coefficients as the positional displacement amount.

[0011] A third aspect of the present invention is irradiation control, which controls the irradiation of a radiation beam from a radiation beam source to a subject based on a treatment plan for the subject and a fluoroscopic image of the subject. Device operation A method wherein the treatment plan includes a CT image of the subject at a specific respiratory phase and isocenter coordinates for identifying the irradiation position of the radiation beam at the subject at the respiratory phase, the radiation beam source is provided in a gantry and is capable of irradiating the subject with the radiation beam when the subject is located on the rotation axis of the gantry, and the fluoroscopic image is generated by a fluoroscopic image generating device positioned substantially coaxially with the rotation axis, operation The method is, The treatment plan acquisition unit of the irradiation control device The first step is to obtain the aforementioned treatment plan, The DRR image generation unit of the irradiation control device is controlled by the treatment plan acquisition unit. A second step involves generating DRR images of the subject at predetermined angles of the gantry based on the CT images and isocenter coordinates included in the acquired treatment plan, The fluoroscopic image acquisition unit of the irradiation control device, A third step of obtaining the perspective image and the rotation angle of the gantry when the perspective image was generated, The positional displacement amount calculation unit of the irradiation control device, A fourth step of calculating the positional displacement between the position of the subject's diaphragm in the generated DRR image and the position of the subject's diaphragm in the acquired fluoroscopic image for the same rotation angle, The irradiation permission determination unit of the irradiation control device, The fourth step includes, when the amount of displacement is within a predetermined value, permission is granted to irradiate the subject with the radiation beam from the radiation beam source, and in the fourth step, The aforementioned positional displacement calculation unit is: With respect to the fluoroscopic image, multiple shift images are generated by shifting it within a predetermined range in the head-to-tail direction of the subject, using a predetermined step width along the head-to-tail direction. For each of the generated multiple shift images, a normalized correlation coefficient with the DRR image is calculated, and the amount of shift in the head-to-tail direction relative to the fluoroscopic image of the shift image corresponding to the largest normalized correlation coefficient among the calculated multiple normalized correlation coefficients is determined as the positional displacement amount.

[0012] A fourth aspect of the present invention is irradiation control according to the third aspect. Device operation Method The irradiation control device It is a program that is executed by a computer.

[0013] The fifth aspect of the present invention is a storage medium that stores the program of the fourth aspect.

Advantages of the Invention

[0014] In the present invention, since the shift amount corresponding to the maximum normalized correlation coefficient is determined as the amount of positional deviation, the amount of positional deviation can be calculated with higher precision. As a result, based on the determined amount of positional deviation, it is possible to accurately determine whether to permit or not permit the irradiation of the radiation beam from the radiation beam source to the subject. Consequently, the irradiation of the radiation beam from the radiation beam source to the subject can be more accurately controlled.

Brief Description of the Drawings

[0015] [Figure 1] FIG. 1 is a schematic configuration diagram of a radiation therapy system according to the present embodiment. [Figure 2] FIG. 2 is a flowchart of the operation (operation method of the irradiation control device) of the irradiation control device. [Figure 3] FIG. 3 is a flowchart showing the specific processing of step S5 in FIG. 2. [Figure 4] FIG. 4 is a diagram showing an example of a DRR image. [Figure 5] FIG. 5 is a diagram showing an example of a fluoroscopic image. [Figure 6] FIG. 6 is a diagram showing an example of a partial image region of a DRR image. [Figure 7] FIG. 7 is a diagram showing an example of a partial image region of a fluoroscopic image. [Figure 8] FIG. 8 is a diagram showing an example of a fluoroscopic image. [Figure 9] FIG. 9 is a diagram showing an example of an image obtained by raising the pixel value of each pixel constituting the fluoroscopic image in FIG. 8 to the 60th power. [Figure 10] FIG. 10 is a diagram showing an example of a DRR image. [Figure 11] FIG. 11 is a diagram showing an example of an image obtained by squaring the pixel value of each pixel constituting the DRR image in FIG. 10. [Figure 12]FIG. 12 is a diagram showing an example of an image obtained by subtracting 52000 from the pixel value of each pixel constituting the perspective image of FIG. 8, replacing negative pixel values with 0, and further raising the pixel value of each pixel to the 16th power. [Figure 13] FIG. 13 is a diagram showing an example of an image obtained by subtracting 55000 from the pixel value of each pixel constituting the perspective image of FIG. 8, replacing negative pixel values with 0, and further raising the pixel value of each pixel to the 16th power. [Figure 14] FIG. 14 is a diagram showing an example of a screen display. [Figure 15] FIG. 15 is a diagram showing an example of a screen display. [Figure 16] FIG. 16 is a diagram showing an example of a reference image. [Figure 17] FIG. 17 is an explanatory diagram for searching for the position of an optimal partial image area with respect to the search image area of a DRR image. [Figure 18] FIG. 18 is an explanatory diagram showing the position of an optimal partial image area. [Figure 19] FIG. 19 is an explanatory diagram showing the positions of optimal partial image areas at different gantry angles. [Figure 20] FIG. 20 is an explanatory diagram showing a partial image area of a perspective image including strip-shaped noise. [Figure 21] FIG. 21 is an explanatory diagram showing a state where strip-shaped noise is removed from the perspective image of FIG. 20 using a median filter.

Mode for Carrying Out the Invention

[0016] FIG. 1 is a schematic configuration diagram of a radiation therapy system 10 according to the present embodiment. The radiation therapy system 10 is provided, for example, in a medical institution (not shown). The radiation therapy system 10 performs radiation therapy on a subject (not shown). The subject is a patient's body or the like. That is, the radiation therapy system 10 performs radiation therapy on a treatment target site within the subject. More specifically, the radiation therapy system 10 performs radiation therapy on a tumor, which is the treatment target site, by irradiating the tumor with a therapeutic radiation beam (radiation beam).

[0017] The radiotherapy system 10 includes a CT scanner 12, a treatment planning device 14, a linear accelerator for radiotherapy 16 (radiation therapy device), a fluoroscopic image generation device 18, and an irradiation control device 20. The irradiation control device 20, the treatment planning device 14, and the linear accelerator for radiotherapy 16 (fluoroscopic image generation device 18) are configured to communicate bidirectionally.

[0018] The CT scanner 12 generates a CT image of each patient at a specific respiratory phase within the patient. The CT scanner 12 then transfers the generated CT image to the treatment planning device 14. The CT scanner 12 can be, for example, a treatment planning CT scanner 12 equipped with a respiratory sensor. This allows the CT scanner 12 to acquire a CT image of the patient at a specific respiratory phase (for example, while the patient is holding their breath).

[0019] The treatment planning device 14 formulates a treatment plan for radiation therapy for the subject. Specifically, the treatment planning device 14 uses CT images transferred from the CT device 12 to identify the location of the tumor during specific respiratory movements of the subject. The treatment planning device 14 formulates a treatment plan that includes the CT images and isocenter coordinates for identifying the irradiation site of the therapeutic radiation beam. The irradiation site is the tumor, which is the treatment target site.

[0020] The specific respiratory phase may be the subject's deep exhalation or deep inhalation. Deep exhalation includes the subject's maximum exhalation. Deep inhalation includes the subject's maximum inhalation. Since the treatment site temporarily stops moving during deep exhalation and deep inhalation, it is preferable to have the subject hold their breath during deep exhalation or deep inhalation. By irradiating the treatment site with a therapeutic radiation beam during such a respiratory phase, radiation therapy can be performed with high precision. However, in the radiation therapy system 10 according to this embodiment, it is also possible to repeatedly maintain the subject's breath-holding state during a specific phase between deep exhalation and deep inhalation.

[0021] The treatment planning device 14 transmits the devised treatment plan (CT image, isocenter coordinates) to the irradiation control device 20 as DICOM-RT standard data.

[0022] The radiotherapy linear accelerator 16 comprises a gantry (not shown) and a radiotherapy beam source (not shown). In the following description, the radiotherapy beam source will be referred to as the radiation beam source. The radiation beam source is located in the gantry.

[0023] During radiation therapy, the patient is positioned on the rotation axis (not shown) of the gantry. Specifically, a patient bed (not shown) is positioned along the rotation axis of the linear accelerator 16 for radiation therapy. The patient lies on the patient bed. The patient bed is movable along the rotation axis. During radiation therapy, the patient bed is moved to a position where the patient faces the radiation beam source. The radiation beam source irradiates the patient with a therapeutic radiation beam from an arbitrary rotation angle of the gantry, while the radiation beam source is facing the patient and the gantry is rotating around its rotation axis.

[0024] The fluoroscopic image generation device 18 is installed alongside the radiotherapy linear accelerator 16 so as to be positioned approximately coaxially with the rotation axis. The fluoroscopic image generation device 18 is a radiography device. The fluoroscopic image generation device 18 comprises a radiation source (not shown) and a radiation detector (not shown). The radiation source and the radiation detector are positioned on either side of the rotation axis.

[0025] The radiation source irradiates the subject with radiation, such as X-rays, when the examination table moves along a rotation axis and the radiation source and radiation detector face each other with the subject in between. The radiation detector generates a fluoroscopic image of the subject by converting the radiation that has passed through the subject into an electrical signal (image signal).

[0026] The fluoroscopic image generation device 18 streams the fluoroscopic image, the rotation angle of the gantry at the time the fluoroscopic image was generated, and the deflection correction amount. The deflection correction amount is a correction amount to compensate for distortion of the fluoroscopic image caused by the deflection of the radiation detector due to gravity. The streamed fluoroscopic image, rotation angle, and deflection correction amount are transmitted to the irradiation control device 20 via a Gigabit Ethernet line. The streaming output is in approximately real time with a delay of about 100ms to 200ms. The deflection correction amount may also be stored in advance in the memory 22 (storage medium) of the irradiation control device 20. In this case, the memory 22 stores the deflection correction amount for each predetermined angle of the gantry. In this case, the predetermined angle can be any angle within the range of 0.5° to 5°, for example. It is more preferable that the predetermined angle is 1°.

[0027] When the fluoroscopic image generation device 18 is installed in the gantry, the radiation source and radiation detector are positioned within the gantry at a rotational position of ±90° relative to the radiation beam source. When the gantry rotates, the radiation source and radiation detector rotate together with the radiation beam source around the axis of rotation. In this case, the deflection correction amount is a correction amount that depends on the rotation angle of the gantry. In addition, the deflection correction amount is a correction amount due to the deflection of the radiation detector due to gravity. In the following explanation, the deflection correction amount will be described as a correction amount that depends on the rotation angle of the gantry.

[0028] The irradiation control device 20 is a computer. The irradiation control device 20 has a memory 22, a control processing unit 24, a display unit 26, and an operation unit 28. The control processing unit 24 reads and executes a program stored in the memory 22 to realize the functions of the treatment plan acquisition unit 30, the DRR image generation unit 32, the fluoroscopic image acquisition unit 34, the positional displacement amount calculation unit 36, the irradiation permission determination unit 38, the irradiation permission signal output unit 40, and the display processing unit 42. In other words, the irradiation control device 20 functions as a control device for controlling the irradiation of a therapeutic radiation beam from a radiation beam source to a patient.

[0029] The treatment plan acquisition unit 30 acquires (receives) the treatment plan (CT image, isocenter coordinates) transmitted from the treatment planning device 14. The fluoroscopic image acquisition unit 34 acquires (receives) the fluoroscopic image, rotation angle, and deflection correction amount streamed from the fluoroscopic image generation device 18.

[0030] The DRR image generation unit 32 generates DRR images of the subject at predetermined angles around the rotation axis from the CT images and isocenter coordinates included in the treatment plan. Specifically, the DRR image generation unit 32 generates DRR images at any angle (predetermined angle) within the range of 0.5° to 5°. A predetermined angle of 1° is more preferable. The DRR image is a fluoroscopic image of the inside of the subject, simulated from the CT images used for treatment planning. That is, the DRR image generation unit 32 generates DRR images using CT images under breath-holding for a specific respiratory phase used in radiotherapy.

[0031] Furthermore, the reception of fluoroscopic images, rotation angles, and deflection correction amounts by the fluoroscopic image acquisition unit 34 is performed repeatedly in approximately real time during radiotherapy for the subject. Therefore, the treatment plan acquisition unit 30 needs to receive CT images and isocenter information before the start of radiotherapy for the subject. In addition, the DRR image generation unit 32 needs to generate DRR images before the start of radiotherapy for the subject.

[0032] Incidentally, the location of a tumor within a subject can shift by several centimeters in the head-to-tail direction due to the subject's breathing. Furthermore, fluoroscopic images are two-dimensional images given as the intensity distribution of radiation that has passed through the subject. Therefore, it is often difficult to confirm relatively small tumors in fluoroscopic images. The diaphragm also displaces due to the subject's breathing. However, the diaphragm is relatively large compared to a tumor. Also, the diaphragm is located between the low-density lungs and the high-density liver. Therefore, the diaphragm is easily visible in fluoroscopic images.

[0033] Therefore, the displacement calculation unit 36 ​​uses the fluoroscopic image and the DRR image at the same rotation angle of the gantry to calculate the displacement between the position of the subject's diaphragm in the DRR image and the position of the subject's diaphragm in the fluoroscopic image. In other words, the displacement calculation unit 36 ​​uses the position of the subject's diaphragm in the DRR image as a reference and calculates the displacement of the subject's diaphragm in the fluoroscopic image at the respiratory origin relative to the reference diaphragm position. That is, the displacement calculation unit 36 ​​calculates the displacement of the diaphragm by comparing the position of the diaphragm in the fluoroscopic image and the DRR image at the same rotation angle. This makes it possible to determine whether or not a tumor is actually present at the irradiation position of the therapeutic radiation beam predetermined in the treatment plan.

[0034] Furthermore, the radiation detector experiences deflection due to gravity, which depends on the rotation angle of the gantry. Therefore, fluoroscopic images may be affected by this deflection. Accordingly, the positional displacement calculation unit 36 ​​corrects the position of the subject's diaphragm in the fluoroscopic image using a deflection correction amount. The positional displacement calculation unit 36 ​​then calculates the respiratory-derived positional displacement using the corrected fluoroscopic image and DRR image.

[0035] The positional displacement calculation unit 36 ​​generates multiple shifted images by shifting the fluoroscopic image in the craniocaudal direction relative to the anatomical shape of the subject, for example, within a range of ±6.5 mm at intervals of 1 mm (step width), at the same rotation angle.

[0036] The predetermined range for shifting the fluoroscopic image should be within ±2mm to ±10mm relative to the original fluoroscopic image, following the craniocaudal direction in relation to the subject's anatomical shape. Specifically, the predetermined range should be an integer or half-integer value within ±2mm to ±10mm. The step size for shifting the fluoroscopic image should be within the range of 0.5mm to 1.5mm. Specifically, the step size should be an integer or half-integer value (0.5mm, 1.0mm, 1.5mm) within the range of 0.5mm to 1.5mm. Within the predetermined range, the positive direction is the head direction of the subject relative to the original fluoroscopic image, and the negative direction is the foot direction of the subject relative to the original fluoroscopic image.

[0037] The positional displacement calculation unit 36 ​​calculates a normalized correlation coefficient between each of the multiple shift images and the DRR image. For the shift image corresponding to the shift image with the largest normalized correlation coefficient, the positional displacement calculation unit 36 ​​determines the amount of shift in the head-to-tail direction relative to the original fluoroscopic image as the positional displacement amount described above. For example, if there is no positional displacement of the diaphragm between the fluoroscopic image and the DRR image, the normalized correlation coefficient will be maximized with a shift amount of 0 mm. The specific method for calculating the normalized correlation coefficient will be described later.

[0038] The irradiation permission determination unit 38 permits irradiation of the therapeutic radiation beam from the radiation beam source to the subject when the positional displacement is within a predetermined value. If the positional displacement is within a predetermined value, the position of the tumor in the DRR image and the position of the tumor in the fluoroscopic image will roughly coincide. In this state, if the therapeutic radiation beam is irradiated from the radiation beam source towards the subject, it becomes possible to perform radiation therapy on the tumor with high accuracy.

[0039] The irradiation permission signal output unit 40 transmits an irradiation permission signal to the radiotherapy linear accelerator 16 when the irradiation permission determination unit 38 determines that irradiation with the therapeutic radiation beam is permitted. The irradiation permission signal output unit 40 can communicate via software, for example, using a USB terminal. This ensures that the irradiation permission signal is reliably transmitted to the radiotherapy linear accelerator 16. The radiotherapy linear accelerator 16 can only irradiate the patient with the therapeutic radiation beam from the radiation beam source when it receives the irradiation permission signal.

[0040] The display processing unit 42 performs display processing to display various images such as DRR images and perspective images on the display unit 26. The display processing unit 42 also performs display processing to display images on the display unit 26 for setting the predetermined range, step width, and predetermined values. The display unit 26 is a display. The display unit 26 displays the images created by the display processing unit 42 on the screen.

[0041] The control unit 28 includes various control units such as a touch panel, keyboard, and mouse. The operator of the radiation therapy system 10 can set or change at least one value from a predetermined range, step size, and predetermined value by checking the display contents of the display unit 26 and operating the control unit 28.

[0042] Next, the operation of the radiotherapy system 10 including the irradiation control device 20 (irradiation control Operation of device 20 The method will be explained with reference to Figures 2 to 21. Figure 1 will also be referenced as needed during this explanation. This section describes the procedure when radiation therapy is performed on a patient's tumor in a medical institution employing the radiation therapy system 10.

[0043] Prior to the administration of radiotherapy, the CT scanner 12 (see Figure 1) generates CT images of the subject at a specific respiratory phase (e.g., deep exhalation or deep inhalation). Alternatively, the fluoroscopy image generator 18 may generate the CT images of the subject.

[0044] Next, the treatment planning device 14 acquires a CT image of the subject from the CT scanner 12 or the fluoroscopy image generation device 18. The treatment planning device 14 uses the acquired CT image to formulate a treatment plan for the subject. Specifically, the treatment planning device 14 determines the location of the tumor in the CT image as the target area for irradiation with the therapeutic radiation beam. Next, the treatment planning device 14 identifies the isocenter coordinates, which are the coordinates of the tumor's location (the target area for irradiation). This allows the device to formulate a treatment plan that includes the CT image and the isocenter coordinates.

[0045] Next, in order to perform radiation therapy, the patient is positioned on the rotation axis of the gantry. Specifically, the patient is placed lying on a bed positioned parallel to the rotation axis of the gantry, with the patient lying on their side along the rotation axis. Then, the bed is moved along the rotation axis until the radiation beam source, radiation source and radiation detector are facing the patient.

[0046] After the examination table has been moved, radiation is emitted from the radiation source of the fluoroscopic image generation device 18 onto the subject. The radiation detector converts the emitted radiation into an image signal. As a result, the radiation that has passed through the subject is converted into an image signal corresponding to the radiation intensity. This generates a fluoroscopic image.

[0047] The fluoroscopic image generation device 18 streams the fluoroscopic image of the subject, the rotation angle of the gantry, and the deflection correction amount. The deflection correction amount, which corresponds to the rotation angle, does not change over a long period of time. Therefore, the deflection correction amount can also be stored in the memory 22 of the irradiation control device 20.

[0048] Then, in step S1 (first step) in Figure 2, the treatment plan acquisition unit 30 (see Figure 1) receives the treatment plan for the subject transferred from the treatment planning device 14. As described above, the treatment plan includes the subject's CT image and isocenter coordinates at a specific respiratory phase.

[0049] In the next step S2 (second step), the DRR image generation unit 32 generates DRR images for predetermined angles of the gantry from the CT images and isocenter coordinates included in the treatment plan. The DRR image generation unit 32 outputs the generated DRR images to the positional displacement calculation unit 36. It is desirable that the processes in steps S1 and S2 be performed before the therapeutic radiation beam is irradiated onto the subject.

[0050] The DRR image generation unit 32 calculates a DRR image, for example, every 1°. Figure 4 shows an example of a DRR image. Figure 4 shows a DRR image of the subject's chest. In Figure 4, the vertical direction is the head-to-tail direction of the subject. That is, the top of Figure 4 is the head direction of the subject, and the bottom of Figure 4 is the feet direction of the subject. The diaphragm 50 is also visible in the DRR image of Figure 4. Note that Figure 4 is a DRR image at the same rotation angle as the fluoroscopic image in Figure 5, which will be described later. Furthermore, the images in Figures 4 and 5 are images used in radiotherapy for lung cancer.

[0051] The streamed rotation angle output includes decimal values. Therefore, in practice, the DRR image is used for the rotation angle that has been rounded to the first decimal place and converted to an integer.

[0052] To explain step S2 in Figure 2 in more detail, the DRR image generation unit 32 (see Figure 1) simulates and calculates a DRR image from a CT image using the mechanical parameters of the fluoroscopy image generation device 18. The mechanical parameters include the distance from the focal point of the radiation source to the isocenter, the distance from the focal point to the radiation detector, and the size and pixel size of the radiation detector. The DRR image generation unit 32 ray-traces each beam of radiation (X-rays) diverging from the focal point of the radiation source and calculates a DRR image equivalent to a fluoroscopy image by adding the CT values ​​along the straight line.

[0053] Next, in step S3 (third step), the fluoroscopic image acquisition unit 34 receives the fluoroscopic image, the rotation angle of the gantry, and the deflection correction amount streamed from the fluoroscopic image generation device 18. The fluoroscopic image acquisition unit 34 outputs the received fluoroscopic image, the rotation angle of the gantry, and the deflection correction amount to the positional displacement calculation unit 36.

[0054] Figure 5 shows an example of a fluoroscopic image received by the fluoroscopic image acquisition unit 34 (see Figure 1). Figure 5 shows a fluoroscopic image of the subject's chest. In Figure 5, the vertical direction is the head-to-tail direction of the subject. The diaphragm 52 is also visible in the fluoroscopic image of Figure 5.

[0055] In step S4 of Figure 2, the positional displacement calculation unit 36 ​​(see Figure 1) corrects the coordinates of the perspective image using the deflection correction amount. Specifically, the deflection correction amount can be divided into a head-to-tail component and a left-to-right component on the perspective image. The positional displacement calculation unit 36 ​​translates the streamed perspective image in the head-to-tail direction by only the head-to-tail component. The positional displacement calculation unit 36 ​​also translates the perspective image in the left-to-right direction by only the left-to-right component. As described above, the deflection correction amount depends on the rotation angle of the gantry. Therefore, the positional displacement calculation unit 36 ​​refers to the rotation angle for multiple perspective images and corrects the perspective image according to the rotation angle.

[0056] In the next step S5 (fourth step), the displacement calculation unit 36 ​​uses the fluoroscopic image and the DRR image of the gantry at the same rotation angle to calculate the displacement between the subject's diaphragm 50 in the DRR image and the subject's diaphragm 52 in the fluoroscopic image. More specifically, the displacement calculation unit 36 ​​uses, for example, the position of the diaphragm 50 in the DRR image in Figure 4 as a reference to calculate the displacement of the diaphragm 52 in the fluoroscopic image in Figure 5 relative to the reference position of the diaphragm 50.

[0057] In other words, DRR images and fluoroscopic images should be nearly identical if acquired at the same rotation angle. However, due to the subject's respiratory movements, the position of the diaphragm 52 in the fluoroscopic image (the actual position at the respiratory phase) may be shifted by several centimeters in the craniocaudal direction relative to the position of the diaphragm 50 in the reference DRR image (the position at a specific respiratory phase), due to the subject's anatomical shape. Therefore, the positional displacement calculation unit 36 ​​calculates the amount of positional displacement of the diaphragms 50 and 52 between the DRR image and the fluoroscopic image at the same rotation angle, as described above. The specific method for calculating the positional displacement will be described later.

[0058] In the next step S6, the irradiation permission determination unit 38 determines whether the positional displacement amount calculated by the positional displacement amount calculation unit 36 ​​is within a predetermined value. If the positional displacement amount is within the predetermined value (step S6: YES), the irradiation permission determination unit 38 determines that it is possible to accurately irradiate the tumor with a therapeutic radiation beam. Next, the irradiation permission determination unit 38 permits the irradiation of the therapeutic radiation beam and outputs the determination result indicating permission for irradiation to the irradiation permission signal output unit 40.

[0059] In the next step S7 (5th step), the irradiation permission signal output unit 40 transmits an irradiation permission signal to the radiotherapy linear accelerator 16 based on the irradiation permission determination result from the irradiation permission determination unit 38. As a result, the radiotherapy linear accelerator 16 irradiates the subject with a therapeutic radiation beam from the radiation beam source based on the received irradiation permission signal.

[0060] In step S6, if the misalignment exceeds a predetermined value (step S6: NO), the irradiation permission determination unit 38 determines that it is not possible to accurately irradiate the tumor with the therapeutic radiation beam. Next, the irradiation permission determination unit 38 outputs the determination result of not permitting irradiation to the irradiation permission signal output unit 40. Based on the input determination result, the irradiation permission signal output unit 40 does not output an irradiation permission signal. Therefore, the radiotherapy linear accelerator 16 stops irradiating the therapeutic radiation beam from the radiation beam source.

[0061] The overview of the operation of this embodiment is as described above. Next, a specific example of the process in step S5 of Figure 2 will be explained with reference to Figures 3 and 6 to 21. In this specific example, the amount of positional displacement is calculated using the following methods (1) to (5).

[0062] (1) The positional displacement calculation unit 36 ​​calculates the normalized correlation coefficient for the perspective image (see Figure 5) and the DRR image (see Figure 4) with the same rotation angle.

[0063] (2) The positional displacement calculation unit 36 ​​selects a single sub-image region in the DRR image that includes the diaphragm 50 for the fluoroscopic image and DRR image with the same rotation angle. The positional displacement calculation unit 36 ​​selects a sub-image region in the fluoroscopic image that has the same coordinates as the sub-image region in the DRR image. The positional displacement calculation unit 36 ​​calculates the normalized correlation coefficient between the sub-image region in the DRR image and the sub-image region in the fluoroscopic image.

[0064] (3) The positional displacement calculation unit 36 ​​raises the pixel values ​​of at least multiple pixels constituting the perspective image to a power from the perspective image and DRR image of the same rotation angle. As a result, the positional displacement calculation unit 36 ​​generates a new image with improved contrast of the perspective image. The positional displacement calculation unit 36 ​​calculates the normalized correlation coefficient using the generated new image, etc.

[0065] (4) The positional displacement calculation unit 36 ​​calculates the average value of the pixel values ​​of multiple pixels that make up each of the perspective image and DRR image with the same rotation angle. The positional displacement calculation unit 36 ​​generates a new image by subtracting the calculated average value from the pixel values ​​of the multiple pixels that make up the image. The positional displacement calculation unit 36 ​​calculates the normalized correlation coefficient using the generated new image, etc.

[0066] (5) The positional displacement amount calculation unit 36 ​​generates multiple shift images by shifting the fluoroscopic image within a predetermined range in the head-to-tail direction and by a predetermined step width along the head-to-tail direction. The positional displacement amount calculation unit 36 ​​calculates a normalized correlation coefficient with the DRR image for each of the multiple shift images generated. The positional displacement amount calculation unit 36 ​​determines the amount of shift in the head-to-tail direction relative to the fluoroscopic image as the positional displacement amount for the shift image corresponding to the shift image with the largest normalized correlation coefficient among the multiple normalized correlation coefficients calculated.

[0067] Figure 3 is a flowchart showing the specific processing steps in step S5 of Figure 2. In the flowchart of Figure 3, the positional displacement calculation unit 36 ​​calculates the positional displacement by combining the methods (1) to (5) described above. The specific processing operations in Figure 3 will be explained below.

[0068] In step S10 of Figure 3, the displacement calculation unit 36 ​​(see Figure 1) calculates the normalized correlation coefficient for fluoroscopic and DRR images with the same rotation angle. However, in step S5, the purpose is to calculate the displacement of the diaphragms 50 and 52. The displacement of the entire subject is expected to be spatially different. Therefore, the displacement calculation unit 36 ​​needs to focus on a small region including the diaphragms 50 and 52 to calculate the displacement.

[0069] Therefore, it is desirable for the positional displacement calculation unit 36 ​​to calculate the positional displacement using the partial image regions 54 and 56 in Figures 6 and 7, instead of the DRR image in Figure 4 and the fluoroscopic image in Figure 5. Figure 6 shows the partial image region 54 selected from the DRR image in Figure 4. The partial image region 54 (first partial image region) in Figure 6 is a single partial image region including the diaphragm 50, selected from the DRR image. The partial image region 56 (second partial image region) in Figure 7 shows the partial image region selected from the fluoroscopic image in Figure 5. The partial image region 56 in Figure 7 is a single partial image region including the diaphragm 52, selected from the fluoroscopic image. The respective partial image regions 54 and 56 in Figures 6 and 7 need to be cropped at the same coordinates. Note that if there is no positional displacement between the fluoroscopic image and the DRR image, the positional displacement calculation unit 36 ​​can calculate the positional displacement as 0.

[0070] As shown in Figure 8, the fluoroscopic image received by the fluoroscopic image acquisition unit 34 (see Figure 1) may have poor contrast due to the inclusion of scattered radiation. This fluoroscopic image is used for radiotherapy for liver cancer. In this fluoroscopic image, the irradiation volume of fluoroscopic X-rays in the visceral region is increased compared to the fluoroscopic image in Figure 5. Therefore, the fluoroscopic image in Figure 8 is significantly more affected by scattered radiation than the fluoroscopic image in Figure 5.

[0071] Figure 9 is a new perspective image with improved contrast from the perspective image in Figure 8. The perspective image in Figure 9 was obtained by raising the pixel values ​​of multiple pixels that make up the perspective image in Figure 8 to the power of 60. In other words, by raising the pixel values ​​of multiple pixels to a power, the signal level of pixels with relatively small pixel values ​​becomes even smaller. Conversely, the signal level of pixels with relatively large pixel values ​​becomes even larger. Therefore, the perspective image in Figure 9 has improved contrast compared to the perspective image in Figure 8.

[0072] The contrast of the DRR image received by the treatment planning unit 30 (see Figure 1) can also be improved by raising the pixel values ​​of multiple pixels that make up the DRR image to a power. However, the DRR image does not contain scattered radiation in calculations. Therefore, an exponent (power) of 2 to 4 is sufficient for the DRR image.

[0073] Figure 10 shows an example of a DRR image. Figure 11 shows a new DRR image obtained, for example, by squaring the pixel values ​​of multiple pixels that make up the DRR image in Figure 10. The DRR image in Figure 11 has improved contrast compared to the DRR image in Figure 10.

[0074] As described above, the positional displacement calculation unit 36 ​​(see Figure 1) does not calculate the positional displacement of the diaphragms 50 and 52 (see Figures 4 and 5) between the entire fluoroscopic image and the entire DRR image. The positional displacement calculation unit 36 ​​extracts (selects) a single partial image region 54 or 56 (see Figures 6 and 7) containing the diaphragms 50 and 52 for each of the fluoroscopic image and DRR image at the same rotation angle. The positional displacement calculation unit 36 ​​calculates the positional displacement of the diaphragms 50 and 52 for the extracted partial image region 56 of the fluoroscopic image and the partial image region 54 of the DRR image.

[0075] This allows for accurate calculation of the displacement of the diaphragm 50 and 52. Furthermore, it significantly reduces the number of pixels required when calculating the normalized correlation coefficient. As a result, the calculation of the normalized correlation coefficient can be sped up.

[0076] Here, let A(i, j) be the pixel values ​​of multiple pixels that constitute the partial image region 56 of the perspective image. Let Am be the average value of each pixel value A(i, j). Let B(i, j) be the pixel values ​​of multiple pixels that constitute the partial image region 54 of the DRR image. Let Bm be the average value of each pixel value B(i, j). In this case, the normalized correlation coefficients R1 and R2 between the partial image region 56 of the perspective image and the partial image region 54 of the DRR image are expressed by equations (1) and (2) below, respectively.

number

number

[0077] Equation (1) shows the formula for the normalized correlation coefficient using the values ​​obtained by subtracting the mean values ​​Am and Bm from each pixel value A(i, j) and B(i, j). Equation (2) shows the formula for the normalized correlation coefficient without subtracting the mean values ​​Am and Bm from each pixel value A(i, j) and B(i, j). Note that i represents the horizontal (left-right) coordinate value of each sub-image region 54 and 56. j represents the vertical (head-to-tail) coordinate value of each sub-image region 54 and 56. Also, Σ is a mathematical symbol that shows the sum of pixel values, etc., for each pixel at coordinate (i, j).

[0078] Numerical simulations comparing two normalized correlation coefficients, R1 and R2, revealed that the normalized correlation coefficient R1 showed a larger rate of change in the amount of displacement due to respiratory origin than the normalized correlation coefficient R2. In other words, the normalized correlation coefficient R1 has higher sensitivity in detecting displacement than the normalized correlation coefficient R2 and is therefore more useful.

[0079] Each pixel value A(i, j) may be either the pixel value of multiple pixels constituting the fluoroscopic image received by the fluoroscopic image acquisition unit 34, or the pixel value obtained by raising each pixel value to a power. Similarly, each pixel value B(i, j) may be either the pixel value of multiple pixels constituting the DRR image received by the treatment plan acquisition unit 30, or the pixel value obtained by raising each pixel value to a power.

[0080] Another way to improve the contrast of the perspective image is to limit the display range of the perspective image. For example, the pixel values ​​of multiple pixels that make up the perspective image are offset by scattered rays. Therefore, the positional displacement calculation unit 36 ​​can set an appropriate cutoff value. By doing so, the contrast of the perspective image can be improved by displaying only pixel values ​​greater than or equal to the cutoff value on the screen of the display unit 26.

[0081] Furthermore, perspective images are typically displayed as 16-bit non-negative integer images. In this case, the maximum pixel value of the perspective image is 65535. Therefore, the positional displacement calculation unit 36 ​​may subtract 52000 from each pixel value and replace the negative pixel values ​​with 0. Figure 12 shows a new perspective image obtained by raising the pixel values ​​of multiple pixels constituting the perspective image, after such a replacement, to the power of 16. By performing the above replacement and raising the pixel values ​​to a power, the contrast of the perspective image can be improved.

[0082] The perspective image in Figure 12 has a significantly smaller exponent compared to the perspective image in Figure 9 (exponent of power: 60). Therefore, the positional displacement calculation unit 36 ​​(see Figure 1) can perform the calculations to generate the perspective image in Figure 12 without overflowing using normal double-precision calculations. Moreover, by significantly reducing the exponent of power, the calculation can be sped up. Figure 13 shows a new perspective image when the value subtracted from each pixel value is changed to 55000. The contrast of the perspective image can be improved by changing the integer being subtracted.

[0083] In other words, conventionally, in order to increase the contrast of a perspective image, it was necessary to raise the exponent of the power very large, such as raising the pixel values ​​of multiple pixels constituting the perspective image to the power of 40. In this case, the calculation of the perspective image could overflow, resulting in calculation errors. In contrast, in this embodiment, a numerical value such as 50000 is subtracted from the pixel values ​​of multiple pixels constituting the perspective image, and any negative pixel values ​​are replaced with 0. As a result, even when the pixel values ​​of multiple pixels constituting the perspective image are raised to a power of, for example, around the power of 8, the same level of contrast as conventional methods can be obtained. In short, in this embodiment, the contrast of the perspective image can be increased while avoiding the risk of overflow. This makes it possible to accurately calculate the amount of positional displacement.

[0084] As described above, it is preferable to subtract the pixel values ​​of multiple pixels constituting the perspective image by an integer of about 40,000 to 55,000, raise each pixel value to a power of about 1 to 16, and then cut out the partial image region 56. Alternatively, it is preferable to raise the pixel values ​​of multiple pixels constituting the DRR image to a power of about 1 to 4, and then cut out the partial image region 54.

[0085] Then, in step S10, the positional displacement calculation unit 36 ​​generates multiple shifted images by shifting the fluoroscopic image in a predetermined step size within a predetermined range in the head-to-tail direction relative to the anatomical shape of the subject, at the same rotation angle. In this case, a new shifted image (first shifted image) may be generated for each of the multiple shifted images by calculating the power of the pixel value and replacing any negative pixel values ​​with 0. Furthermore, it is preferable to extract a partial image region 56 from each of the multiple shifted images or each of the multiple first shifted images.

[0086] Furthermore, in step S10, the positional displacement calculation unit 36 ​​uses equation (1) or (2) above to calculate the normalized correlation coefficient between each of the multiple shift images, multiple first shift images, or multiple partial image regions 56 and the DRR image or partial image region 54.

[0087] In step S11 of Figure 3, the positional displacement calculation unit 36 ​​(see Figure 1) determines the positional displacement using the calculation result of the normalized correlation coefficient in step S10. In step S11, the positional displacement calculation unit 36 ​​determines the shift amount of the shifted image corresponding to the largest normalized correlation coefficient among several normalized correlation coefficients as the positional displacement.

[0088] Figure 14 shows the screen display of the display unit 26 (see Figure 1) in steps S10 and S11. On the screen of the display unit 26, the image displayed on the left is the DRR image. The image displayed on the right is a perspective image with the same rotation angle as the DRR image. Partial image regions 54 and 56 are displayed in the perspective image and the DRR image, respectively.

[0089] The normalized correlation coefficient is calculated, for example, for each sub-image region 54 and 56, as described above. Since deflection correction is applied to the perspective image, the coordinates of the sub-image region 56 of the perspective image are corrected by the amount of deflection. In the example in Figure 14, the amount of deflection correction is approximately a few millimeters.

[0090] On the screen of the display unit 26, an image 58 is displayed in the center, showing a predetermined range and the calculation result of the normalized correlation coefficient. In the image 58, the predetermined range from -6.5 mm to +6.5 mm and the calculation result of the normalized correlation coefficient when the perspective image is shifted in the head-to-tail direction in 1 mm increments are displayed vertically.

[0091] In the example in Figure 14, the maximum normalized correlation coefficient is 0.951. Furthermore, the shift amount corresponding to the maximum normalized correlation coefficient is -0.5 mm. This means that the fluoroscopic image and the DRR image best match when the fluoroscopic image is shifted by -0.5 mm in the foot direction. Therefore, we can conclude that the positional displacement is -0.5 mm.

[0092] In this case, the display unit 26 highlights the maximum normalized correlation coefficient and the shift amount corresponding to the maximum normalized correlation coefficient for image 58. For example, the maximum normalized correlation coefficient and the shift amount corresponding to the maximum normalized correlation coefficient are highlighted in red. In Figure 14, the maximum normalized correlation coefficient and the shift amount corresponding to the maximum normalized correlation coefficient are enclosed in a thick frame. By highlighting in this way, the operator of the radiotherapy system 10 can visually monitor the amount of positional displacement until the irradiation of the therapeutic radiation beam is completed.

[0093] Furthermore, the display unit 26 may highlight predetermined values. In Figure 14, the predetermined value is set to, for example, ±5 mm. In this case, for example, the position of the predetermined value is indicated by a red line. In Figure 14, the position of ±5 mm, which is the predetermined value, is highlighted by a dashed line. Therefore, when the positional deviation is within the range of -5 mm to +5 mm, irradiation with a therapeutic radiation beam is permitted.

[0094] Figure 15 is an image showing an example of a GUI (Graphical User Interface) of application software to which the irradiation control device 20 is applied. This image shows a touch panel display.

[0095] In Figure 15, multiple widgets are displayed on the screen. On the left side of the screen, a widget 60 for setting predetermined values ​​is displayed. This widget 60 corresponds to the central image 58 in Figure 14. This widget 60 is used to set the position of the dashed line indicating the predetermined value. An OK button 62 and a Cancel button 64 are displayed at the bottom of the screen.

[0096] The operator of the radiation therapy system 10 can set or change a predetermined value by operating this widget 60 and then pressing the OK button 62. When a predetermined value is set or changed, the range of the set or changed predetermined value is displayed on the right side of the screen. The operator can also cancel a selected predetermined value by pressing the Cancel button 64 after selecting it.

[0097] For example, if the predetermined value is set to ±3 mm, irradiation with a therapeutic radiation beam is permitted when the positional displacement is 3 mm or less. Furthermore, considering the movement of the diaphragm 50 and 52 (see Figures 4 and 5) in the craniocaudal direction, it is also possible to set different predetermined values ​​for the head and foot directions. Figure 15 illustrates the case where the predetermined value is set in the range of -3 mm to +5 mm. In Figure 15, the left side of the screen shows the case where the fluoroscopic image is shifted in 1 mm increments within the predetermined range of -6.5 mm to 6.5 mm. The upper and lower limits of the predetermined value, +5 mm and -3 mm, are indicated by dashed lines on the left side of the screen.

[0098] Furthermore, if the positional displacement calculation unit 36 ​​determines that the positional displacement is 3 mm, the normalized correlation coefficient will be the same and maximum for both a 2.5 mm shift and a 3.5 mm shift. In other words, by setting the shift amount to a half-integer (integer + 0.5), it becomes possible to easily perform the determination process for permission to irradiate with a therapeutic radiation beam.

[0099] Furthermore, both DRR images and fluoroscopic images have their image dimensions determined as projection images onto the isocenter plane. Therefore, the amount of displacement due to respiration is also discussed in terms of displacement on the isocenter plane.

[0100] The above-mentioned partial image regions 54 and 56 (see Figures 6 and 7) were fixed coordinates regardless of the gantry rotation angle. In a typical treatment plan, the center of gravity of the tumor coincides with the center of the image. Therefore, it is desirable to include the diaphragm 50 and 52 in the partial image regions 54 and 56 while bringing them closer to the center of the image. However, depending on the gantry rotation angle, the partial image regions 54 and 56 may overlap with the cardiac image region, resulting in a significant decrease in the contrast of the diaphragm 50 and 52. Therefore, it is desirable to optimally position the partial image regions 54 and 56 according to the rotation angle.

[0101] Figure 16 shows the reference image 66. The reference image 66 is set to the same size as the partial image region 54 of the DRR image. The positional displacement calculation unit 36 ​​calculates the normalized correlation coefficient (first normalized correlation coefficient) between the reference image 66 and the DRR image, and sets the partial image region 54 at the position with the largest normalized correlation coefficient.

[0102] The reference image 66 in Figure 16 is a rectangular image region. In reference image 66, the pixel values ​​in one part along the head-to-tail direction are set lower than the pixel values ​​in the other part. Specifically, in reference image 66, the pixel value of the upper part 68 in the head direction is set to 0. Also, in reference image 66, the pixel value of the lower part 70 in the foot direction is set to 1. That is, the pixel value of the diaphragm 50 in the head direction is relatively small. Conversely, the pixel value of the diaphragm 50 in the foot direction is relatively large. Therefore, as described above, the pixel values ​​of reference image 66 are set to different pixel values ​​in the upper and lower parts.

[0103] The positional displacement calculation unit 36 ​​uses the reference image 66 as a template and sets the image region in the DRR image that is most similar to the reference image 66 as the partial image region 54. Specifically, as shown in Figure 17, the positional displacement calculation unit 36 ​​sets a search image region 72 in the DRR image for searching for the partial image region 54. Within the search image region 72, the positional displacement calculation unit 36 ​​calculates the normalized correlation coefficient between the image region and the reference image 66 while moving the image region for search in the head-to-tail direction and left-to-right direction. From among the multiple image regions in the search image region 72, the positional displacement calculation unit 36 ​​selects the image region with the maximum normalized correlation coefficient with the reference image 66 as the partial image region 54. In other words, the position of the image region with the maximum becomes the optimal placement position for the partial image region 54.

[0104] Figure 18 shows the optimized partial image region 54 for a DRR image at an arbitrary rotation angle. Figure 19 shows the optimized partial image region 54 for a DRR image at a different rotation angle. As shown in Figures 18 and 19, the optimal position of the partial image region 54 differs depending on the rotation angle of the gantry.

[0105] As shown in Figure 20, fluoroscopic images generated during irradiation with therapeutic radiation beams may contain band-like noise. If this noise is large, the accuracy of calculating the normalized correlation coefficient between the DRR image and the fluoroscopic image may decrease.

[0106] Therefore, the positional displacement calculation unit 36 ​​(see Figure 1) applies a one-dimensional median filter to the perspective image in Figure 20 in the left-right direction (horizontal direction). Figure 21 shows the perspective image after the one-dimensional median filter processing. It can be seen that the band-like noise in the perspective image in Figure 21 has been reduced compared to the perspective image in Figure 20.

[0107] In median filtering, large fluctuations (noise) are removed by selecting the median value of a pixel sequence (a list of pixel values) of a predetermined length. This is a commonly used filtering method in the field of signal processing. Therefore, applying one-dimensional median filtering is useful because it can suppress band-like noise contained in perspective images.

[0108] The positional displacement calculation unit 36 ​​calculates the normalized correlation coefficient using the perspective image after one-dimensional median filtering. Alternatively, the positional displacement calculation unit 36 ​​may apply one-dimensional median filtering to the shift image, the first shift image, or the partial image region 56.

[0109] Furthermore, the present invention is not limited to the embodiments described above, and various configurations can be taken without departing from the spirit of the invention.

[0110] The inventions that can be understood from the above embodiments are described below.

[0111] A first aspect of the present invention is an irradiation control device (20) that controls the irradiation of a radiation beam from a radiation beam source to a subject based on a treatment plan for the subject and a fluoroscopic image of the subject, wherein the treatment plan includes a CT image of the subject at a specific respiratory phase and isocenter coordinates for specifying the irradiation position of the radiation beam at the respiratory phase of the subject, the radiation beam source is provided in a gantry and is capable of irradiating the subject with the radiation beam when the subject is located on the rotation axis of the gantry, the fluoroscopic image is generated by a fluoroscopic image generation device (18) arranged substantially coaxially with the rotation axis, and the irradiation control device includes a treatment plan acquisition unit (30) that acquires the treatment plan, a DRR image generation unit (32) that generates DRR images of the subject at predetermined angles of the gantry based on the CT image and isocenter coordinates included in the acquired treatment plan, and the fluoroscopic image and the fluoroscopic image The system includes a fluoroscopic image acquisition unit (34) that acquires the rotation angle of the gantry when it is generated, a positional displacement amount calculation unit (36) that calculates the amount of positional displacement between the position of the subject's diaphragm (50) in the generated DRR image and the position of the subject's diaphragm (52) in the acquired fluoroscopic image for the same rotation angle, and an irradiation permission determination unit (38) that permits irradiation of the subject with the radiation beam from the radiation beam source when the amount of positional displacement is within a predetermined value. The positional displacement amount calculation unit generates a plurality of shifted images by shifting the fluoroscopic image within a predetermined range in the head-to-tail direction of the subject by a predetermined step width along the head-to-tail direction, calculates a normalized correlation coefficient with the DRR image for each of the plurality of generated shifted images, and determines the amount of shift in the head-to-tail direction of the shifted image with respect to the fluoroscopic image corresponding to the largest normalized correlation coefficient among the plurality of calculated normalized correlation coefficients as the amount of positional displacement.

[0112] According to the present invention, the shift amount corresponding to the maximum normalized correlation coefficient is determined as the positional displacement, allowing for more accurate calculation of this positional displacement. This enables a more accurate determination of whether or not to permit radiation beam irradiation from the radiation beam source to the subject, based on the determined positional displacement. As a result, the irradiation of the radiation beam from the radiation beam source to the subject can be controlled more precisely.

[0113] The effects of the present invention will be explained in more detail.

[0114] In fluoroscopic images, the area ratio between the liver and the lung region above the liver fluctuates with respiration. Numerical simulations have shown that by calculating the normalized correlation coefficient so that the image correlation between the fluoroscopic image and the DRR image is maximized when this area ratio is approximately 1:1, the amount of positional displacement corresponding to the maximum normalized correlation coefficient can be calculated with high accuracy. In other words, when the fluoroscopic image is shifted in the craniocaudal direction, the maximum normalized correlation coefficient is obtained when the area ratio between the liver and lung region is 1:1. As a result, the accuracy of calculating the amount of positional displacement is improved.

[0115] In contrast, in DRR images, the area ratio of the liver and lung regions in the image area used to calculate image correlation is pre-set to approximately 1:1. When the DRR image is shifted in the craniocaudal direction and the amount of displacement corresponding to the maximum normalized correlation coefficient is calculated, the maximum normalized correlation coefficient is obtained when the area ratio is not 1:1. This actually reduces the accuracy of the displacement calculation.

[0116] Furthermore, in this invention, multiple shifted images are generated by shifting the fluoroscopic image within a predetermined range using a predetermined step size, and the normalized correlation coefficient between each of the multiple shifted images and the DRR image is calculated. This makes it possible to directly determine the shift amount corresponding to the maximum normalized correlation coefficient. Moreover, since the calculated shift amount is determined as the positional displacement amount, the positional displacement amount can be calculated accurately. Furthermore, by directly comparing the positional displacement amount with a predetermined value, the determination process for permitting irradiation with a radiation beam can be performed with high accuracy.

[0117] In a first embodiment of the present invention, the positional displacement calculation unit selects a single first partial image region (54) including the diaphragm from the DRR image, selects a second partial image region (56) at the same coordinate position as the first partial image region from the fluoroscopic image, and generates a plurality of shifted images by shifting the selected second partial image region within the predetermined range and by the step size, and calculates a normalized correlation coefficient with the first partial image region for each of the plurality of shifted images generated.

[0118] This allows for accurate calculation of diaphragm displacement. Furthermore, the number of pixels required for calculating the normalized correlation coefficient is significantly reduced, thus speeding up the calculation.

[0119] In a first embodiment of the present invention, the positional displacement calculation unit sets a search image region (72) within the DRR image, searches for an image region including the diaphragm from the set search image region, and selects the searched image region as the first partial image region.

[0120] This allows for the extraction of the first partial image region, including the diaphragm, in a short amount of time.

[0121] In a first aspect of the present invention, the positional displacement calculation unit calculates a first normalized correlation coefficient between each of the plurality of image regions in the search image region and a predetermined reference image (66), and selects the image region with the maximum first normalized correlation coefficient as the first partial image region.

[0122] This allows for efficient and rapid extraction of the first partial image region.

[0123] In a first embodiment of the present invention, the reference image is a rectangular image region, and the pixel values ​​(68) of one portion along the head-to-tail direction are lower than the pixel values ​​of the other portion (70).

[0124] If the calculation domain for image correlation is fixed, depending on the rotation angle of the gantry, the liver and the heart located above the liver may overlap in the fluoroscopic image, causing the contrast of the upper edge of the liver to disappear. Therefore, by setting the position where the image correlation with the reference image (first normalized correlation coefficient) is maximized as the first partial image domain, the partial image domain can be set according to the rotation angle. As a result, the disappearance of the contrast of the upper edge of the liver in the fluoroscopic image can be avoided.

[0125] In a first aspect of the present invention, the positional displacement calculation unit calculates a first shift image by subtracting a constant value from the pixel values ​​of multiple pixels constituting the shift image for each of the multiple shift images, replacing any pixels with negative pixel values ​​with 0, and further raising the pixel values ​​of the multiple pixels to a power. The unit calculates a first average value of the pixel values ​​of the multiple pixels constituting the first shift image, and generates a first DRR image by raising the pixel values ​​of the multiple pixels constituting the DRR image to a power. The unit calculates a second average value of the pixel values ​​of the multiple pixels constituting the first DRR image for each of the multiple first shift images, and calculates the normalized correlation coefficient using the deviation between each of the pixel values ​​of the multiple pixels constituting the first shift image and the first average value, and the deviation between each of the pixel values ​​of the multiple pixels constituting the first DRR image and the second average value.

[0126] This allows for the calculation of the normalized correlation coefficient without overflow. Furthermore, it speeds up the calculation of the normalized correlation coefficient. Additionally, it can improve the contrast of perspective images.

[0127] In a first embodiment of the present invention, the positional displacement calculation unit applies a median filter to the perspective image, the shift image, or the first shift image.

[0128] When generating fluoroscopic images during radiotherapy, if the radiation beam's field is large, excessive scattered radiation generated within the patient can be mixed into the fluoroscopic image as linear noise. As a result, the calculation of the normalized correlation coefficient between the fluoroscopic image and the DRR image may be inaccurate. Therefore, by applying a median filter to the fluoroscopic image, shift image, or first-shift image prior to calculating the normalized correlation coefficient, noise can be reduced and the accuracy of the normalized correlation coefficient calculation can be improved.

[0129] In a first embodiment of the present invention, the positional displacement calculation unit sets the constant value to 0, selects a number from 1 to 70 as an exponent for raising the pixel values ​​of the multiple pixels constituting the shift image to a power for each of the multiple shift images, and selects a number from 1 to 4 as an exponent for raising the pixel values ​​of the multiple pixels constituting the DRR image to a power for each of the multiple pixels.

[0130] This can improve the contrast of perspective images.

[0131] In a first embodiment of the present invention, the positional displacement calculation unit selects a number from 40000 to 55000 as the constant value, selects a number from 1 to 30 as an exponent for raising the pixel values ​​of the multiple pixels constituting the shift image to a power for each of the multiple shift images, and selects a number from 1 to 4 as an exponent for raising the pixel values ​​of the multiple pixels constituting the DRR image to a power.

[0132] In this case, it is possible to speed up the calculation of the normalized correlation coefficient while avoiding overflow, and also to improve the contrast of the perspective image.

[0133] In a first embodiment of the present invention, the predetermined range is a range of ±2 mm to ±10 mm along the head-to-tail direction in relation to the anatomical shape of the subject with respect to the fluoroscopic image, and the step width is within the range of 0.5 mm to 1.5 mm.

[0134] This allows for a more accurate calculation of the positional displacement.

[0135] In a first embodiment of the present invention, the predetermined range is an integer or half-integer value within the range of ±2 mm to ±10 mm, and the step width is an integer or half-integer value within the range of 0.5 mm to 1.5 mm.

[0136] This allows for a more accurate calculation of the positional displacement.

[0137] In a first embodiment of the present invention, the treatment plan acquisition unit acquires the CT image and the isocenter coordinates before the fluoroscopic image acquisition unit acquires the fluoroscopic image and the rotation angle.

[0138] This allows for the pre-calculation of DRR images using CT images and isocenter coordinates, making it possible to calculate the normalized correlation coefficient during radiotherapy treatment for the subject.

[0139] In a first embodiment of the present invention, the perspectival image acquisition unit acquires the perspectival image and the rotation angle from the perspectival image generation device via a Gigabit Ethernet line.

[0140] This allows for the reception of each piece of information being streamed in near real-time.

[0141] In a first embodiment of the present invention, the treatment plan acquisition unit acquires the CT image in DICOM-RT format and the isocenter coordinate data from the treatment planning device (14) that creates the treatment plan.

[0142] This allows for the reception of CT images and isocenter coordinates using existing equipment.

[0143] In a first embodiment of the present invention, the DRR image generation unit generates the DRR image at predetermined angles of 0.5° to 5°.

[0144] This allows for the generation of DRR images at, for example, 1° intervals for all gantry rotation angles, and these DRR images can be used as reference images for fluoroscopic images. As a result, when applied to intensity-modulated rotational irradiation (VMAT), radiotherapy can be performed with high precision, even under repeated breath-holding.

[0145] A second aspect of the present invention is a radiotherapy system (10) comprising: a treatment planning device for formulating a treatment plan for radiotherapy for a subject; a gantry; a radiation beam source provided in the gantry, wherein the radiation beam can be irradiated from the radiation beam source to the subject when the subject is located on the rotation axis of the gantry; a fluoroscopic image generating device arranged substantially coaxially with the rotation axis and generating a fluoroscopic image of the subject; and an irradiation control device that controls the irradiation of the radiation beam from the radiation beam source to the subject based on the treatment plan and the fluoroscopic image, wherein the treatment plan includes a CT image of the subject at a specific respiratory phase and isocenter coordinates for identifying the irradiation position of the radiation beam at the respiratory phase of the subject, and the irradiation control device includes a treatment plan acquisition unit that acquires the treatment plan from the treatment planning device, and based on the CT image and isocenter coordinates included in the acquired treatment plan, the DRR image of the subject is irradiated from the gantry The system includes a DRR image generation unit that generates images at predetermined angles, a fluoroscopic image acquisition unit that acquires the fluoroscopic image and the rotation angle of the gantry when the fluoroscopic image was generated from the fluoroscopic image generation device, a positional displacement amount calculation unit that calculates the amount of positional displacement between the position of the subject's diaphragm in the generated DRR image and the position of the subject's diaphragm in the acquired fluoroscopic image for the same rotation angle, and an irradiation permission determination unit that permits irradiation of the subject with the radiation beam from the radiation beam source when the positional displacement amount is within a predetermined value. The positional displacement amount calculation unit generates a plurality of shifted images by shifting the fluoroscopic image within a predetermined range in the head-to-tail direction of the subject by a predetermined step width along the head-to-tail direction, calculates a normalized correlation coefficient with the DRR image for each of the plurality of generated shifted images, and determines the amount of shift in the head-to-tail direction relative to the fluoroscopic image of the shifted image corresponding to the largest normalized correlation coefficient among the plurality of calculated normalized correlation coefficients as the positional displacement amount.

[0146] The present invention also provides the same effects as the first embodiment.

[0147] A third aspect of the present invention is irradiation control, which controls the irradiation of a radiation beam from a radiation beam source to a subject based on a treatment plan for the subject and a fluoroscopic image of the subject. Device operation A method wherein the treatment plan includes a CT image of the subject at a specific respiratory phase and isocenter coordinates for identifying the irradiation position of the radiation beam at the subject at the respiratory phase, the radiation beam source is provided in a gantry and is capable of irradiating the subject with the radiation beam when the subject is located on the rotation axis of the gantry, and the fluoroscopic image is generated by a fluoroscopic image generating device positioned substantially coaxially with the rotation axis, operation The method is, The treatment plan acquisition unit of the irradiation control device The first step is to obtain the aforementioned treatment plan, The DRR image generation unit of the irradiation control device is controlled by the treatment plan acquisition unit. A second step involves generating DRR images of the subject at predetermined angles of the gantry based on the CT images and isocenter coordinates included in the acquired treatment plan, The fluoroscopic image acquisition unit of the irradiation control device, A third step of obtaining the perspective image and the rotation angle of the gantry when the perspective image was generated, The positional displacement amount calculation unit of the irradiation control device, A fourth step of calculating the positional displacement between the position of the subject's diaphragm in the generated DRR image and the position of the subject's diaphragm in the acquired fluoroscopic image for the same rotation angle, and when the positional displacement is within a predetermined value, The irradiation permission determination unit of the irradiation control device, The fourth step includes a fifth step of allowing the radiation beam from the radiation beam source to irradiate the subject with the radiation beam, The aforementioned positional displacement calculation unit is: With respect to the fluoroscopic image, multiple shift images are generated by shifting it within a predetermined range in the head-to-tail direction of the subject, using a predetermined step width along the head-to-tail direction. For each of the generated multiple shift images, a normalized correlation coefficient with the DRR image is calculated, and the amount of shift in the head-to-tail direction relative to the fluoroscopic image of the shift image corresponding to the largest normalized correlation coefficient among the calculated multiple normalized correlation coefficients is determined as the positional displacement amount.

[0148] The present invention also provides the same effects as the first embodiment.

[0149] A fourth aspect of the present invention is irradiation control according to the third aspect. Device operation Method The irradiation control device This is a program to be executed by the computer (20).

[0150] The present invention also provides the same effects as the first embodiment.

[0151] A fifth aspect of the present invention is a storage medium (22) for storing the program of the fourth aspect.

[0152] The present invention also provides the same effects as the first embodiment. [Explanation of Symbols]

[0153] 10…Radiation therapy system 14…Treatment planning system 16…Radiation therapy linac (radiation therapy equipment) 18…Fluoroscopy image generation device 20…Irradiation control device (computer) 22...Memory (storage medium) 30...Treatment plan acquisition unit 32...DRR image generation unit 34...Periscopic image acquisition unit 36... Positional displacement calculation unit 38... Irradiation permission determination unit 50, 52... Diaphragm

Claims

1. An irradiation control device that controls the irradiation of a radiation beam from a radiation beam source to a subject based on a treatment plan for the subject and a fluoroscopic image of the subject, The treatment plan includes a CT image of the subject at a specific respiratory phase and isocenter coordinates for identifying the irradiation position of the radiation beam at the subject at the respiratory phase. The radiation beam source is provided in the gantry and is capable of irradiating the subject with the radiation beam when the subject is positioned on the rotation axis of the gantry. The aforementioned perspective image is generated by a perspective image generation device positioned substantially coaxially with the rotation axis. The irradiation control device is A treatment plan acquisition unit that acquires the aforementioned treatment plan, A DRR image generation unit generates DRR images of the subject at predetermined angles of the gantry based on the CT images and isocenter coordinates included in the acquired treatment plan, A perspective image acquisition unit that acquires the perspective image and the rotation angle of the gantry when the perspective image was generated, A positional displacement calculation unit calculates the amount of positional displacement between the position of the subject's diaphragm in the generated DRR image and the position of the subject's diaphragm in the acquired fluoroscopic image for the same rotation angle, An irradiation permission determination unit that permits irradiation of the radiation beam from the radiation beam source to the subject when the amount of positional displacement is within a predetermined value, Equipped with, The aforementioned positional displacement calculation unit is: Multiple shifted images are generated by shifting the aforementioned fluoroscopic image within a predetermined range in the head-to-tail direction of the subject, using a predetermined step width along the head-to-tail direction. For each of the multiple shift images generated, calculate the normalized correlation coefficient with the DRR image. An irradiation control device that determines the amount of shift in the head-to-tail direction relative to the fluoroscopic image of the shift image corresponding to the largest normalized correlation coefficient among the calculated multiple normalized correlation coefficients as the amount of positional displacement.

2. In the irradiation control device according to claim 1, The aforementioned positional displacement calculation unit is: From the aforementioned DRR images, a single first partial image region including the diaphragm is selected. From the aforementioned perspective image, select a second partial image region at the same coordinate position as the first partial image region. Multiple shifted images are generated by shifting the selected second partial image region within the predetermined range and by the specified step width. An irradiation control device that calculates a normalized correlation coefficient between each of the multiple shifted images generated and the first partial image region.

3. In the irradiation control device according to claim 2, The irradiation control device comprises a positional displacement calculation unit which sets a search image region within the DRR image, searches for an image region including the diaphragm from the set search image region, and selects the searched image region as the first partial image region.

4. In the irradiation control device according to claim 3, The irradiation control device comprises a positional displacement calculation unit which calculates a first normalized correlation coefficient between each of a plurality of image regions within the search image region and a predetermined reference image, and selects the image region with the maximum first normalized correlation coefficient as the first partial image region.

5. In the irradiation control device according to claim 4, The aforementioned reference image is a rectangular image region, and the pixel values ​​of one portion along the head-to-tail direction are lower than the pixel values ​​of the other portion, in this irradiation control device.

6. In the irradiation control device according to any one of claims 1 to 5, The aforementioned positional displacement calculation unit is: For each of the multiple shift images, the pixel values ​​of the multiple pixels constituting the shift image are subtracted by a fixed value, and if there are pixels with negative pixel values ​​among the multiple pixels, the negative pixel values ​​are replaced with 0, and furthermore, the pixel values ​​of the multiple pixels are raised to a power to generate a first shift image. The first average value of the pixel values ​​of the multiple pixels constituting the first shift image is calculated, A first DRR image is generated by raising the pixel values ​​of multiple pixels constituting the DRR image to a power. The second average value of the pixel values ​​of the multiple pixels constituting the first DRR image is calculated, An irradiation control device that calculates the normalized correlation coefficient for each of the plurality of first shift images, using the deviation between each of the pixel values ​​of the plurality of pixels constituting the first shift image and the first mean value, and the deviation between each of the pixel values ​​of the plurality of pixels constituting the first DRR image and the second mean value.

7. In the irradiation control device according to claim 6, The positional displacement calculation unit applies median filtering to the transmission image, the shifted image, or the first shifted image, in the irradiation control device.

8. In the irradiation control device according to claim 6, The aforementioned positional displacement calculation unit is: Set the aforementioned constant value to 0, For each of the multiple shift images, select a number from 1 to 70 as the exponent for raising the pixel values ​​of the multiple pixels constituting the shift image to a power. An irradiation control device that selects one of 1 to 4 as an exponent for raising the pixel values ​​of multiple pixels constituting the DRR image to a power.

9. In the irradiation control device according to claim 6, The aforementioned positional displacement calculation unit is: Select any number between 40,000 and 55,000 as the aforementioned constant value. For each of the multiple shift images, select a number from 1 to 30 as the exponent for raising the pixel values ​​of the multiple pixels constituting the shift image to a power. An irradiation control device that selects one of 1 to 4 as an exponent for raising the pixel values ​​of multiple pixels constituting the DRR image to a power.

10. In the irradiation control device according to any one of claims 1 to 5, The predetermined range is a range of ±2 mm to ±10 mm along the head-to-tail direction in relation to the anatomical shape of the subject relative to the fluoroscopic image. The aforementioned step width is within the range of 0.5 mm to 1.5 mm, in the irradiation control device.

11. In the irradiation control device according to claim 10, The predetermined range is an integer or half-integer value within ±2 mm to ±10 mm. The aforementioned step width is an integer or half-integer value within the range of 0.5 mm to 1.5 mm, in the irradiation control device.

12. In the irradiation control device according to any one of claims 1 to 5, The treatment plan acquisition unit is an irradiation control device that acquires the CT image and the isocenter coordinates before the fluoroscopic image acquisition unit acquires the fluoroscopic image and the rotation angle.

13. In the irradiation control device according to any one of claims 1 to 5, The above-mentioned fluoroscopic image acquisition unit is an irradiation control device that acquires the fluoroscopic image and the rotation angle from the fluoroscopic image generation device via a Gigabit Ethernet line.

14. In the irradiation control device according to any one of claims 1 to 5, The treatment plan acquisition unit is an irradiation control device that acquires the CT image and isocenter coordinate data in the DICOM-RT standard from the treatment planning device that creates the treatment plan.

15. In the irradiation control device according to any one of claims 1 to 5, The DRR image generation unit is an irradiation control device that generates the DRR image at predetermined angles of 0.5° to 5°.

16. A treatment planning device for formulating a treatment plan for radiation therapy for a subject, A radiotherapy apparatus comprising a gantry and a radiation beam source provided in the gantry, wherein when the subject is positioned on the rotation axis of the gantry, a radiation beam can be irradiated from the radiation beam source to the subject. A fluoroscopic image generating device, which is arranged substantially coaxially with the rotation axis and generates a fluoroscopic image of the subject, An irradiation control device that controls the irradiation of the radiation beam from the radiation beam source to the subject based on the treatment plan and the fluoroscopic image, A radiotherapy system having, The treatment plan includes a CT image of the subject at a specific respiratory phase and isocenter coordinates for identifying the irradiation position of the radiation beam at the subject at the respiratory phase. The irradiation control device is A treatment plan acquisition unit that acquires the treatment plan from the treatment planning device, A DRR image generation unit generates DRR images of the subject at predetermined angles of the gantry based on the CT images and isocenter coordinates included in the acquired treatment plan, A transparent image acquisition unit that acquires the transparent image and the rotation angle of the gantry when the transparent image was generated from the transparent image generation device, A positional displacement calculation unit calculates the amount of positional displacement between the position of the subject's diaphragm in the generated DRR image and the position of the subject's diaphragm in the acquired fluoroscopic image for the same rotation angle, An irradiation permission determination unit that permits irradiation of the radiation beam from the radiation beam source to the subject when the amount of positional displacement is within a predetermined value, Equipped with, The aforementioned positional displacement calculation unit is: Multiple shifted images are generated by shifting the aforementioned fluoroscopic image within a predetermined range in the head-to-tail direction of the subject, using a predetermined step width along the head-to-tail direction. For each of the multiple shift images generated, calculate the normalized correlation coefficient with the DRR image. A radiotherapy system that determines the amount of head-to-tail shift of the shift image with respect to the fluoroscopic image, based on the amount of the largest normalized correlation coefficient among the calculated normalized correlation coefficients, as the amount of positional displacement.

17. A method for operating an irradiation control device that controls the irradiation of a radiation beam from a radiation beam source to a subject based on a treatment plan for the subject and a fluoroscopic image of the subject, The treatment plan includes a CT image of the subject at a specific respiratory phase and isocenter coordinates for identifying the irradiation position of the radiation beam at the subject at the respiratory phase. The radiation beam source is provided in the gantry and is capable of irradiating the subject with the radiation beam when the subject is positioned on the rotation axis of the gantry. The aforementioned perspective image is generated by a perspective image generation device positioned substantially coaxially with the rotation axis. The aforementioned operation method is, The first step is for the treatment plan acquisition unit of the irradiation control device to acquire the treatment plan, The DRR image generation unit of the irradiation control device generates a DRR image of the subject at predetermined angles of the gantry based on the CT image and isocenter coordinates included in the treatment plan acquired by the treatment plan acquisition unit, in a second step, The third step involves the fluoroscopic image acquisition unit of the irradiation control device acquiring the fluoroscopic image and the rotation angle of the gantry when the fluoroscopic image was generated. The fourth step involves the positional displacement calculation unit of the irradiation control device calculating the positional displacement between the position of the subject's diaphragm in the generated DRR image and the position of the subject's diaphragm in the acquired fluoroscopic image for the same rotation angle. When the amount of displacement is within a predetermined value, the irradiation permission determination unit of the irradiation control device permits the irradiation of the radiation beam from the radiation beam source to the subject in a fifth step, It has, In the fourth step described above, The aforementioned positional displacement calculation unit is: Multiple shifted images are generated by shifting the aforementioned fluoroscopic image within a predetermined range in the head-to-tail direction of the subject, using a predetermined step width along the head-to-tail direction. For each of the multiple shift images generated, calculate the normalized correlation coefficient with the DRR image. A method for operating an irradiation control device, wherein the amount of shift in the head-to-tail direction relative to the fluoroscopic image of the shift image corresponding to the largest normalized correlation coefficient among the calculated multiple normalized correlation coefficients is determined as the positional displacement amount.

18. A program that causes a computer, which is the irradiation control device, to execute the operation method of the irradiation control device described in claim 17.

19. A storage medium for storing the program described in claim 18.

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