Dose distribution calculation system, program, and dose distribution calculation method

Abstract

A dose distribution calculation system 113 calculates the transmittance of a multi-leaf collimator for each transport particle and calculates a dose distribution using the transmittance. In calculation of the transmittance: a high-importance condition is set on the basis of the opening shape of the multi-leaf collimator or the generation process of the particles; a determination is made as to whether the transport particles satisfy the high-importance condition; and, for a transport particle determined to not satisfy the high-importance condition, the weight of the transport particle is increased or decreased so that the expected value of the weight of all the transport particles is preserved. There are thereby provided a dose distribution calculation system, a program, and a dose distribution calculation method with which it is possible to obtain a highly accurate result of calculating dose distribution in a shorter time than before.

Description

Dose distribution calculation system, program, and dose distribution calculation method

[0001] The present invention relates to a dose distribution calculation system, a program, and a dose distribution calculation method that are suitable for use in a radiation therapy system for treating an affected area such as a tumor by irradiating it with radiation.

[0002] Non-Patent Document 1 describes a multi-leaf collimator particle transport model using a semi-analytical computational method that is accurate, fast, and efficient for use in Monte Carlo dose distribution calculations, and is therefore applicable to iterative intensity modulated radiation therapy dose distribution calculations.

[0003] JV Siebers et al., “A method for photon beam Monte Carlo multileaf collimator particle transport”, Phys. Med. Biol. 47, 3225-3249 (2002).

[0004] Radiation therapy is a local therapy that kills cancer cells by irradiating the cancer lesion with radiation and damaging the DNA of the cancer cells.

[0005] X-rays are the predominant type of radiation used, and treatment is carried out using high-precision irradiation methods that irradiate radiation from multiple irradiation directions and improve dose concentration, such as intensity-modulated radiation therapy (IMRT), volumetric modulated arc therapy (VMAT), and dynamic wave arc (DWA), which performs continuous non-coplanar irradiation with a wave-like trajectory.

[0006] In VMAT, continuous rotational irradiation is performed while continuously modulating three parameters: gantry angle, multi-leaf collimator (MLC) aperture shape, and dose rate, making it possible to deliver a high dose to the tumor while reducing the dose to normal tissue.

[0007] In radiation therapy, the dose distribution inside the patient's body is simulated using treatment planning software based on the patient's CT images and the prescribed dose set by the doctor, and irradiation parameters such as the treatment device's equipment parameters (such as the collimator opening shape), irradiation angle, irradiation amount per irradiation angle, and energy are determined.

[0008] Here, the determined irradiation parameters and the dose distribution that is the basis for calculating the irradiation parameters are called a treatment plan, and the irradiation device irradiates the beam based on the treatment plan to form a desired dose distribution on the affected area.

[0009] To ensure the soundness of the treatment plan, it is necessary to check whether the equipment can deliver the dose distribution specified in the treatment plan before irradiating the patient. This is called patient QA. The most common method for patient QA is to actually irradiate a homogeneous phantom, such as a solid phantom, and measure the dose distribution.

[0010] Through patient QA, clinical staff (1) verify the calculation accuracy of the dose distribution calculation algorithm of the treatment planning software, (2) verify the effect of irradiation errors of the irradiation device on the dose distribution within a homogeneous phantom, and (3) confirm the validity of the device operation in relation to the treatment plan.

[0011] Items (1) and (2) are verified by comparing the actual measurement results with the calculation results of the treatment planning software. Each treatment facility sets its own patient QA evaluation items and criteria, and once it has been confirmed that each evaluation item meets the criteria, it can be used as a treatment plan. If the criteria are not met, patient QA will be re-performed and the treatment plan will be re-created.

[0012] Radiation therapy involves irradiating the affected area once a day for several days to several weeks, which can cause the tumor to expand or shrink and changes to the internal structures around the tumor during the treatment period.

[0013] When such changes occur, there is a growing need for adaptive therapy, which recreates a treatment plan and performs treatment using the recreated treatment plan.One type of adaptive therapy, in which a treatment plan is recreated immediately before treatment while the patient is lying on the treatment bed, is called online adaptive therapy (hereinafter referred to as OART).

[0014] OART is a treatment method that optimizes the radiation dose according to the patient's body shape, which varies from treatment day to treatment day, such as the condition of the nasal cavity or intestines. By optimizing the dose distribution on each treatment day, the area outside the target to be irradiated as a margin can be made smaller, making it possible to reduce damage to normal tissue.

[0015] However, OART poses various development challenges, such as an increase in treatment time due to the significant increase in processes performed during treatment, such as recreating treatment plans. One of the challenges is patient QA during OART.

[0016] In OART, patient QA of the re-planned treatment plan must be performed quickly while the patient is lying on the bed, which makes it difficult to perform the actual measurement-based patient QA that has been performed up until now. Therefore, in recent years, a method has been proposed for patient QA in which dose distribution calculations are performed using high-precision dose distribution calculation methods such as the Monte Carlo method, and whether the dose distribution can be formed as planned is confirmed.

[0017] The Monte Carlo method uses random numbers to probabilistically reproduce the behavior of individual radiation beams, and repeats this process many times (several million times or more) to estimate the average behavior of radiation and calculate the dose distribution. Performing a Monte Carlo simulation across the entire process, from the stage where electrons accelerated in the accelerator generate X-rays, to the stage where the X-rays pass through the MLC to form the irradiation field, and finally the X-rays that have passed through the MLC deliver the dose inside the patient, requires an enormous amount of calculation time.

[0018] Therefore, when calculating the dose distribution in X-ray therapy, it is common to calculate the dose distribution by dividing it into two parts: (A) the process up to the formation of the irradiation field by the MLC (transport calculation within the irradiation device), and (B) the process in which the X-rays impart a dose inside the patient's body (internal transport calculation).

[0019] The present invention relates to (A) transport within the irradiation device. In VMAT irradiation and the like, the leaf shape changes in a complex manner according to the rotation of the gantry, so in order to efficiently perform transport calculations for the MLC irradiation field formation process, it is common to simulate the irradiation field formation process using analytical or semi-analytical methods.

[0020] Non-Patent Document 1 discloses a calculation method for the irradiation field calculation process using a semi-analytical method. In the semi-analytical method, particle species information (particle type, position, energy, and traveling direction information) upstream of the MLC is generated in advance by calculating using Monte Carlo simulation or modeling, and this particle species information is used as input to calculate the path length within the MLC for each particle, and the transmittance calculated from the calculated path length is set as the particle weight, thereby taking into account the influence of the MLC shape.

[0021] The number of particles used in the Monte Carlo simulation ranges from tens to hundreds of millions, and the semi-analytical method involves calculating the path length of the MLC several times for each particle, and then calculating the transmittance for each particle. Therefore, the calculation for particles passing through the closed part of the MLC takes longer than the calculation for particles passing through the open part of the MLC.

[0022] In high-precision irradiation, such as VMAT and DWA, the dose distribution is formed by overlapping irradiation fields irradiated from multiple irradiation directions, so the low-dose region of the MLC closure becomes more important. Therefore, there was a concern that the calculation accuracy would decrease if the calculation of particles passing through the closure was ignored. In particular, in small-field cases, the opening area is small, so the calculation cost for transmittance calculation increases, which was an issue.

[0023] The present invention provides a dose distribution calculation system, a program, and a dose distribution calculation method that can obtain highly accurate dose distribution calculation results in a shorter time than conventional methods.

[0024] The present invention includes multiple means for solving the above-mentioned problems. One example is a dose distribution calculation system that calculates the transmittance of a multi-leaf collimator for each transport particle and calculates a dose distribution using the transmittance. When calculating the transmittance, a high importance condition is set based on the aperture shape of the multi-leaf collimator or the particle generation process, and it is determined whether the transport particle satisfies the high importance condition. For transport particles that are determined not to satisfy the high importance condition, the weight of the transport particle is increased and the number of transport particles is reduced so that the expected value of the weight of all transport particles is preserved.

[0025] According to the present invention, it is possible to obtain highly accurate calculation results of dose distribution in a shorter time than in the past. Problems, configurations and effects other than those described above will become clear from the following description of the embodiments.

[0026] 1 is an overall configuration diagram of a radiation therapy system according to an embodiment of the present invention. FIG. 2 is a diagram illustrating a flow of a dose distribution calculation process using the dose distribution evaluation system according to an embodiment of the present invention. FIG. 3 is a diagram illustrating a flow of an MLC transport calculation process when the dose distribution evaluation system according to an embodiment of the present invention is used. FIG. 4 is a diagram illustrating a process flow of performing a variance reduction process of transported particles based on an MLC aperture shape when the dose distribution evaluation system according to an embodiment of the present invention is used. FIG. 5 is an explanatory diagram of a process of generating a high importance region when a rectangle circumscribing the MLC aperture is used as a reference aperture region. FIG. 6 is a diagram illustrating a process of generating a high importance region of transported particles when a rectangle circumscribing the MLC aperture is used as a reference aperture region between two control points. FIG. 7 is a diagram illustrating a process of generating a high importance region of transported particles when a collimator aperture shape is used as a reference aperture region between two control points. FIG. 8 is a diagram illustrating a process of generating a high importance region of transported particles when a collimator aperture shape is used as a reference aperture region between two control points. FIG. 9 is a diagram illustrating a process of performing a variance reduction process of transported particles based on a generation process of transported photons when the dose distribution evaluation system according to an embodiment of the present invention is used. FIG. 10 is a comparison diagram of fluence distribution at a downstream position of the MLC with and without the dispersion reduction treatment of transported particles in this embodiment.

[0027] Embodiments of the dose distribution calculation system, program, and dose distribution calculation method of the present invention will be described with reference to Figures 1 to 10. In the drawings used in this specification, identical or similar reference numerals will be used to designate identical or corresponding components, and repeated explanations of these components may be omitted.

[0028] The present invention can be suitably applied to an X-ray irradiation system that uses X-rays as the radiation used, but can also be suitably applied to a radiation irradiation system that uses neutrons other than X-rays. In the following embodiments, an X-ray irradiation system will be described as an example.

[0029] First, the overall configuration of the X-ray therapy system of the present invention will be described with reference to Fig. 1. Fig. 1 is a diagram showing an outline of the overall configuration of the X-ray therapy system.

[0030] The X-ray therapy system shown in FIG. 1 includes an X-ray irradiation system 110 , a data server 111 , a treatment planning device 112 , and a dose distribution calculation system 113 .

[0031] The X-ray irradiation system 110 includes an X-ray irradiation device 100, a radiation irradiation control device 104, a communication device 105, a storage device 106, and an input device 107. The X-ray irradiation device 100 includes a ring-type gantry 101, an irradiation nozzle 102, and a bed 103.

[0032] In the X-ray irradiation system 110, when an extraction start signal is output from the radiation irradiation control device 104, the linear accelerator in the irradiation nozzle 102 accelerates the electron beam, and the accelerated electron beam is irradiated onto tungsten to generate X-rays.

[0033] The irradiation nozzle 102 is equipped with a collimator (hereinafter referred to as a multi-leaf collimator (MLC)) consisting of multiple plate-shaped shields (hereinafter referred to as leaves) arranged on the left and right sides, and the generated X-rays are shaped into a desired distribution by arbitrarily changing the position of each leaf based on instruction values ​​from the radiation irradiation control device 104.

[0034] In addition, a dose monitor that measures the X-ray dose is provided inside the irradiation nozzle 102, and the detected measurement value is output to the radiation irradiation control device 104 and used for control during radiation irradiation.

[0035] The bed on which the irradiation target A is placed is called the bed 103. The bed 103 can be moved in the directions of three orthogonal axes and can also be rotated independently around each axis based on instructions from the radiation irradiation control device 104. These movements and rotations allow the irradiation target A to be moved to a desired position.

[0036] The irradiation nozzle 102 is mounted on a ring-type gantry 101, and receives instructions from a radiation irradiation control device 104 to set the gantry rotation angle and the ring rotation angle, making it possible to irradiate X-rays from any angle onto an irradiation target A on a bed 103. The rotation center of the gantry rotation and the ring rotation is called the isocenter.

[0037] The radiation irradiation control device 104 is connected to the ring-type gantry 101, the irradiation nozzle 102, the bed 103, the communication device 105, the memory device 106, the input device 107, etc., and controls the operation of the equipment inside the irradiation nozzle 102, the ring-type gantry 101, the bed 103, etc.

[0038] The communication device 105 is connected to a data server 111 via a network, and acquires treatment plan data from the data server 111, which stores irradiation parameters (gantry angle, irradiation dose, leaf position information, etc.) created by a treatment planning device 112 via the network before irradiation, and stores the irradiation parameters in a storage device 106.

[0039] The input device 107 is connected to the radiation irradiation control device 104 and displays information on a monitor based on signals acquired from the radiation irradiation control device 104. The input device 107 also receives input signals from a medical professional operating the X-ray irradiation system 110 and transmits various control signals to the radiation irradiation control device 104. When the radiation irradiation control device 104 receives an instruction to start radiation irradiation via the input device 107, it starts irradiation based on the irradiation parameters stored in the storage device 106.

[0040] The dose distribution calculation system 113 is a system that calculates the transmittance of a multi-leaf collimator for each transported particle and calculates the dose distribution using the transmittance, and is composed of an MLC transport calculation unit 120, a dose distribution calculation unit 121, a display unit 122, an input unit 123, and a storage unit 124. The dose distribution calculation system 113 is connected to the data server 111 via a network and acquires treatment plan information for calculating the dose distribution.

[0041] The MLC transport calculation unit 120 includes a transmittance calculation unit 120A and a dispersion reduction unit 120B.

[0042] The transmittance calculation unit 120A receives particle species information on the upper surface of the MLC stored in the memory unit 124, calculates the path length within the MLC for each particle, and sets the transmittance calculated from the path length as the particle weight, thereby generating particle species data downstream of the MLC. Here, the particle species data includes position information, travel vector, energy, particle species (X-ray, electron, positron), and statistical weight for each particle / photon. This transmittance calculation unit 120A preferably executes the procedure for calculating the transmittance of the multi-leaf collimator for each transported particle.

[0043] In order to reduce the calculation cost of the transmittance calculation unit 120A, the variance reduction unit 120B sets a high importance condition based on the aperture shape of the multi-leaf collimator or the particle generation process when calculating the transmittance, determines whether transport particles satisfy the high importance condition, and increases the weight of transport particles determined to not satisfy the high importance condition and reduces the number of transport particles so that the expected value of the weight of all transport particles is preserved. In this way, by reducing transport particles that do not satisfy the high importance condition, the number of particles for which transport calculations are performed within the MLC is reduced, and the calculation load of the transmittance calculation is reduced.

[0044] Here, the "high importance condition" in the present invention refers to, for example, a rectangular area (irradiation field size) circumscribing the opening shape of the MLC, and an area obtained by expanding the irradiation field size, and the determination of whether the transported particles satisfy the high importance condition can be made by whether the transported particles pass through the high importance area.

[0045] The dose distribution calculation unit 121 receives the particle species data and transmittance downstream of the MLC generated by the MLC transport calculation unit 120 as input and calculates the dose distribution inside the patient's body or in a numerical phantom. Here, the particle species data includes position information, travel vector, energy, particle species (X-ray, electron, positron), and statistical weight for each particle / photon. This dose distribution calculation unit 121 preferably executes the procedure for calculating the dose distribution using transmittance.

[0046] The display unit 122 is a display that displays the dose distribution calculated by the dose distribution calculation unit 121 .

[0047] The radiation irradiation control device 104 and the dose distribution calculation system 113 each include a central processing unit (CPU) and a memory connected to the CPU.

[0048] The control processes for the operations to be executed may be integrated into one program, or may be divided into multiple programs, or may be a combination of these.

[0049] Some or all of the programs stored in each device may be implemented using dedicated hardware or may be modularized. Furthermore, various programs may be installed in each device from a program distribution server or external storage media, or may be used to update existing devices.

[0050] Furthermore, each device may be an independent device connected to a wired or wireless network, or two or more devices may be integrated.

[0051] Next, a flow of calculating the dose distribution in the patient's body using the dose distribution calculation system 113 of this embodiment will be described. The overall flow of the dose distribution calculation is shown in Figure 2, and the flow of the transport calculation part in the MLC using the variance reduction unit 120B, which is a characteristic process of the present invention, is shown in Figure 3. The dose distribution can be calculated during online adaptive treatment or when calculating the dose distribution during treatment planning, but is not limited to these timings.

[0052] 2 , first, the dose distribution calculation system 113 acquires treatment plan data for verifying the dose distribution from the data server 111 (S201). At this time, the treatment plan data to be read by the dose distribution calculation system 113 can be selected by the operator from the treatment plan information 111A in the data server 111 using the input unit 123, or the dose distribution calculation system 113 can automatically search the treatment plan information 111A in the data server 111 for the treatment plan data read by the radiation irradiation control device 104 and read it.

[0053] Next, the MLC transport calculation unit 120 of the dose distribution calculation system 113 extracts plan parameters to be used for dose distribution calculation and MLC transport calculation from the treatment plan data read in step S201, and stores them in the storage unit 124 (S202).

[0054] Next, the MLC transport calculation unit 120 calculates the total number of samples at the MLC upstream position using the plan parameters of the treatment plan data read in step S202 (S203).

[0055] Here, the number of samples is the number of particles used in dose distribution calculation, and since the X-ray irradiation dose is determined by the MU value of a dose monitor installed in the irradiation nozzle in the X-ray irradiation device 100, the number of particles per MU value is often used. Here, an example will be described using the number of samples [particles / (MU / Gy)] calculated per unit dose at the normalization point.

[0056] The MLC transport calculation unit 120 acquires dose values ​​and MU value information at normalization points from the treatment plan information and calculates the MU value per unit dose [MU / Gy] at the normalization point. The number of particles per MU value per unit dose [particles / (MU / Gy)] is stored in advance in the storage unit 124, and the total number of samples is calculated by multiplying the MU value per unit dose at the normalization point by the number of particles per MU value per unit dose. Here, a value designated by the operator using the input unit 123 can also be used as the number of particles per MU value per unit dose [particles / (MU / Gy)].

[0057] Next, the MLC transport calculation unit 120 reads particle species data upstream of the MLC and performs transport calculations within the MLC to generate particle species data downstream of the MLC (S204).

[0058] After the transport calculation is completed, the dose distribution calculation unit 121 of the dose distribution calculation system 113 reads the particle species data downstream of the MLC generated by the MLC transport calculation unit 120 and performs dose distribution calculation (S205). In the dose distribution calculation, the statistical weight of the input particle k is set as s ｋ , the dose delivered by particle k to voxel i is d ｉ，ｋ Then, the dose D of voxel i is ｉ is calculated by the following formula (1).

[0059]

[0060] Here, C is a correction coefficient for converting the dose calculated based on the number of calculation samples into the dose given based on the actual number of particles.

[0061] The present invention is characterized by the calculation method of the MLC transport calculation portion in step S204. During the MLC transport calculation, the variance reduction unit 120B sets high importance conditions, which mean conditions under which the importance of transported particles in dose evaluation is increased, for transported particles. For transported particles that do not satisfy the high importance conditions, the variance reduction unit 120B increases the weight of the transported particles and reduces the number of particles while preserving the expected value of the weight of all transported particles. This makes it possible to reduce the calculation load of the transmittance calculation unit 120A. A flow diagram of the MLC transport calculation using the variance reduction unit 120B is shown in Figure 3.

[0062] As shown in FIG. 3, first, the MLC transport calculation unit 120 reads the system data of the multi-leaf collimator used in the X-ray irradiation device 100 and constructs a calculation system (step S301).

[0063] A multi-leaf collimator has a number of plate-shaped shields (hereinafter referred to as leaves) that can be driven in one axial direction and arranged in a row. The direction in which the leaves are driven is called the leaf drive direction, and the direction in which the leaves are arranged is called the leaf arrangement direction. In the leaf arrangement direction, a frame is placed outside the leaves, which serves to shield the radiation outside the irradiation field.

[0064] Next, the MLC transport calculation unit 120 reads the MLC upstream particle species information database 125 stored in the storage unit 124, and acquires MLC upstream particle data that will be used as input for the MLC transport calculation (step S302).

[0065] Next, the transmittance calculation unit 120A of the MLC transport calculation unit 120 reads particles one by one from the particle species data upstream of the MLC read in step S302 (step S303) and starts the transmittance calculation process. Next, the variance reduction unit 120B performs variance reduction processing to reduce the calculation load of the transmittance calculation process (step S304).

[0066] The flow of the variance reduction process performed by the variance reduction unit 120B will be described below with reference to Fig. 4. Here, for the purpose of reducing the calculation load in small radiation field cases, the high importance condition is whether or not the particle passes through a high importance region determined based on the aperture shape of the multi-leaf collimator, and an example is shown in which increase and decrease are performed on transport particles determined to pass outside the high importance region.

[0067] 4, first, a reference aperture region is determined based on the treatment plan information read in step S202 (step S401), and a high importance region is set based on the reference aperture region (step S402). A schematic diagram of the high importance region setting and the reduction of light rays is shown in FIG.

[0068] Here, an example will be described in which a rectangle circumscribing the aperture shape of the MLC is defined as the reference aperture region.

[0069] When acquiring the aperture shape information of the MLC, it is necessary to first determine only the leaves that are being driven. ｉ，Ｌ and the x-direction position x of the i-th right leaf ｉ，Ｒ Refer to the left leaf standby position x ｓｔａｎｄｂｙ，Ｌ and the standby position x of the right leaf ｓｔａｎｄｂｙ，Ｒ The driven leaf is determined by comparing these values, and the length of the reference opening area in the y direction is calculated.

[0070] Next, calculation of the reference aperture region length in the x direction will be described. As shown in FIG. 5, the negative side position of the reference aperture region in the x direction through which the beam 505 passes is determined by the left leaf forming the MLC aperture shape 501, and the positive side position is determined by the right leaf. Therefore, the minimum position x of the left leaf among the driven leaves is ｍｉｎ，Ｌ and the maximum position x of the right leaf ｍａｘ，Ｒ By calculating the above, the reference opening area 502 in the x direction is defined.

[0071] Here, an example is shown in which the reference opening region 502 is expanded by an arbitrary length δL to set a high importance region 503.

[0072] In X-ray therapy, the position of the 50% dose edge of the dose distribution generated from each irradiation direction is basically determined by the collimator opening position. Therefore, if you want to emphasize the peripheral region of the dose distribution (the 50%-20% dose region) in dose evaluation, you need to set an area that is an extension of the reference opening region as the high-importance region.

[0073] Next, it is determined whether the transport ray passes through a high importance region (step S403). Here, an example is shown in which it is determined whether the transport ray passes through a high importance region at the isocenter position.

[0074] In an X-ray treatment device, assuming that the beam from the X-ray source spreads out in a fan beam, the source-to-isocenter distance is SAD and the source-to-collimator center distance is l, the high-importance region at the isocenter position is SAD / l times the high-importance region at the leaf position.

[0075] The dispersion reduction unit 120B reads particle species information of the transport ray irradiated upstream of the MLC and calculates the travel vector (v ｘ , v ｙ , v ｚ ) and location information (x ０ , y ０ , z ０ ) and, under the assumption that the transport ray travels straight from the upstream of the MLC, it is determined whether the ray passes through a high importance region.

[0076] If it is determined in step S403 that the ray passes through the high importance region, the flow of FIG. 4 is terminated and the flow returns to the flow of FIG. 3, and the transmittance calculation for each ray is performed (step S305). On the other hand, if it is determined that the ray does not pass through the high importance region, the weight of the ray is set to the initial weight w of the ray. ０ High weight compared to w ｇｅｏｍ is set (step S404).

[0077] Next, a process of reducing the transport rays is performed so that the expected value of the weight of all transport rays is preserved (step S405). Therefore, for particles that do not pass through the high importance region, w ０ / w ｇｅｏｍ A uniform random number r is generated for each transport ray so that the transport particles are reduced with a probability of r>w. ０ / w ｇｅｏｍ If this is the case, then the particles that fall under this condition are eliminated.

[0078] This makes it possible to reduce the number of transport rays that pass outside of the highly important regions while maintaining the expected weight of all transport rays, allowing computational resources to be concentrated in the highly important regions, and enabling efficient calculation of transmittance.

[0079] 3, it is confirmed whether the transport ray has been eliminated by the variance reduction process (step S305). If the transport ray has been eliminated, the transport for this transport ray is terminated and the process moves to the processing for the next transport ray (step S309). On the other hand, if a transport ray exists, the path length of the MLC for each transport ray is calculated, and the transmittance is calculated (step S306).

[0080] 2, the particle species data with statistical weights created in step S204 is read, and the dose distribution is calculated using the Monte Carlo method (step S205). Therefore, if the particle species data read in step S205 contains many particles with small statistical weights, the efficiency of the dose distribution calculation is significantly reduced.

[0081] Therefore, in order to improve the calculation efficiency of step S205, a step (step S307) is provided in which low-weight particles are reduced using the Russian roulette method, which is one of the variance reduction methods, as shown in FIG.

[0082] In the Russian roulette method, the cutoff value is ｃｕｔ Set w ｃｕｔ For the following particles, we used uniform random numbers to calculate w / w ｃｕｔ The probability of reducing particles is ｃｕｔ For particles with statistical weights below the cutoff value w ｃｕｔ is set as the statistical weight.

[0083] Next, the coordinate values ​​of the particles at the downstream position of the MLC are calculated, and then written and saved as particle species data downstream of the MLC (step S308). After that, it is determined whether there are any transported particles to be calculated next (step S309). If it is determined that there are, the process returns to step S303, and if it is determined that there are no transported particles, the flow of FIG. 3 ends.

[0084] If it is necessary to consider the influence of the rotation of the treatment device (gantry angle information, irradiation ring angle information, couch angle information), it is possible to reflect this by performing coordinate conversion on the progress vector information and position information of the particle species data using a rotation matrix representing the device rotation before writing the particle species data downstream of the MLC.

[0085] So far, the setting of high importance regions and the variance reduction process have been described using an example in which X-rays are emitted discretely for each irradiation port.

[0086] Next, we will explain how to set high importance areas when the multi-leaf collimator shape is changed without interrupting radiation irradiation between control points, i.e., in the case of VMAT irradiation in which irradiation is performed while continuously changing the MLC opening shape.

[0087] In the VMAT irradiation conditions, an irradiation control point is set for each irradiation angle, and the leaf position and cumulative irradiation amount are recorded for each irradiation control point. Therefore, in the transport calculation of VMAT irradiation, the irradiation amount and the number of transported rays are determined for each irradiation control point from the cumulative irradiation amount, and the transport calculation is performed for each control point. Here, the gantry angle θ Ａ Irradiation control point A, gantry angle θ Ｂ A method for setting a high importance region will be described using an example of MLC transport calculation between irradiation control points B.

[0088] FIG. 6 shows an explanatory diagram of a method for setting the MLC opening shape and high importance area at control points A and B.

[0089] 6, similar to the case of discrete X-ray irradiation, a reference aperture region 602 of control point A is represented by a rectangle circumscribing an MLC aperture shape 601 of control point A, and a reference aperture region 612 of control point B is represented by a rectangle circumscribing an MLC aperture shape 611 of control point B. When considering the MLC transport of particles irradiated between control points A and B, each leaf moves within the reference aperture regions of control points A and B, and therefore the reference aperture region (= high importance region) between control points A and B is represented by the union of the two reference aperture regions.

[0090] This union can be set by taking the union of the reference opening shapes at each control point A and control point B and then expanding them over a specified area, or it can be set by expanding the reference opening shapes at each control point A and control point B over a specified area and then taking the union of the reference opening shapes expanded over the specified area, and the union can be selected as appropriate.

[0091] FIG. 6 shows an example in which the reference opening area 622 between control points A and B is more simply represented by a rectangle circumscribing the two opening areas.

[0092] Next, similarly to the case of discretely irradiating X-rays, a high importance region 621 is set based on the reference aperture region. Here, an example is shown in which the reference aperture region is expanded by an arbitrary length δL to create a high importance region. Here, the same expansion width δL is set in the y direction and the x direction, but different expansion widths δL in the x direction and the y direction can also be set. ｘ , δL ｙ You can also set

[0093] So far, we have explained an example in which a rectangle circumscribing the opening shape of the MLC is defined as a reference opening area, and a high importance area is set based on the reference opening area, but the method for setting the high importance area is not limited to this setting method as long as it is a method that uses the opening shape of the MLC.

[0094] Next, the aperture shape of the MLC itself is defined as a reference aperture area, and the reference aperture area is defined as an arbitrary length δL in the x and y directions.ｘ , δL ｙ Here, we will explain the procedure for setting a high importance region by widening the gantry angle θ Ａ Irradiation control point A, gantry angle θ Ｂ An example is an MLC transport calculation between irradiation control points B. A flow chart of the procedure for setting high importance regions is shown in FIG. 7, and an explanatory diagram is shown in FIG.

[0095] First, as shown in Fig. 7, the leaf positions of irradiation control points A and B are read (step S701). In Fig. 7, the leaf positions of control points A and B are represented by dashed and dotted lines.

[0096] Next, for the right leaf, the maximum leaf position xL at each control point A and B ｍａｘ，ｉ The widest leaf position is the widest leaf position, and the left leaf position is the minimum leaf position xL at each control point A and B. ｍｉｎ，ｉ is set as the widest leaf position, and the aperture area defined by this leaf position is defined as the reference aperture area (step S702).

[0097] Next, ±δL in the y direction from the reference opening area ｙ , ±δL in the x direction ｘ The enlarged region is generated and set as a high importance region.

[0098] First, the expansion width ±δL ｙ Using the above, an area obtained by expanding the reference opening area in the y direction is calculated (step S703). ｙ The reference opening area expanded in the y direction is formed by calculating the minimum value of the left leaf position and the maximum value of the right leaf position of the leaves existing in the range of ｉ and right leaf position rpos ｉ is defined by the following equations (2) and (3).

[0099]

[0100]

[0101] Here, n ｄｏｗｎ，ｉ and n ｕｐ，ｉ is -δL from the i-th leafｙ and +δL ｙ represents the number of leaves forming the reference aperture area that exist in the range of

[0102] Next, in order to expand the reference opening area in the x direction, δL is calculated from the left side position of all leaves forming the reference opening area expanded in the y direction. ｘ Subtract δL from the right side ｘ (Step S704). Finally, the enlarged reference opening region is set as a high importance region (Step S705). An example of the set high importance region is shown in FIG.

[0103] As shown in Figure 8, it can be seen that the opening position 803 of the left leaf and the opening position 805 of the right leaf at control point A, and the opening position 804 of the left leaf and the opening position 806 of the right leaf at control point B are included within the high importance area between the left leaf 801 that forms the high importance area between control points A and B and the right leaf 802 that forms the high importance area between control points A and B.

[0104] In addition, in this variance reduction process, the expansion width δL from the reference opening region, which is a setting parameter of the high importance region, and the weight w of particles irradiated outside the high importance region are set. ｇｅｏｍ There are calculation parameters such as the above. Therefore, the values ​​of these parameters may be set by an operator using a GUI, or a parameter file in which the calculation parameters are recorded may be recorded in the storage unit 124, and each calculation parameter may be read during the variance reduction process.

[0105] It is also possible to display the extent of the enlargement width δL on a screen such as that shown in FIG. 5 or FIG. 10 described later.

[0106] So far, we have described methods for reducing the calculation cost far outside the irradiation field. However, another factor that increases the calculation load in MLC transport calculations is the consideration of Compton photons.

[0107] Compton photons are photons generated within the MLC by Compton scattering. Compared to primary photons, they are scattered at larger angles and are more likely to pass through multiple MLC leaves. This requires the calculation of the MLC path length using multiple leaves, which increases the calculation time. On the other hand, since the Compton scattering component mainly contributes to the low-dose range of 20% or less, its importance from the perspective of dose distribution verification is lower than that of primary photons.

[0108] Therefore, the high importance condition is whether the transport particle is a primary photon or a Compton photon, and the weight can be increased and particles can be reduced for transport particles that are determined to be Compton photons.

[0109] In other words, by introducing a similar method into the generation process of Compton scattering, it is thought that it is possible to reduce the number of Compton photons generated and reduce the calculation load of the MLC transport calculation code. Therefore, the fact that it is a primary photon is set as a high importance condition, and the process of reducing Compton photons is explained below.

[0110] An example of dispersion reduction processing for Compton photons is shown in FIG.

[0111] 9, first, it is determined whether the transport photon is a primary photon or a Compton photon (step S901). If it is determined to be a primary photon, the process proceeds directly to a step of calculating the transmittance of the light ray. On the other hand, if it is determined to be a Compton photon, the process proceeds to a process of reducing the number of transport photons.

[0112] Here, regarding the reduction process of Compton photons, the weight w of the Compton photon generated in the MLC by a certain primary photon is ｃ０ (In the case of Compton photons, this weight w ｃ０ differs for each Compton photon), the threshold weight w ｃｏｍｐｔｏｎ This will be explained using:

[0113] Compton photon weight ｃ０ is the threshold weight of the variance reduction process w ｃｏｍｐｔｏｎ That's all (lol ｃ０ ≧w ｃｏｍｐｔｏｎ ) if the original w ｃ，ｊ (Yes in step S902), whereas,ｃ０ <w ｃｏｍｐｔｏｎ In this case, the corresponding Compton particle is deleted (step S903) so that the expected value of the weight of the Compton photon is preserved, and then w is set as the weight of the Compton photon. ｃ０ =w ｃｏｍｐｔｏｎ is set (step S904).

[0114] This makes it possible to reduce the number of Compton photons while maintaining the expected value of the weight of all transport rays.

[0115] It is possible to use the following two conditions together: determining whether or not the Compton photons are independent of the irradiation field as a high importance condition; and setting a high importance region as a high importance condition, since the smaller the irradiation field, the greater the number of particles outside the region (i.e., the number of particles to be reduced) and therefore the greater the shortening effect.

[0116] When dispersion reduction processing is performed based on the MLC aperture shape and the transport photon generation process, a comparison of the fluence distribution calculation results downstream of the MLC with and without dispersion reduction processing is shown in Figures 10(a) and (b).

[0117] As shown in Figure 10, it can be confirmed that the calculation results of Geant4, a general-purpose particle simulation code, are reproduced very well, regardless of whether the variance reduction method is used. This shows that by using the variance reduction process, particle transmittance calculations can be accelerated and dose distribution calculations can be performed at high speed without causing a significant change in calculation accuracy.

[0118] Next, the effects of this embodiment will be described.

[0119] The dose distribution calculation system 113 of the present embodiment described above is a dose distribution calculation system that calculates the transmittance of a multi-leaf collimator for each transport particle and calculates the dose distribution using the transmittance. When calculating the transmittance, a high importance condition is set based on the aperture shape of the multi-leaf collimator or the particle generation process, and it is determined whether or not a transport particle satisfies the high importance condition. For transport particles that are determined not to satisfy the high importance condition, the weight of the transport particle is increased and the number of transport particles is reduced so that the expected value of the weight of all transport particles is preserved.

[0120] According to the present invention, the low dose region is no longer ignored, and it is therefore possible to provide highly accurate calculation results of dose distribution at high speed.

[0121] Furthermore, the high importance condition is whether or not the particle passes through a high importance region determined based on the aperture shape of the multi-leaf collimator, and an increase or decrease is performed on transport particles that are determined to pass outside the high importance region. Therefore, the smaller the radiation field, the greater the number of particles outside the region (i.e., particles to be decreased), and the greater the reduction effect, which makes it possible to significantly speed up transmittance calculations in small radiation field cases, for example.

[0122] Furthermore, a high-importance condition is whether the transport particle is a primary photon or a Compton photon. By increasing or decreasing the number of transport particles determined to be Compton photons, it becomes possible to take into account the effect of the dose in areas other than the irradiation field, thereby improving the accuracy of dose distribution calculations outside the irradiation field.

[0123] Furthermore, when the multi-leaf collimator shape is changed between control points without interrupting radiation irradiation, the high importance region is set by taking the union of the reference aperture shapes at each control point and then expanding the union by a predetermined area, or when the multi-leaf collimator shape is changed between control points without interrupting radiation irradiation, the high importance region is set by expanding the reference aperture shape at each control point by a predetermined area and then summing the reference aperture shapes expanded by the predetermined area. This makes it possible to speed up transmittance calculations without reducing the accuracy under complex conditions where the multi-leaf collimator shape is changed between control points without interrupting radiation irradiation.

[0124] Furthermore, by determining the dose distribution during online adaptive treatment, highly accurate irradiation can be achieved in a short time, thereby reducing the burden on patients compared to conventional methods.

[0125] Furthermore, by determining the dose distribution when calculating the dose distribution during treatment planning, the burden on doctors before treatment begins can be reduced.

[0126] <Others> The present invention is not limited to the above-described embodiments, and various modifications and applications are possible. The above-described embodiments have been described in detail to clearly explain the present invention, and the present invention is not necessarily limited to those having all of the described configurations.

[0127] The embodiment of the present invention may be in the following form.

[0128] (1) A dose distribution calculation system that calculates the transmittance of a multi-leaf collimator for each transport particle and calculates a dose distribution using the transmittance, and when calculating the transmittance, sets a high importance condition based on the aperture shape of the multi-leaf collimator or the particle generation process, determines whether the transport particle satisfies the high importance condition, and for the transport particle that is determined not to satisfy the high importance condition, increases the weight of the transport particle and reduces the transport particle so that the expected value of the weight of all transport particles is preserved.

[0129] (2) In the dose distribution calculation system described in (1), the high importance condition is whether or not the transported particles pass through a high importance area determined based on the opening shape of the multi-leaf collimator, and the increase and decrease are performed on the transported particles that are determined to pass outside the high importance area.

[0130] (3) In the dose distribution calculation system described in (1), the high importance condition is whether the transport particle is a primary photon or a Compton photon, and the increase and decrease are performed on the transport particle determined to be the Compton photon.

[0131] (4) In the dose distribution calculation system described in (2), when the shape of the multi-leaf collimator is changed between control points without interrupting radiation irradiation, the high importance area is set by taking the union of the reference aperture shapes at each control point and then expanding the specified area.

[0132] (5) In the dose distribution calculation system described in (2), when the shape of the multi-leaf collimator is changed between control points without interrupting radiation irradiation, the high importance area is set by expanding the reference opening shape by a predetermined area at each control point, and then calculating the union of the reference opening shapes expanded by the predetermined area.

[0133] (6) In the dose distribution calculation system according to any one of (1) to (5), the dose distribution is calculated during online adaptive therapy.

[0134] (7) In the dose distribution calculation system according to any one of (1) to (5), the dose distribution is calculated when calculating the dose distribution during treatment planning.

[0135] DESCRIPTION OF SYMBOLS A...Irradiation target 100...X-ray irradiation device 101...Ring-type gantry 102...Irradiation nozzle 103...Bed 104...Radiation irradiation control device 105...Communication device 106...Storage device 107...Input device 110...X-ray irradiation system 111...Data server 111A...Treatment plan information 112...Treatment planning device 113...Dose distribution calculation system 120...MLC transport calculation unit 120A...Transmittance calculation unit 120B...Dispersion reduction unit 121...Dose distribution calculation unit 122...Display unit 123...Input unit 124...Storage unit 125...Particle species information database 501...MLC aperture shape 502...Reference aperture region 503...High importance region 504...MLC leaf viewed from beam irradiation direction 505...Beam 601...MLC aperture shape of control point A 602...Reference opening area of ​​control point A 611...MLC opening shape of control point B 612...Reference opening area of ​​control point B 621...High importance area between control points A and B 622...Reference opening area between control points A and B 801...Left leaf forming the high importance area between control points A and B 802...Right leaf forming the high importance area between control points A and B 803...Opening position of left leaf at control point A 804...Opening position of left leaf at control point B 805...Opening position of right leaf at control point A 806...Opening position of right leaf at control point B

Claims

1. A dose distribution calculation system that calculates the transmittance of a multi-leaf collimator for each transported particle and calculates a dose distribution using the transmittance, wherein when calculating the transmittance, a high importance condition is set based on the aperture shape of the multi-leaf collimator or a particle generation process, and it is determined whether the transported particle satisfies the high importance condition, and for the transported particle determined not to satisfy the high importance condition, the weight of the transported particle is increased and the transported particle is reduced so that the expected value of the weight of all transported particles is preserved.

2. A dose distribution calculation system as described in claim 1, wherein the high importance condition is whether or not the transported particles pass through a high importance area determined based on the aperture shape of the multi-leaf collimator, and the increase and decrease are implemented for the transported particles that are determined to pass outside the high importance area.

3. A dose distribution calculation system as described in claim 1, wherein the high importance condition is whether the transport particle is a primary photon or a Compton photon, and the increase and decrease are performed on the transport particle determined to be a Compton photon.

4. A dose distribution calculation system as claimed in claim 2, wherein, when the shape of a multi-leaf collimator is changed between control points without interrupting radiation irradiation, the high importance area is set by taking the union of the reference aperture shapes at each control point and then expanding the specified area.

5. A dose distribution calculation system as described in claim 2, wherein, when the shape of a multi-leaf collimator is changed between control points without interrupting radiation irradiation, the high importance area is set by expanding a reference aperture shape by a specified area at each control point, and then calculating a union of the reference aperture shapes expanded by the specified area.

6. A dose distribution calculation system according to claim 1, wherein said dose distribution is calculated during online adaptive therapy.

7. A dose distribution calculation system according to claim 1, wherein the dose distribution is determined when calculating the dose distribution during treatment planning.

8. A program that causes a calculation system to execute the steps of: calculating the transmittance of a multi-leaf collimator for each transported particle; and calculating a dose distribution using the transmittance, the program causing a calculation system to set a high importance condition based on the aperture shape of the multi-leaf collimator or a particle generation process when calculating the transmittance; determining whether or not the transported particle satisfies the high importance condition; and for the transported particle determined not to satisfy the high importance condition, increasing the weight of the transported particle and reducing the number of the transported particles so that the expected value of the weight of all transported particles is preserved.

9. A dose distribution calculation method for calculating the transmittance of a multi-leaf collimator for each transported particle and calculating a dose distribution using said transmittance, comprising the steps of: when calculating said transmittance, setting a high importance condition based on the aperture shape of said multi-leaf collimator or a particle generation process; determining whether or not said transported particle satisfies said high importance condition; and for said transported particle determined not to satisfy said high importance condition, increasing the weight of said transported particle and reducing said transported particle so as to preserve the expected value of the weight of all transported particles.

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