Treatment planning system

Abstract

Provided is a treatment planning system for making a treatment plan for charged particle radiation therapy. In the treatment planning system, optimization processing is executed for creating a weight map indicating the distribution of the weight of the dose of a charged particle beam to be delivered to each irradiation site of a tumor, and in optimization calculations using an iterative method performed in the optimization processing, a constraint condition is set so that the weight constraint fluctuates during the iterative calculations.

Description

Treatment Planning System

[0001] The present disclosure relates to treatment planning systems.

[0002] Charged particle beam therapy devices that treat a patient's affected area by irradiating the area with a charged particle beam are known. Patent Document 1 describes a charged particle beam therapy device that divides the irradiated object into multiple layers and irradiates each layer with a charged particle beam by a scanning method. Such a charged particle beam therapy device irradiates the charged particle beam based on a treatment plan created by a treatment planning system.

[0003] Japanese Patent Publication No. 2020-065655

[0004] In this type of treatment planning system, the beam current is set so that the irradiation location requiring the smallest dose in the affected area can be irradiated with that dose. This results in longer irradiation times for irradiation locations requiring a larger dose, resulting in a longer overall irradiation time. Therefore, it is conceivable to perform a cutoff process for irradiation locations requiring a smaller dose, but such a cutoff process reduces the dose irradiated to the irradiated body, raising concerns about a deterioration in the dose volume histogram (DVH). Therefore, an object of the present disclosure is to provide a treatment planning system capable of shortening the irradiation time of a charged particle beam.

[0005] The gist of the present disclosure lies in the following [1] to [7].

[0006] [1] A treatment planning system for creating a treatment plan for charged particle beam therapy, which executes an optimization process to create a weight map showing the distribution of weights of the dose of the charged particle beam to be irradiated to each irradiation location of the irradiated body, and in the optimization calculations performed in the optimization process, a constraint condition is set such that the constraints on the weight values ​​change.

[0007] [2] The treatment planning system according to [1], wherein an iterative method is used in the optimization calculation, and the constraints on the weight values ​​vary during the iterative calculation according to the constraint conditions.

[0008] [3] The treatment planning system according to [1] or [2], wherein the constraint specifies a dynamic range of the weights in the weight map during the iterative calculation.

[0009] [4] The treatment planning system according to any one of [1] to [3], wherein the constraint condition includes a dynamic range factor relating to the dynamic range.

[0010] [5] The treatment planning system according to any one of [1] to [4], wherein the constraint condition is that the weight assigned to each irradiation location during the iterative calculation is limited to a value greater than the product of the maximum weight value in the weight map during the iterative calculation and the dynamic range factor, which is a constant.

[0011] [6] The treatment planning system according to any one of [1] to [5], wherein in the optimization calculation, the ratio of the maximum weight value to the minimum weight value in the weight map converges to the value of the dynamic range factor.

[0012] [7] A treatment planning system according to any one of [1] to [6], wherein in the iterative calculation, the minimum weight value is determined by the ratio to the maximum weight value in the weight map according to the constraint condition.

[0013] [8] A treatment planning system according to any one of [1] to [7], further comprising: a beam current determination process for determining the beam current of the charged particle beam to be irradiated to the subject based on the weight map created by the optimization process; wherein in the beam current determination process, the beam current is determined based on the maximum scanning speed of the charged particle beam in a charged particle beam therapy device used for the charged particle beam therapy and the minimum weight in the weight map.

[0014] According to the present disclosure, it is possible to provide a treatment planning system that can shorten the irradiation time of a charged particle beam.

[0015] 1 is a schematic diagram showing a charged particle beam therapy device that performs charged particle beam therapy based on a treatment plan planned by a treatment planning system according to an embodiment of the present disclosure. It is a schematic diagram showing the vicinity of an irradiation unit of the charged particle beam therapy device of FIG. 1. (a) is a diagram showing layers set for a tumor, and (b) is a plan view of one of the layers. It is a block diagram showing a treatment planning system. (a) is a flowchart showing an example of a treatment planning procedure by the treatment planning system, and (b) is an example of a weight map created by the treatment plan. It is a graph showing an example of DVH. (a) is an enlarged schematic diagram showing the vicinity of the minimum weight of a histogram obtained by conventional optimization calculation, and (b) is an enlarged schematic diagram showing the vicinity of the minimum weight of a histogram obtained by optimization calculation of this embodiment.

[0016] Various embodiments will be described in detail below with reference to the drawings. Note that the same or equivalent parts in each drawing are denoted by the same reference numerals, and redundant explanations will be omitted.

[0017] FIG. 1 is a schematic diagram illustrating a charged particle beam therapy system that performs charged particle beam therapy based on a treatment plan created by a treatment planning system according to an embodiment of the present disclosure. The charged particle beam therapy system 1 shown in FIG. 1 is a system used for cancer treatment using radiation therapy, etc. The charged particle beam therapy system 1 includes an ion source 50 that generates charged particles, an accelerator 3 that accelerates the charged particles generated in the ion source 50 and emits them as a charged particle beam, an irradiation unit 2 that irradiates a target body with the charged particle beam, a beam transport line 21 that transports the charged particle beam emitted from the accelerator 3 to the irradiation unit 2, and an energy adjustment unit 20 provided on the beam transport line 21 between the accelerator 3 and the irradiation unit 2. The irradiation unit 2 is attached to a rotating gantry 5 that surrounds a treatment table 4. The irradiation unit 2 is configured to be rotatable around the treatment table 4 by the rotating gantry 5.

[0018] FIG. 2 is a schematic diagram of the vicinity of the irradiation unit of the charged particle beam therapy device of FIG. 1. In the following description, the terms "X direction," "Y direction," and "Z direction" are used. The "Z direction" is the direction in which the base axis (irradiation axis) AX of the charged particle beam B extends, and is the depth direction of irradiation of the charged particle beam B. The "base axis AX" is the irradiation axis of the charged particle beam B when not deflected by the irradiation position adjustment unit 60 described below. FIG. 2 shows the charged particle beam B being irradiated along the base axis AX. The "X direction" is one direction in a plane perpendicular to the Z direction. The "Y direction" is a direction perpendicular to the X direction in a plane perpendicular to the Z direction.

[0019] First, the schematic configuration of the charged particle beam therapy system 1 will be described with reference to Fig. 2. The following description will be given assuming that the charged particle beam therapy system 1 is an irradiation system using a scanning method. The scanning method is not particularly limited, and line scanning, raster scanning, spot scanning, etc. may be employed. As shown in Fig. 2, the charged particle beam therapy system 1 includes an accelerator 3, an irradiation unit 2, a beam transport line 21, and a control unit 7.

[0020] The accelerator 3 is a device that accelerates charged particles and emits a charged particle beam B with a predetermined energy. Examples of the accelerator 3 include a cyclotron, a synchrocyclotron, and a linac. When a cyclotron that emits a charged particle beam B with a predetermined energy is used as the accelerator 3, the energy adjustment unit 20 can be used to adjust (reduce) the energy of the charged particle beam B sent to the irradiation unit 2. The accelerator 3 is connected to a control unit 7, which controls the current supplied. The charged particle beam B generated by the accelerator 3 is transported to the irradiation unit 2 by a beam transport line 21. The beam transport line 21 connects the accelerator 3, the energy adjustment unit 20, and the irradiation unit 2, and transports the charged particle beam B extracted from the accelerator 3 to the irradiation unit 2.

[0021] The irradiation unit 2 irradiates a tumor (irradiated object) 14 inside the body of a patient 15 with a charged particle beam B. The charged particle beam B is electrically charged particles accelerated to high speed, such as a proton beam, a heavy particle (heavy ion) beam, or an electron beam. Specifically, the irradiation unit 2 is a device that irradiates the tumor 14 with the charged particle beam B emitted from an accelerator 3 that accelerates charged particles generated by an ion source (not shown) and transported via a beam transport line 41. The irradiation unit 2 includes a scanning electromagnet (scanning unit) 6, a quadrupole electromagnet 8, a profile monitor 11, a dose monitor 12, flatness monitors 13a and 13b, and a degrader 30. The scanning electromagnet 6, the monitors 11, 12, 13a, and 13b, the quadrupole electromagnet 8, and the degrader 30 are housed in an irradiation nozzle 9.

[0022] The scanning electromagnets 6 include an X-direction scanning electromagnet 6a and a Y-direction scanning electromagnet 6b. Each of the X-direction scanning electromagnet 6a and the Y-direction scanning electromagnet 6b is composed of a pair of electromagnets, and changes the magnetic field between the pair of electromagnets in accordance with a current supplied from the control unit 7, thereby scanning the charged particle beam B passing between the electromagnets. The X-direction scanning electromagnet 6a scans the charged particle beam B in the X direction, and the Y-direction scanning electromagnet 6b scans the charged particle beam B in the Y direction. These scanning electromagnets 6 are arranged in this order on the base axis AX, downstream of the accelerator 3 in the direction of the charged particle beam B.

[0023] The quadrupole electromagnets 8 include an X-direction quadrupole electromagnet 8a and a Y-direction quadrupole electromagnet 8b. The X-direction quadrupole electromagnets 8a and the Y-direction quadrupole electromagnets 8b focus and converge the charged particle beam B in accordance with the current supplied from the control unit 7. The X-direction quadrupole electromagnet 8a focuses the charged particle beam B in the X direction, and the Y-direction quadrupole electromagnet 8b focuses the charged particle beam B in the Y direction. The beam size of the charged particle beam B can be changed by changing the amount of focusing (amount of convergence) by changing the current supplied to the quadrupole electromagnets 8. The quadrupole electromagnets 8 are arranged on the base axis AX between the accelerator 3 and the scanning electromagnet 6, in this order. The beam size refers to the size of the charged particle beam B in the XY plane. The beam shape refers to the shape of the charged particle beam B in the XY plane.

[0024] The profile monitor 11 detects the beam shape and position of the charged particle beam B for alignment during initial setup. The profile monitor 11 is disposed on the base axis AX between the quadrupole electromagnet 8 and the scanning electromagnet 6. The dose monitor 12 detects the intensity of the charged particle beam B and transmits a signal to the control unit 7. The dose monitor 12 is disposed on the base axis AX downstream of the scanning electromagnet 6. The flatness monitors 13a and 13b detect and monitor the beam shape and position of the charged particle beam B. The flatness monitors 13a and 13b are disposed on the base axis AX downstream of the dose monitor 12 with respect to the charged particle beam B. Each monitor 11, 12, 13a, and 13b outputs the detected results to the control unit 7.

[0025] The degrader 30 reduces the energy of the passing charged particle beam B to finely adjust the energy of the charged particle beam B. In this embodiment, the degrader 30 is provided at the tip 9 a of the irradiation nozzle 9. Note that the tip 9 a of the irradiation nozzle 9 is the end on the downstream side of the charged particle beam B. The degrader 30 in the irradiation nozzle 9 may be omitted.

[0026] The control unit 7 is configured with, for example, a CPU, a ROM, a RAM, etc. The control unit 7 controls the accelerator 3, the quadrupole electromagnet 8, the scanning electromagnet 6, and the degrader 30 based on the detection results output from the profile monitor 11, the dose monitor 12, and the irradiation position adjustment unit 60.

[0027] The control unit 7 of the charged particle beam therapy device 1 is also connected to a treatment planning system 200 that creates a treatment plan for charged particle beam therapy. The treatment planning system 200 measures the tumor 14 of the patient 15 using a CT or the like before treatment and plans a dose distribution (dose distribution of the charged particle beam B to be irradiated) at each position on the tumor 14. Specifically, the treatment planning system 200 creates a treatment plan map for the tumor 14. The treatment planning system 200 transmits the created treatment plan map to the control unit 7. The treatment plan map created by the treatment planning system 200 plans the scanning path of the charged particle beam B.

[0028] When irradiating the tumor 14 with a charged particle beam by the scanning method, the tumor 14 is virtually divided into multiple layers in the Z-axis direction, and the charged particle beam is scanned and irradiated in one layer along a scanning path defined in the treatment plan. After the irradiation of the one layer with the charged particle beam is completed, the next adjacent layer is irradiated with the charged particle beam B.

[0029] When the charged particle beam B is irradiated by the scanning method using the charged particle beam therapy system 1 shown in FIG. 2, the quadrupole electromagnet 8 is activated (ON) so that the passing charged particle beam B is converged.

[0030] 3(a) and 3(b) will be used to explain the image of the irradiation of the scanning electromagnet 6 with the charged particle beam in response to the control of the control unit 7. Fig. 3(a) shows an image of the irradiation target virtually sliced ​​into multiple layers in the depth direction, and Fig. 3(b) shows an image of the scanning of the charged particle beam in one layer as viewed from the depth direction.

[0031] As shown in FIG. 3A, the object to be irradiated is virtually sliced ​​into a plurality of layers in the irradiation depth direction. In this example, the layers are arranged in order from the deepest layer (the layer having the longest range of the charged particle beam B), as follows: １ , layer L ２ ,...Layer L ｎ－１ , layer L ｎ , layer L ｎ＋１ ,...Layer L Ｎ－１ , layer L Ｎ As shown in FIG. 3B, the charged particle beam B is irradiated in a layer L while tracing a beam trajectory along a scanning path TL. In the case of continuous irradiation (line scanning or raster scanning), the charged particle beam B is irradiated in a layer L. ｎ In the case of spot scanning, the layer L ｎ That is, the charged particle beam B emitted from the irradiation unit 2 controlled by the control unit 7 moves on a scanning path TL.

[0032] The dose of the charged particle beam B required for the tumor 14 varies depending on the location. For example, a lower dose is required near the boundary of the tumor 14 than near the center of the tumor 14. The charged particle beam B is scanned with a constant current value. Therefore, the dose for a specific location increases as the scanning speed of the charged particle beam B decreases and the irradiation time increases. The charged particle beam B moves at a low scanning speed to locations requiring a high dose, and at a high scanning speed to locations requiring a low dose. The charged particle beam B moves at the highest scanning speed to locations requiring the lowest dose.

[0033] Next, the treatment planning system 200 will be described with reference to FIG. 4. As shown in FIG. 4, the treatment planning system 200 includes a calculation unit 110, a display unit 101, and an input unit 102. In terms of hardware, the treatment planning system 200 is configured, for example, by a computer connected to the control unit 7. Note that the treatment planning system 200 may not only be configured by a single processing device, but may also be configured as a system workstation using multiple processing devices. The display unit 101 displays various information to the user. The display unit 101 is configured by a display or the like. The input unit 102 receives input through user operation. The input unit 102 is configured by a mouse, keyboard, touch panel, or the like.

[0034] The calculation unit 110 is a part that performs various calculations for the treatment plan, and in terms of hardware, it is configured to include, for example, a CPU, ROM, and RAM. The calculation unit 110 performs calculations to optimize the number of layers for treatment, the operation path, the scanning speed, etc., based on data of the tumor 14 obtained from CT images, etc. The calculation unit 110 also calculates the dose distribution for the optimized treatment plan.

[0035] Next, an example of a treatment planning procedure using the treatment planning system 200 according to this embodiment will be described with reference to Fig. 5. Fig. 5(a) is a flowchart showing an example of a treatment plan, and Fig. 5(b) is a diagram showing an example of a weight map, which will be described later. The treatment plan in this example includes an optimization process S101, a cutoff process S102, and a beam current determination process S103, which will be described next.

[0036] (Optimization Process S101) As described above, the dose of the charged particle beam B required varies depending on the location in the tumor 14. Therefore, in the optimization process S101, a spatial distribution of the weight W of the dose of the charged particle beam B to be irradiated to each irradiation location P in the tumor 14 is created. Here, for example, as shown in FIG. 3B, ｎ , the irradiation point P along the scanning path TL １ , P ２ , ..., P ｍ is set. Then, each irradiation point P １ , P ２ , ..., P ｍ The weight W of the dose of the charged particle beam to be irradiated １ , W ２ , ..., W ｍ In this way, the irradiation point P １ , P ２ , ..., P ｍ Weight W for each １ , W ２ , ..., W ｍ The information representing the distribution of the weights is hereinafter referred to as a "weight map." FIG. 5B is a diagram showing an example of a weight map 40. As shown in this figure, the weight map (distribution of weights) is １ , P ２ , ..., P ｍ and weight W １ , W ２ , ..., W ｍ The weight map contains information that associates each irradiation point P １ , P ２ , ..., P ｍ Further information may be included, such as the position coordinates of each of the above.

[0037] In the optimization process S101, the weight W１ , W ２ , ..., W ｍ is calculated to optimize the treatment of the tumor 14, and a weight map is created. An iterative method is used for the optimization calculation. The iterative algorithm used here may be a known one, and examples of algorithms that can be used include gradient methods (SQP method, augmented Lagrangian method, nonlinear interior point method) and direct search methods (downhill simplex method, genetic algorithm method, differential evolution method). Linear optimization algorithms such as interior point methods and simplex methods may also be used. The objective function in the optimization calculation is created in a preparatory step prior to the optimization process S101. In the preparatory step prior to the optimization process S101, the objective function is created by calculation based on data on the tumor 14 acquired from, for example, CT images.

[0038] The objective function may be, for example, a function that optimizes DVH. DVH can be represented as a graph such as that shown in FIG. 6. The horizontal axis of the graph indicates the dose, and the vertical axis indicates the percentage of the volume of the object to which a dose equal to or greater than the dose indicated on the horizontal axis has been administered relative to the entire volume of the object. From the graph, it is possible to determine what percentage of the object's volume has been administered with a dose equal to or greater than the dose indicated on the horizontal axis for a specific dose. For example, in FIG. 8, L1 is a graph showing DVH for the tumor 14 to be irradiated, and L2 and L3 are graphs showing DVH for other organs. For the tumor 14, it is preferable that the volume be close to 100% at a high dose. For organs not to be irradiated, it is preferable that the volume to which the dose has been administered is as low as possible, and that the dose is also as low as possible.

[0039] The constraints set in the optimization calculation in this embodiment will be described. The optimization calculation is set with a constraint that changes the constraint on the value of the weight W during the iterative calculation. More specifically, the constraint is set with a constraint that changes the constraint on the lower limit value of the weight W during the iterative calculation. More specifically, the constraint specifies the dynamic range of the weight W in the weight map during the iterative calculation. More specifically, the constraint includes a dynamic range factor (DRF) related to the dynamic range. The constraint limits the weight W assigned to each irradiation point P during the iterative calculation to a value greater than the product of the maximum weight W in the weight map during the iterative calculation and the dynamic range factor DRF, which is a constant. Furthermore, during the iterative calculation in the optimization calculation, the constraint determines the value of the minimum weight W as a ratio to the maximum weight W in the weight map. According to this optimization calculation, the ratio of the maximum weight W to the minimum weight W in the weight map converges to the value of the dynamic range factor DRF.

[0040] An example of the constraint condition is as follows. For example, in the optimization calculation of this embodiment, the constraint condition expressed by the following formula (1) is set: W ｊ >DRF·Wmax ... (1) j = 1, 2, 3, ... Wmax: Maximum weight DRF: Dynamic range factor (constant)

[0041] Equation (1) means that the weight W assigned to the irradiation point P is limited to a value greater than the maximum weight Wmax multiplied by the DRF. According to the constraints of equation (1), during the iterative calculation, each irradiation point Pj is assigned a weight Wj greater than the product of the maximum weight Wmax and the DRF. In other words, this means that the dynamic range of the weight W is limited to the DRF. The DRF is a constant set by the user, and is set appropriately based on the planning concept of the treatment plan, and is input, for example, by the user in advance to the treatment planning system 200. Note that equation (1) can also be expressed as the following equation (2): Wmin = DRF · Wmax ... (2) Wmin: minimum weight

[0042] In the optimization process S101 in this embodiment, the weight W １ , W ２ , ..., W ｍ is calculated and a weight map is created.

[0043] (Cutoff Process S102) In the weight map created in the optimization process S101, there may be irradiation locations with weights that are extremely smaller than the DRF·Wmax. In this case, the cutoff process S102 is executed to forcibly change weights in the weight map that are equal to or smaller than a predetermined value to zero. When irradiating the tumor 14 with the charged particle beam B, the beam is temporarily stopped at such irradiation locations with a weight of zero. If there are no irradiation locations with extremely small weights like those described above, the cutoff process S102 may be skipped. Note that the cutoff process S102 may be incorporated into the optimization calculation in the optimization process S101 as a constraint for the optimization calculation.

[0044] (Beam current determination process S103) Next, in the beam current determination process S103, the beam current of the n-th layer L is determined based on the weight map created as described above. ｎ As described above, the beam current when irradiating the n-th layer L with the charged particle beam is determined. ｎ The beam current of the charged particle beam B is constant while the irradiation point P is irradiated with the charged particle beam. The scanning speed of the charged particle beam B is slower for irradiation points P with larger weights W, and faster for irradiation points P with smaller weights W, so that each irradiation point P is irradiated with a dose according to the weight W. Within this layer, the scanning speed of the irradiation point Pmin with the smallest weight Wmin is the highest.

[0045] In order to shorten the treatment time in charged particle beam therapy and reduce the burden on the patient, it is preferable to shorten the scanning time of the charged particle beam B over the entire tumor 14. To achieve this, it is preferable to increase the beam current and the scanning speed as much as possible in each layer. However, the scanning speed is limited by the performance of the irradiation unit 2 (FIG. 2), and the scanning speed and beam current are determined within this limit. The specific determination method is as follows.

[0046] nth layer L ｎ In the above equation, the irradiation point P that is scanned at the fastest speed within the layer is the irradiation point Pmin with the smallest weight Wmin. Therefore, the scanning speed may be set so that the scanning speed at the irradiation point Pmin matches the maximum scanning speed within the limit set by the irradiation unit 2 (FIG. 2). This makes it possible to maximize the scanning speed throughout the layer. Then, the beam current is determined so that the planned irradiation dose is irradiated to the irradiation point Pmin when scanned at the maximum scanning speed. Specifically, the beam current I is determined by the following equation (3): I=Vmax·Wmin / L (3) I: beam current [MU / ms] Vmax: maximum scanning speed [mm / ms] L: length of the irradiation point Pmin in the scanning direction [mm]

[0047] As described above, it is preferable to increase the beam current, but as can be seen from equation (3), the upper limit of the determined beam current depends on the minimum weight Wmin in the layer. According to this beam current determination process S103, the beam current value is determined to a value that enables irradiation of the irradiation point Pmin with the minimum weight Wmin with the dose determined by the weight map at the maximum scanning speed limited by the performance of the irradiation unit 2 (FIG. 2), etc.

[0048] In the treatment planning by the treatment planning system 200 according to this embodiment, the optimization process S101, the cutoff process S102, and the beam current determination process S103 described above are performed for each layer L of the tumor 14. １ ~L Ｎ By performing １ ~L Ｎ In the subsequent cancer treatment of the patient, the weight map and beam current created in this treatment plan are transferred to the control unit 7, and the tumor 14 is irradiated with the charged particle beam B in accordance with the weight map and beam current information.

[0049] Next, the functions and effects of the treatment planning system 200 according to this embodiment will be described.

[0050] In the optimization process S101 executed by the treatment planning system 200, a weight map is created by an optimization calculation, and in this optimization calculation, the constraint condition expressed by the above-mentioned mathematical formula (1) is set. According to the optimization calculation with such constraint condition set, the minimum weight Wmin in the weight map converges to the DRF·Wmax. Therefore, by appropriately setting the DRF, it is possible to increase the minimum weight Wmin compared to the case where such constraint condition is not present.

[0051] As described above, the upper limit of the beam current to be determined depends on the minimum weight Wmin in the layer, and the beam current can be increased by increasing the minimum weight Wmin. Increasing the beam current shortens the scanning time of the charged particle beam B, thereby shortening the treatment time in charged particle beam therapy and reducing the burden on the patient.

[0052] 7(a) and 7(b) are diagrams showing enlarged schematic views of only the area near the minimum weight in a histogram of weights in a weight map. The horizontal axis represents weight, and the vertical axis represents frequency. Of these, FIG. 7(b) is a schematic enlarged view of the area near the minimum weight in a histogram obtained by optimization calculation of this embodiment in which a constraint (Equation (1)) is set. Also, for comparison, FIG. 7(a) is a schematic enlarged view of the area near the minimum weight in a histogram obtained by conventional optimization calculation in which the above-mentioned constraint is not set.

[0053] As can be seen by comparing Figures 7(a) and 7(b), an irradiation point that existed in a small weight region in the conventional case (Figure 7(a)) is considered to move to a weight region greater than DRF·Wmax in this embodiment (Figure 7(b)). As a result, in this embodiment (Figure 7(b)), it is considered that there are almost no irradiation points that appear in a weight region below DRF·Wmax. In the conventional case, as shown in Figure 7(a), in order to ensure scanning speed, cutoff processing is performed by setting a cutoff reference CV within a range that allows for DVH degradation. In contrast, in this embodiment, as shown in Figure 7(b), the cutoff reference CV can be set to a larger weight within a range that allows for DVH degradation. For example, DRF·Wmax may be used as the cutoff reference CV. Therefore, the minimum weight Wmin after cutoff processing increases. As a result, as described above, the beam current can be increased, and the treatment time in charged particle beam therapy can be shortened.

[0054] Furthermore, the constraints in the optimization calculation change the constraint on the weight W during the iterative calculation, and the constraint on the lower limit of the weight W changes during the iterative calculation. Therefore, compared to the case where a constraint is set such that the lower limit of the weight W is a constant, a reasonable constraint on the lower limit of the weight W is efficiently implemented, and as a result, the minimum weight Wmin becomes a reasonable value.

[0055] Furthermore, since the minimum weight Wmin increases as described above, the process of cutting off irradiation locations with extremely small weights (cutoff process S102) is often unnecessary. Therefore, according to the treatment planning system 200 of this embodiment, deterioration of the DVH is suppressed compared to when the cutoff process as performed in the aforementioned Patent Document 1 is performed. Even if an irradiation location with an extremely small weight occurs as a result of the optimization calculation, the frequency of such occurrence is low, so performing the cutoff process S102 does not result in extreme deterioration of the DVH. As described above, according to the treatment planning system 200 of this embodiment, it is possible to shorten the irradiation time of the charged particle beam while suppressing deterioration of the DVH.

[0056] As mentioned above, the value of the dynamic range factor DRF may be set appropriately based on the planning concept of the treatment plan, but it may also be set to a value that includes a margin relative to the originally appropriate DRF (a value larger by the margin). In this case, it is possible to further reduce the number of irradiation points that occur in the weighted region below the originally appropriate DRF·Wmax.

[0057] The present disclosure is not limited to the above-described embodiments.

[0058] For example, in the above-described embodiment, the treatment planning system 200 created a treatment plan in which the current value of the charged particle beam B is kept constant for one layer of the tumor 14, but there may be areas where the current value changes.

[0059] In addition, the calculation unit 110 of the treatment planning system 200 may be configured by a combination of multiple processing devices, for example, a processing device that performs optimization calculations within the treatment plan and a processing device that performs calculations related to cutoff processing may be separate.

[0060] 1...charged particle beam therapy device, 14...tumor (irradiated body), 40...weight map, 200...treatment planning system, B...charged particle beam, P...irradiation location, W...weight.

Claims

1. A treatment planning system for creating a treatment plan for charged particle beam therapy, which executes an optimization process to create a weight map showing the distribution of dose weights of the charged particle beam to be irradiated to each irradiation location of the irradiated body, and in the optimization calculations performed in the optimization process, a constraint condition is set that changes the constraint on the weight value.

2. A treatment planning system as described in claim 1, wherein the optimization calculation uses an iterative method, and the constraint on the weight value varies during the iterative calculation according to the constraint condition.

3. The treatment planning system of claim 2, wherein said constraint specifies a dynamic range of said weights in said weight map during said iterations.

4. The treatment planning system of claim 3, wherein the constraints include a dynamic range factor relating to the dynamic range.

5. The treatment planning system of claim 4, wherein the constraint condition limits the weight assigned to each irradiation point during the iterative calculation to a value greater than the product of the maximum weight value in the weight map during the iterative calculation and the dynamic range factor, which is a constant.

6. The treatment planning system according to claim 5, wherein in the optimization calculation, a ratio between a maximum weight value and a minimum weight value in the weight map converges to the value of the dynamic range factor.

7. A treatment planning system as described in claim 2, wherein in the iterative calculation, the constraint condition determines a minimum weight value based on a ratio to a maximum weight value in the weight map.

8. The treatment planning system of claim 1, further comprising a beam current determination process for determining a beam current of the charged particle beam to be irradiated to the subject based on the weight map created by the optimization process, wherein in the beam current determination process, the beam current is determined based on a maximum scanning speed of the charged particle beam in a charged particle beam therapy device used for the charged particle beam therapy and a minimum weight in the weight map.