JP7574142B2 - Particle beam arrival position monitor, particle beam therapy system, and particle beam arrival position monitor method - Google Patents

Description

The present invention relates to a particle beam arrival position monitor, a particle beam therapy system, and a particle beam arrival position monitoring method, and in particular to a technology for detecting prompt gamma rays emitted from an irradiated body.

Particle beam therapy is widely used. In particle beam therapy, tumors are treated by irradiating the affected area with particle beams of protons, carbon, etc. Particle beams are generated by accelerating ions in an accelerator. The particle beams are guided to a treatment room by a beam transport system, and are irradiated to the patient's treatment area from an irradiation device in the treatment room. Generally, particle beams such as proton beams and heavy particle beams irradiated inside a patient's body stop at a specified depth, and impart energy to tissues inside the body just before stopping, destroying the tissue. The depth to which the particle beam reaches is determined by the energy of the particle beam.

In particle beam therapy, the properties of particle beams described above are utilized to adjust the position where the particle beam arrives by changing the energy of the particle beam. In other words, the particle beam is irradiated to the affected area with energy adjusted so that the particle beam stops at the affected area. This provides the affected area with sufficient energy just before it stops, destroying the tissue in the affected area. However, the actual position where the particle beam arrives does not necessarily match the designed position due to factors on the treatment device side or the patient side. For this reason, when treating an affected area with particle beams, it is necessary to confirm the actual position where the particle beam arrives.

A technology has been proposed to determine the arrival position of a particle beam. With this technology, the dose distribution in the depth direction (irradiation axis direction) of the particle beam is measured, and the position of the Bragg peak is determined as the arrival position of the particle beam. The Bragg peak is defined as the peak of the dose distribution when the energy of the particle beam is expressed as a dose distribution in the depth direction. The position of the Bragg peak corresponds to the arrival position of the particle beam.

The following Patent Documents 1 and 2 describe a technique for measuring the dose distribution in the depth direction by measuring the radiation generated by the interaction between a particle beam and an irradiated object. That is, as the particle beam travels through the inside of the irradiated object, prompt gamma rays are generated by collisions between the particle beam and atoms inside the irradiated object. In the technique described in Patent Document 1, the position of the Bragg peak is found by using a gamma ray pinhole camera to measure the dose distribution in the depth direction of prompt gamma rays generated by irradiation with a particle beam.

In the technology described in Patent Document 2, the dose distribution in the depth direction of prompt gamma rays generated by irradiation with a particle beam is measured based on the position where electron pairs are generated when prompt gamma rays collide with a detector, the direction of travel of the electrons and positrons, and information on the energy of the electrons and positrons. Then, the position of the Bragg peak is determined based on the dose distribution.

Special Publication No. 2011-526504 JP 2019-88597 A

In conventional technology, if there is a lot of noise in the measurement of prompt gamma rays used to identify the position irradiated by the particle beam, the Bragg peak does not appear properly in the dose distribution, making it difficult to measure the position of the particle beam with high accuracy. Noise sources include neutrons generated within the irradiated body due to particle beam irradiation, and particles generated by nuclear reactions between neutrons inside the irradiated body or with the detection device.

The objective of the present invention is to determine the arrival position of a particle beam with high accuracy.

The present invention is characterized in that a particle beam arrival position monitor for determining the arrival position of a particle beam incident on an irradiated object comprises a shield having a transmission opening, a front shield arranged in front of the shield to slow down or absorb radiation, or to slow down and absorb radiation, and a gamma ray detector provided behind the front shield, the gamma ray detector comprising a plurality of detection elements arranged in the irradiation axis direction of the particle beam.

The present invention also includes a particle beam arrival position monitoring method for determining the arrival position of a particle beam incident on an irradiated object, which includes arranging a gamma ray detector having a plurality of detection elements so that the arrangement direction of the detection elements is aligned with the irradiation axis direction of the particle beam, arranging a shielding body having a transmission opening through which gamma rays pass in a direction intersecting the irradiation axis between the irradiation axis and the gamma ray detector, arranging a front shielding body between the irradiation axis and the shielding body, and arranging a shielding detector between the shielding body and the gamma ray detector for detecting noise that has passed through the shielding body, invalidating the main detection value output from the detection element when a reference detection value is output from the shielding detector, determining a dose distribution of the gamma rays along the irradiation axis direction based on the main detection value that has not been invalidated among the main detection values output from each of the detection elements and the positional relationship between each of the detection elements and the transmission opening, and determining the arrival position based on the dose distribution.

The present invention makes it possible to determine the arrival position of a particle beam with high accuracy.

1 is a diagram showing an overall configuration of a particle beam therapy system. FIG. 2 is a diagram conceptually showing a dose distribution in the irradiation axis direction of a particle beam. 4A and 4B are diagrams showing outlines of time waveforms of a reference detection signal and a detection signal. 13 is a flowchart of a process for determining the arrival position of a particle beam. 10 is a flowchart showing the operation of a calibration mode. FIG. 13 is a diagram showing an external trigger signal input to a shielding detector and a gamma ray detector. 13 is a flowchart showing an example of operation in a measurement mode. 13A and 13B are diagrams showing modified examples of a shield and a shielding detector. FIG. 13 is a diagram showing a modified example of a shielding detector. FIG. 1 shows an embodiment in which multiple detection units are arranged around an illumination axis. 1 is a diagram showing a particle beam therapy system according to a first embodiment of the present invention. FIG. 1 is a diagram showing a particle beam therapy system according to a second embodiment of the present invention. FIG. 13 is a diagram showing a modified example of the particle beam therapy system. FIG. 13 is a diagram showing a modified example of the particle beam therapy system. FIG. 13 is a diagram showing a modified example of the particle beam therapy system.

The basic technology of the present invention and an embodiment of the present invention will be explained with reference to each drawing. Identical items shown in multiple drawings will be given the same reference numerals and their explanation will be omitted. Figure 1 shows the overall configuration of a particle beam therapy system 100 relating to the basic technology of the present invention.

The particle beam therapy system 100 includes a particle beam irradiation device 1, a particle beam arrival position monitor 2, a particle beam control device 10, and a display unit 55. The particle beam irradiation device 1 irradiates a particle beam 11 to an irradiated body 20 based on the control of the particle beam control device 10. The irradiated body 20 is a patient who is the subject of particle beam therapy. The particle beam arrival position monitor 2 monitors the arrival position of the particle beam 11 irradiated to the irradiated body 20. The display unit 55 displays the results of monitoring the arrival position of the particle beam 11 and the operating status of the particle beam arrival position monitor 2.

The particle beam arrival position monitor 2 includes a gamma ray detector 30, a shielding body 40, a shielding detector 42, and a computing device 50. The shielding body 40 is disposed between the gamma ray detector 30 and the irradiated body 20. The shielding detector 42 is disposed between the shielding body 40 and the gamma ray detector 30. The shielding body 40 has a transmission opening 41. The shielding detector 42 has a defect portion 39 at a position corresponding to the transmission opening 41, i.e., at the same position on the irradiation axis z along which the particle beam 11 travels. Note that if the shielding detector 42 is formed of a material that transmits gamma rays, the defect portion 39 does not necessarily have to be provided.

The particle beam irradiation device 1 may include, for example, a particle beam generator, a particle beam transport device, an irradiation unit, etc. (none of which are shown). The particle beam generator has an accelerator such as a cyclotron or a synchrotron, and generates particle beams such as protons or carbon. The particle beam transport device transports the particle beam output from the particle beam generator to the irradiation unit. The irradiation unit controls the irradiation direction and particle beam diameter (irradiation field) of the particle beam 11, and irradiates the particle beam 11 toward a target 21, such as a tumor, in the irradiated body 20.

When the particle beam irradiation device 1 irradiates the subject 20 with the particle beam 11, prompt gamma rays 13 are generated. The gamma ray detector 30 detects the prompt gamma rays 13 that have passed through the transparent opening 41 and the missing portion 39. The calculation device 50 is connected to the gamma ray detector 30 and the shielding detector 42. The calculation device 50 performs calculations related to the detection results of the prompt gamma rays.

The gamma ray detector 30 may include a scintillator and a photoelectric converter that converts the light emitted from the scintillator into an electrical signal. For example, LaBr3, GSO, LYSO, BGO, etc. may be used as the scintillator, or a radiation emitting element having a different composition may be used.

A photomultiplier tube, a photodiode, etc. may be used as the photoelectric converter. Also, instead of a scintillator, a semiconductor detector such as CdTe or CZT may be used, in which case the gamma ray detector 30 uses a preamplifier instead of a photoelectric converter.

The gamma ray detector 30 is an array-type detector in which multiple detection elements are arranged in at least one dimension. The gamma ray detector 30 is arranged so that the array direction of the detection elements is aligned with the irradiation axis z of the particle beam 11. This allows the gamma ray detector 30 to capture prompt gamma rays 13 coming from a direction that is not parallel to the irradiation axis z. The position of the irradiation axis z is determined according to the target position of irradiation set in the treatment plan.

In the particle beam arrival position monitor 2, one gamma ray detector 30 may be used, or, as shown in the example described below, multiple gamma ray detectors 30 may be arranged around the irradiation axis z. This improves the detection accuracy of the prompt gamma rays 13. A detection signal based on the prompt gamma rays 13 detected by each detection element of the gamma ray detector 30 is output to the calculation device 50.

The shield 40 is made of a material that is substantially opaque to gamma rays. Examples of materials that are substantially opaque to gamma rays include substances with large mass numbers and high density, such as materials containing mercury or lead, and tungsten. The shield 40 is placed on the irradiation axis z side of the gamma ray detector 30, and limits the detection range of the gamma ray detector 30. The transparent opening 41 may be made of a material that is transparent to gamma rays, or may be a slit cut into the shield 40 in the vertical direction of the figure.

The shielding detector 42 detects radiation particles, such as neutrons and X-rays, that are generated from the internal tissue of the irradiated subject 20 irradiated with the particle beam 11 and that have passed through the shielding body 40. An organic scintillator that is highly sensitive to neutrons may be used for the shielding detector 42. As the scintillator, for example, a plastic scintillator, an organic liquid scintillator, etc. may be used, or a radiation emitting element with a different composition may be used.

When the particle beam 11 is irradiated onto the irradiated body 20, prompt gamma rays 13 that have passed through the transparent opening 41 and the missing portion 39 reach each detection element of the gamma ray detector 30. The detection signal output by the gamma ray detector 30 indicates the event of prompt gamma rays 13 reaching each detection element through the transparent opening 41 and the missing portion 39, and the calculation device 50 performs one count for each event.

The plot of the counts of the gamma ray detector 30 along the array direction of the detector elements of the gamma ray detector 30 is defined as a profile of the counts or simply a profile. The profile reflects the dose distribution of the particle beam 11.

The arithmetic device 50 analyzes the profile of the gamma ray detector 30 and determines the position where the dose is maximum in the dose distribution in the irradiation axis z direction of the particle beam 11, i.e., the position of the Bragg peak (BP), as the arrival position of the particle beam 11. For this purpose, the arithmetic device 50 includes a signal analysis unit 52 that analyzes the detection signal of the gamma ray detector 30, a reconstruction calculation unit 53, and an arrival position calculation unit 54. The arithmetic device 50 may include a database (not shown) that stores the analysis results, etc. Specific processing performed by the arithmetic device 50 will be described later.

The computing device 50 is, for example, configured as a computer having a processor such as a CPU (Central Processing Unit) or a GPU (Graphics Processing Unit) and a memory.

The CPU reads and executes the computer program stored in the memory, thereby realizing the functions of the signal analysis unit 52, the reconstruction calculation unit 53, and the arrival position calculation unit 54 by software. The calculation device 50 may realize some or all of its functions by hardware.

For example, a custom IC such as an ASIC (Application Specific Integrated Circuit) or a programmable IC such as an FPGA (Field-Programmable Gate Array) may be used. That is, the signal analysis unit 52, the reconstruction calculation unit 53, and the arrival position calculation unit 54 may be configured by a programmable IC, and the circuit may be designed to realize the functions.

A part or all of the processing performed by the calculation device 50 may be executed by an external computer. The external computer may be directly connected to the particle beam arrival position monitor 2, or may be connected to a communication line such as the Internet. One process executed by the calculation device 50 may be executed by multiple external computers that execute distributed processing.

In the particle beam therapy system 100 according to this basic technology, once the position of the target 21 is determined in the treatment plan, the energy of the particle beam 11 is set so that the position of the Bragg peak where the dose distribution along the irradiation axis z direction of the particle beam 11 is maximized coincides with the target position of irradiation on the target 21. The particle beam 11 may be irradiated by dividing the target 21 into a number of small regions and irradiating each small region in a spot shape while changing the irradiation position.

Figure 2 conceptually shows a dose distribution 200 along the irradiation axis z direction of the particle beam 11. The horizontal axis indicates the irradiation axis z of the particle beam 11, and the vertical axis indicates the dose. The dose distribution 200 corresponds to each position in the irradiation axis z direction when the particle beam 11 is irradiated to the irradiation position calculated in the treatment plan. The dose distribution 200 in the irradiation axis z direction becomes maximum at a position according to the energy of the particle beam 11. The Bragg peak is a peak in the dose distribution 200, and the position BP of the Bragg peak corresponds to the position reached by the particle beam 11.

When the particle beam irradiation device 1 irradiates the target 21 with the particle beam 11, the particle beam arrival position monitor 2 starts detecting the prompt gamma ray 13. Based on the events detected by the gamma ray detector 30 and the events detected by the shielding detector 42, the particle beam arrival position monitor 2 identifies events detected by the gamma ray detector 30 and the shielding detector 42 at the same time as noise.

The particle beam arrival position monitor 2 removes noise from the count of the gamma ray detector 30, analyzes the profile, and calculates the position of the Bragg peak, i.e., the arrival position of the particle beam 11. The arrival position of the particle beam 11 calculated by the particle beam arrival position monitor 2 is displayed on the display unit 55. The user may check whether or not the particle beam 11 has reached the target position of irradiation based on the information displayed on the display unit 55. In addition, the arrival position of the particle beam 11 is output to the particle beam control device 10.

When the particle beam control device 10 determines that the particle beam 11 has not hit the target 21, the particle beam control device 10 sends a control signal to the particle beam irradiation device 1 to stop the irradiation of the particle beam 11. Furthermore, the particle beam control device 10 controls the particle beam irradiation device 1 so that, for example, the target position is changed. Alternatively, the particle beam control device 10 controls the particle beam irradiation device 1 so that the position at which the particle beam 11 is irradiated onto the irradiation target 20 is corrected. Furthermore, if necessary, the particle beam control device 10 may display instructions or suggestions for changing the position of the gamma ray detector 30 on the display unit 55.

Next, the process executed by the particle beam arrival position monitor 2 according to the basic technology will be described. The particle beam arrival position monitor 2 measures the range of the particle beam 11 when it enters the irradiated body 20 as the arrival position. Here, the basic technology in which the particle beam arrival position monitor 2 is used as part of the particle beam therapy system 100 is shown, but the particle beam arrival position monitor 2 may be used as a device used in high-energy physics experiments, a device used in neutron capture therapy (BNCT: Boron Neutron Capture Therapy), etc.

When the particle beam is irradiated, not only prompt gamma rays 13 but also particles (radiation) such as neutrons and X-rays are generated in the irradiated body 20. Such radiation is likely to scatter within the irradiated body 20 and around structures, and becomes noise that does not carry positional information about the position where the particle beam arrives. In particular, neutrons generated by particle beam irradiation have high energy and undergo nuclear reactions with structures such as the shielding 40, emitting secondary particles. For this reason, the noise components cannot be completely removed by the shielding 40 alone, and the prompt gamma rays 13 may end up being buried in the noise.

Therefore, in the particle beam arrival position monitor 2, events detected by both the shielding detector 42 and the gamma ray detector 30 are identified as noise and are not counted.

A method for calculating the position of the Bragg peak using the particle beam arrival position monitor 2 is explained. In this basic technology, the signal output by the gamma ray detector 30 that detects the prompt gamma ray 13 is separated from the noise output by the gamma ray detector 30 that detects the neutrons generated in the irradiated body 20 by the particle beam irradiation, and then the profile of the prompt gamma ray 13 is calculated.

That is, when the particle beam 11 is irradiated to the target 21, prompt gamma rays 13 are generated, and the prompt gamma rays 13 are detected by the gamma ray detector 30. On the other hand, when the particle beam 11 is irradiated to the target 21, neutrons are generated in the irradiated body 20, and gamma rays derived from the neutrons are also detected by the gamma ray detector 30. The gamma rays derived from the neutrons are noise that do not contribute to the formation of a Bragg peak, and are therefore distinguished from the detection signal due to the prompt gamma rays 13.

During actual treatment, the gamma ray detector 30 and shield 40 are placed at positions predetermined in the treatment plan. For each target position of particle beam irradiation, the positions x = x1 to xN of the detector elements on the detector axis x parallel to the irradiation axis z are predetermined.

3(a) shows an outline of the time waveform of the reference detection signal output from the shielding detector 42. FIG. 3(b) shows an outline of the time waveform of the detection signal output from one detection element in the gamma ray detector 30. Prompt gamma rays 13 generated in the irradiated body 20, passing through the transparent opening 41 of the shielding body 40 and the missing portion 39 of the shielding detector 42 to reach the gamma ray detector 30 are detected by the gamma ray detector 30. On the other hand, the prompt gamma rays 13 are not detected by the shielding detector 42 due to the shielding by the shielding body 40. In addition, radiation such as neutrons, X-rays, etc. (hereinafter referred to as neutrons, etc.) generated in the irradiated body 20 passes through the shielding body 40 and the shielding detector 42 to reach the gamma ray detector 30 and are detected by the gamma ray detector 30. This radiation is also detected by the shielding detector 42.

The signal analysis unit 52 in the calculation device 50 identifies the detection signal output by the gamma ray detector 30 as noise when the reference detection signal is output from the shielding detector 42. The signal analysis unit 52 invalidates the detection signals output by the gamma ray detector 30 that are identified as noise and does not allow them to contribute to the count value.

Specifically, the signal analysis unit 52 invalidates the detection signal output from the gamma ray detector 30 during the time period Δt from when the reference detection signal is output from the shielding detector 42 to when the time period Δt has elapsed, and does not count this detection signal. In the example shown in Figures 3(a) and (b), when the reference detection signal R is not output from the shielding detector 42, the detection signal A is output from the gamma ray detector 30. Then, during the time period Δt from when the reference detection signal R is output from the shielding detector 42 to when the time period Δt has elapsed, the detection signal B is output from the gamma ray detector 30. Therefore, although the signal analysis unit 52 counts the detection signal A, it identifies the detection signal B as noise and does not count the detection signal B.

Note that there may be a difference between the delay time from when the gamma ray detector 30 detects radiation until the signal analysis unit 52 acquires a detection signal, and the delay time from when the shielding detector 42 detects radiation until the signal analysis unit 52 acquires a reference detection signal. As described below, the signal analysis unit 52 compensates for this difference in delay time before identifying noise and counting the prompt gamma rays 13. To simplify the explanation, it is described here as if the difference in delay time was compensated for.

The signal analysis unit 52 obtains count values for each of the N detection elements located at coordinate values x = x1 to xN on the detector axis x, excluding detection signals identified as noise, and outputs the count values obtained for each detection element to the reconstruction calculation unit 53. The reconstruction calculation unit 53 obtains a measured dose distribution that corresponds the dose of prompt gamma rays 13 on the irradiation axis z of the particle beam 11 by reconstructing a count value distribution that corresponds the count values to the positions of the N detection elements on the detector axis x. Here, the reconstruction is a calculation that obtains a measured dose distribution from the count value distribution based on the geometric relationship between each position on the irradiation axis z, the position of the transmission opening 41, and the position of each detection element on the detector axis x. The reconstruction calculation unit 53 outputs information indicating the measured dose distribution to the arrival position calculation unit 54.

When scanning the particle beam 11, the distance and angle between the particle beam 11 and the gamma ray detector 30 change. Therefore, the signal analysis unit 52 may modify the count value distribution in accordance with the scanning of the particle beam 11.

The arrival position calculation unit 54 stores a reference dose distribution, which is the dose distribution when the particle beam 11 reaches a predetermined reference position (z = 0) on the irradiation axis z. The Bragg peak of the reference dose distribution is at the reference position. The arrival position calculation unit 54 calculates the shift amount of the measured dose distribution relative to the reference dose distribution.

The shift amount indicates the distance by which the measured dose distribution is shifted in the direction of the irradiation axis z relative to the reference dose distribution. The shift amount indicates the position of the Bragg peak on the irradiation axis z. The arrival position calculation unit 54 determines the shift amount of the measured dose distribution relative to the reference dose distribution, i.e., the position of the Bragg peak, and determines the arrival position of the particle beam 11. The shift amount may be determined based on the shift of the maximum value or the shift of the inflection point relative to the reference dose distribution.

Figure 4 shows a flowchart showing the process of determining the arrival position of the particle beam 11. The particle beam irradiation device 1 irradiates the target 21 with the particle beam 11 (S41). The gamma ray detector 30 and the shielding detector 42 detect radiation generated in response to the irradiation of the target 21 with the particle beam 11, and output a detection signal and a reference detection signal, respectively, to the signal analysis unit 52 (S42).

The signal analysis unit 52 identifies the detection signals output from the gamma ray detector 30 during the time period from when the reference detection signal was output from the shielding detector 42 until the time Δt has elapsed as noise (S43). The signal analysis unit 52 invalidates the detection signals output by the gamma ray detector 30 that it identifies as noise and does not allow them to contribute to the count value, and counts the other valid detection signals (S44). The signal analysis unit 52 outputs the count value obtained for each detection element to the reconstruction calculation unit 53.

The reconstruction calculation unit 53 calculates the count value distribution based on the count values calculated for each detection element (S45). The reconstruction calculation unit 53 calculates the measured dose distribution by reconstructing the count value distribution (S46). The arrival position calculation unit 54 calculates the shift amount of the measured dose distribution relative to the reference dose distribution in the irradiation axis z direction, and calculates the arrival position of the particle beam 11 (S47).

The computing device 50 displays the arrival position of the particle beam 11 on the display unit 55. The user may check whether the treatment is being performed appropriately by checking the arrival position displayed on the display unit 55. The user may adjust the particle beam irradiation device 1 while changing the irradiation position of the particle beam 11 so that the actual arrival position is the target position.

In this way, the particle beam arrival position monitor 2 according to the basic technology of the present invention is provided with the following components to determine the arrival position of the particle beam 11 incident on the irradiated object 20. That is, the particle beam arrival position monitor 2 is provided with a shielding body 40, a shielding detector 42, and a gamma ray detector 30. The shielding body 40 has a transmission opening 41. The shielding detector 42 is disposed behind the shielding body 40. Here, the rear indicates the direction when the direction seen from the particle beam arrival position monitor 2 toward the irradiated object 20 is defined as the forward direction. The gamma ray detector 30 is provided behind the shielding detector 42, and is provided with a plurality of detection elements arranged in the irradiation axis z direction of the particle beam 11.

The particle beam arrival position monitor 2 is equipped with a calculation device 50. When a reference detection value is output from the shielding detector 42, the calculation device 50 invalidates the main detection value output from the detection element. Here, the reference detection value and the main detection value are values indicated by the reference detection signal and the detection signal, respectively. The calculation device 50 determines the arrival position based on the main detection values that have not been invalidated among the main detection values output from each detection element.

The calculation device 50 calculates the measured dose distribution (dose distribution) of the prompt gamma rays 13 along the irradiation axis z direction based on the main detection values that have not been invalidated among the main detection values output from each detection element and the positional relationship between each detection element and the transparent opening 41. The calculation device 50 calculates the arrival position based on the measured dose distribution. In other words, the calculation device 50 reconstructs the count value distribution based on the positional relationship between each detection element and the transparent opening 41, and calculates the dose distribution of the gamma rays along the irradiation axis z direction.

The particle beam arrival position monitor 2 executes a particle beam arrival position monitoring method including the following steps (i) to (vi). The particle beam arrival position monitoring method is a method for determining the arrival position of the particle beam 11 incident on the irradiated object 20.

(i) Arrange a gamma ray detector 30 having a plurality of detection elements so that the array direction of the detection elements is aligned with the irradiation axis z direction of the particle beam 11. (ii) Arrange a shielding body 40 having a transmission opening 41 through which the prompt gamma ray 13 passes in a direction intersecting the irradiation axis z between the irradiation axis z and the gamma ray detector 30. (iii) Arrange a shielding detector 42 for detecting noise transmitted through the shielding body 40 between the shielding body 40 and the gamma ray detector 30. (iv) Invalidate the main detection value output from the detection element when the reference detection value is output from the shielding detector 42. (v) Calculate the dose distribution along the irradiation axis z direction of the gamma ray based on the main detection value that was not invalidated among the main detection values output from each detection element and the positional relationship between each detection element and the transmission opening 41. (vi) Calculate the arrival position of the particle beam based on the measured dose distribution.

The particle beam arrival position monitor 2 according to this basic technology suppresses the effects of noise caused by neutrons and the like generated in the irradiated body 20, and the particle beam arrival position can be determined with high accuracy.

Therefore, for example, if the position of the tumor is different from that in the treatment plan due to a change in the patient's posture or body shape during particle beam irradiation, it becomes easy to change the particle beam irradiation position or to stop particle beam irradiation. Also, for example, the possibility of applying particle beam therapy is increased even in cases where organs with high radiation sensitivity are present near the tumor.

The particle beam arrival position monitor 2 according to this basic technology solves the problems found in the technologies described in Patent Documents 1 and 2 above. In the technology described in Patent Document 1, since there is a flight time difference between prompt gamma rays and neutrons, it is described that it is possible to reduce signals (noise) originating from neutrons by performing measurements with a time gate synchronized with the irradiation timing. However, when a particle beam is irradiated once for a time longer than the flight time difference described above, it is not possible to distinguish between signals and noise. On the other hand, the particle beam arrival position monitor 2 according to this basic technology can identify whether a detected signal is noise or not, regardless of the number of times the particle beam is irradiated.

In the technology described in Patent Document 2, by setting the energy of the incoming particles to 10 MeV or more, it is possible to detect only gamma ray signals generated from the irradiation axis of the particle beam. However, depending on the gamma ray energy conditions, the probability of electron pairs being generated in the detector decreases, decreasing the detection sensitivity. On the other hand, the measurement principle of the particle beam arrival position monitor 2 according to this basic technology does not depend on the generation of electron pairs, so such problems do not arise.

As described above, there may be a difference between the delay time from when the gamma ray detector 30 detects radiation until the signal analysis unit 52 acquires a detection signal, and the delay time from when the shielding detector 42 detects radiation until the signal analysis unit 52 acquires a reference detection signal. Here, the operation of the calibration mode for compensating for such a difference in delay time is described. The operation of the calibration mode is performed separately from the operation of the measurement mode for measuring the arrival position of the particle beam.

The flow chart in FIG. 5 shows the operation of the calibration mode (S51 to S53). The operation of the calibration mode is performed for each of the multiple detection elements of the gamma ray detector 30. In the calibration mode, external trigger signals are input simultaneously to the shielding detector 42 and the gamma ray detector 30, and the difference in timing at which each external trigger signal is acquired by the signal analysis unit 52 is measured.

An external trigger signal is input simultaneously to the shielding detector 42 and the gamma ray detector 30 (S51). The signal analysis unit 52 determines the time difference between a first time when the external trigger signal output from the shielding detector 42 is acquired by the signal analysis unit 52 and a second time when the external trigger signal output from the gamma ray detector 30 is acquired by the signal analysis unit 52 (S52). The time difference may be the time obtained by subtracting the second time from the first time, or the time obtained by subtracting the first time from the second time. The signal analysis unit 52 stores the time difference (S53).

The calculation device 50 performs the measurement mode operation after compensating for the difference on the time axis between the detection signal and the reference detection signal based on the time difference stored by the signal analysis unit 52 (S54).

The signal analysis unit 52 may perform the following operations. That is, the signal analysis unit 52 associates the detection signal and the reference detection signal output from the gamma ray detector 30 and the shielding detector 42, respectively, with time and stores them as time data. The signal analysis unit 52 delays the reference detection signal by the time difference on the time axis and stores it. Alternatively, the signal analysis unit 52 advances the detection signal by the time difference on the time axis and stores it. Here, the time difference is the time obtained by subtracting the second time from the first time (the time representing the advance of the reference detection signal relative to the detection signal).

When the time difference is the second time minus the first time, the signal analysis unit 52 delays the detection signal by the time difference on the time axis and stores it. Alternatively, the signal analysis unit 52 advances the reference detection signal by the time difference on the time axis and stores it. In the measurement mode, the process shown in the flowchart of FIG. 4 may be executed based on the time data in which the delay time has been compensated for based on the time difference (S54).

In this way, when the particle beam arrival position monitor 2 operates in the calibration mode, the shielding detector 42 and each detection element detect the same event. When the shielding detector 42 and each detection element detect the same event, the signal analysis unit 52 calculates a time difference, which is the difference between the time at which the main detection value indicated by the detection signal and the reference detection value indicated by the reference detection signal are acquired by the calculation device 50. When the particle beam arrival position monitor 2 operates in the measurement mode, the calculation device 50 calculates the arrival position of the particle beam 11 after compensating for the difference on the time axis between the main detection value and the reference detection value based on the time difference.

Figure 6 shows a conceptual diagram of each external trigger signal input to the shielding detector 42 and the gamma ray detector 30 and acquired by the signal analysis unit 52. Figure 6(a) shows the external trigger signal Tr1 input to the shielding detector 42 and acquired by the signal analysis unit 52. Figure 6(b) shows the external trigger signal Tr2 input to the gamma ray detector 30 and acquired by the signal analysis unit 52. The horizontal axis of each figure indicates time, and the vertical axis indicates the peak value. The time difference δ obtained by subtracting the second time t2 when the external trigger signal Tr2 is acquired by the signal analysis unit 52 from the first time t1 when the external trigger signal Tr1 is acquired is stored in the signal analysis unit 52.

Here, a process is shown in which an external trigger signal is input simultaneously to each detector (42, 30) as the same event for the shielding detector 42 and the gamma ray detector 30. Instead of such a process, the shielding detector 42 and the gamma ray detector 30 may be made to detect radiation simultaneously. For example, a process may be performed in which 22Na decays in the region between the shielding detector 42 and the gamma ray detector 30 to generate gamma rays that are detected simultaneously by the shielding detector 42 and the gamma ray detector 30.

Figure 7 shows a flowchart illustrating another example of operation in the measurement mode. In steps S71, S72, and S75 to S77, the same processing as in steps S41, S42, and S45 to S47 shown in the flowchart in Figure 4 is executed, respectively.

The particle beam irradiation device 1 irradiates the target 21 with the particle beam 11 (S71). The gamma ray detector 30 and the shielding detector 42 detect radiation generated in response to the irradiation of the target 21 with the particle beam 11, and output a detection signal and a reference detection signal, respectively, to the signal analysis unit 52 (S72).

The signal analysis unit 52 generates a gate signal whose signal value is high from the time when the reference detection signal is output from the shielding detector 42 until the time Δt has elapsed, and whose signal value is low during other time periods (S73). The calculation device 50 stops the detection of radiation by the gamma ray detector 30 while the gate signal is high. As a result, the signal analysis unit 52 invalidates the detection signals output by the gamma ray detector 30 that were obtained when the gate signal was high and does not contribute to the count value, and counts other valid detection signals. In other words, the signal analysis unit 52 counts the detection signals output by the gamma ray detector 30 that were obtained when the gate signal was low (S74). The signal analysis unit 52 outputs the count value obtained for each detection element to the reconstruction calculation unit 53.

The reconstruction calculation unit 53 calculates the count value distribution based on the count values calculated for each detection element (S75). The reconstruction calculation unit 53 calculates the measured dose distribution by reconstructing the count value distribution (S76). The arrival position calculation unit 54 calculates the shift amount of the measured dose distribution relative to the reference dose distribution in the irradiation axis z direction, and calculates the arrival position of the particle beam 11 (S77).

When the gate signal is high, there is a possibility that noise such as neutrons may be detected by the shielding detector 42 and the gamma ray detector 30. Therefore, according to this operation, any detection signal output by the gamma ray detector 30 that may be noise is invalidated and does not contribute to the count value. This suppresses the influence of noise due to neutrons and the like generated in the irradiated body 20, and the arrival position of the particle beam 11 can be determined with high accuracy.

Figure 8 shows a modified example of the shielding body 40 and the shielding detector 42. In this modified example, the shielding body 40 is provided with three transmission openings 41. The shielding detector 42 is provided with a cutout 39 at a position corresponding to the transmission openings 41 in the shielding body 40, i.e., at the same position as the transmission openings 41.

In this way, by providing multiple transmission openings 41 in the shielding body 40 and multiple cutouts 39 in the shielding detector 42, the number of radiation events detected by the gamma ray detector 30 increases compared to when one transmission opening 41 and one cutout 39 are provided. This allows the arrival position to be determined from more events, and the arrival position can be determined more accurately.

Figure 9 shows a schematic diagram of a modified shielding detector. This figure shows the shielding detector 60 viewed from the irradiation axis z side with the shielding body 40 shown in Figure 8 removed. The shielding detector 60 has multiple light-emitting sections 62 separated by cutouts 39. Each light-emitting section 62 generates light when it detects radiation.

In the example shown in FIG. 9, the shielding detector 60 has three cutouts 39 and four light-emitting sections 62 separated by the three cutouts 39. The shielding detector 60 also has a photomultiplier tube 66 as a converter that converts light into an electrical signal, and an optical propagation path 64 that connects each light-emitting section 62 to the photomultiplier tube 66.

Each light-emitting unit 62 is composed of a scintillator. The multiple light propagation paths 64 connecting the multiple light-emitting units 62 and the photomultiplier tube 66 have the same optical path length. Each light propagation path 64 may be formed of an optical fiber. When radiation is detected by the light-emitting unit 62, the light-emitting unit 62 generates light. The light generated by the light-emitting unit 62 propagates through the light propagation path 64 and reaches the photomultiplier tube 66.

As described above, the shielding detector 60 includes a photomultiplier tube 66 as a converter, and multiple light propagation paths 64. The photomultiplier tube 66 converts the light output from the multiple light-emitting units 62 into an electrical signal. The multiple light propagation paths 64 are provided between the multiple light-emitting units 62 and the photomultiplier tube 66, and propagate the light in the same propagation time.

The photomultiplier tube 66 converts the light arriving from each of the multiple optical propagation paths 64 into an electrical signal. The electrical signal generated by the photomultiplier tube 66 corresponds to the logical sum (OR) of the light arriving from each of the multiple optical propagation paths 64. The photomultiplier tube 66 outputs the electrical signal as a reference detection signal to the signal analysis unit 52.

Each light-emitting unit 62 and photomultiplier tube 66 are connected by an optical propagation path 64 having the same optical path length. Therefore, light based on the same event detected by each light-emitting unit 62 reaches the photomultiplier tube 66 simultaneously. This allows radiation to be detected more reliably than when a single light-emitting unit 62 is used, and the arrival position of the particle beam 11 can be determined more reliably.

The particle beam arrival position monitor 2 shown in FIG. 1 determines the measurable range on the irradiation axis z of the arrival position of the particle beam 11 according to the relative positions of the irradiated body 20, the shielding body 40, the shielding detector 42, and the gamma ray detector 30. However, the position of the target 21 can exist in a wide range of the irradiated body 20.

The particle beam arrival position monitor 2 may be provided with a movement mechanism that moves the shield 40, the shielding detector 42, and the gamma ray detector 30 parallel to the irradiation axis z direction of the particle beam 11. The movement mechanism may be controlled by the computing device 50 in response to a user's operation. The movement mechanism adjusts the positions of the shield 40, the shielding detector 42, and the gamma ray detector 30 in the irradiation axis z direction so that the target 21 is within the measurable range.

Figure 10 shows a particle beam therapy system in which multiple detection units 25 are arranged around the irradiation axis. Each detection unit 25 includes a shield 40, a shielding detector 42, and a gamma ray detector 30. The calculation device 50 determines the arrival position of the particle beam 11 for each detection unit 25. In the process of determining the arrival position of the particle beam 11 for each detection unit 25, radiation identified as noise is invalidated and does not contribute to the count value. The calculation device 50 determines the overall arrival position based on statistical values such as the average and median of the arrival positions obtained for each detection unit 25.

With this configuration, the overall arrival position is determined based on statistical processing of multiple arrival positions determined by multiple detection units 25. This improves measurement accuracy.

Figure 11 shows a particle beam therapy system 102 according to a first embodiment of the present invention. The particle beam arrival position monitor 3 included in the particle beam therapy system 102 differs from the particle beam arrival position monitor 2 shown in Figure 1 in that a front shield 43 is provided in front of the shield 40. The front shield 43 decelerates or absorbs, or decelerates and absorbs, radiation particles that pass through the front shield 43. Here, decelerating the radiation particles corresponds to suppressing the energy of the radiation, and absorbing the radiation particles corresponds to suppressing the radiation dose.

When the dose of the particle beam 11 irradiated to the irradiated body 20 increases, the noise caused by the radiation, such as neutrons and X-rays, also increases. In this case, in the particle beam arrival position monitor 2 shown in FIG. 1, the number of noise events detected by the shielding detector 42 increases. When the number of noise events occurring per unit time increases and the waveforms of multiple reference detection signals and multiple detection signal waveforms overlap on the time axis, it becomes difficult to individually identify the multiple noise events.

For example, if multiple reference detection signals appear that overlap on the time axis shown in the upper part of Figure 3, and multiple detection signals appear that overlap on the time axis shown in the lower part, it becomes difficult to process the detection signal identified as noise as invalid. In other words, when a reference detection signal is output from the shielding detector 42, it becomes difficult to process the detection signal output from the detection element of the gamma ray detector 30 as invalid.

Therefore, in the particle beam arrival position monitor 3 according to this embodiment, a front shield 43 is provided in front of the shield 40. The front shield 43 has a missing area 44 at a position corresponding to the transmission opening 41 provided in the shield 40. In other words, the missing area 44 is provided at the same position as the transmission opening 41. The missing area 44 is an area where the material forming the front shield 43 is missing. The missing area 44 may be an area where the front shield 43 is cut in the front-rear direction.

The particle beam arrival position monitor 3 according to the embodiment of the present invention includes the following components, and reliably executes a process of invalidating the detection signal output from the detection element of the gamma ray detector 30 when a reference detection signal is output from the shielding detector 42. The particle beam arrival position monitor 3 includes a shield 40, a front shield 43, a shielding detector 42, and a gamma ray detector 30. The front shield 43 is disposed in front of the shield 40, and decelerates or absorbs radiation, or decelerates and absorbs radiation. The gamma ray detector 30 is disposed behind the front shield 43. The shielding detector 42 is disposed behind the front shield 43 and in front of the gamma ray detector 30.

By providing the front shield 43, the number of noise events detected per unit time is reduced, or the energy is suppressed. Therefore, the number of reference detection signals output from the shielding detector 42 is suppressed, or the peak value of the reference detection signal is suppressed. Also, the number of detection signals output from the gamma ray detector 30 is suppressed, or the peak value of the detection signal is suppressed.

In addition, in each detection unit 25 shown in FIG. 10, a front shield 43 may be provided in front of the shield 40.

This prevents the waveforms of multiple reference detection signals and multiple detection signals from overlapping on the time axis, making it easier to identify individual noise events. In other words, it becomes easier to invalidate the detection signal output from the detection element of the gamma ray detector 30 when the reference detection signal is output from the shielding detector 42.

The front shield 43 may be formed from a material that has a large mass number and a high density, as a material that has a large effect of slowing down relatively high-energy neutrons. Such materials include heavy metals. Heavy metals are metals with a specific gravity of 4 or more. The front shield 43 may be formed from a material containing iron, a material containing lead, mercury, tungsten, cadmium, gadolinium, etc. An example of a material containing iron is stainless steel (SUS).

The front shield 43 may be made of hydrogen or a material containing hydrogen (hydrogen compound), which has a large effect of slowing down relatively low-energy neutrons. Such materials include paraffin (hydrocarbon compound).

The front shield 43 may be formed from a material of a light element that has the effect of absorbing neutrons. Light elements are carbon and elements lighter than carbon. Light elements include lithium and boron. The front shield 43 may be formed from a plastic resin such as polyethylene mixed with boron.

The front shield 43 may include multiple layers made of different materials. The frontmost layer of the multiple layers may be made of a heavy metal, and the rearmost layer may be made of a light element. By arranging the layer made of a heavy metal at the front and the layer made of a light element at the rear, the radiation particles that enter the front shield 43 are absorbed after being decelerated. This enhances the effectiveness of the rear layers in absorbing the particles of the particle beam.

The forward shield 43 may include two layers formed of different materials. The forward layer of the two layers may be formed of stainless steel, and the rearward layer may be formed of boron. The forward shield 43 may include three layers formed of different materials. The forwardmost layer of the three layers may be formed of stainless steel, the rearwardmost layer may be formed of boron, and the remaining intermediate layer may be formed of a material containing hydrogen.

In this way, by placing the layer with the greater effect of slowing down radiation particles at the front and the layer with the greater effect of absorbing radiation particles at the rear, the rear layer becomes more effective at absorbing radiation.

Figure 12 shows a particle beam therapy system 104 according to the second embodiment. The particle beam arrival position monitor 4 included in the particle beam therapy system 104 is obtained by adding a front shield 43 in front of the shield 40 shown in Figure 8. The front shield 43 has missing regions 44 at positions corresponding to the multiple transmission openings 41 in the shield 40 and the multiple missing portions 39 in the shielding detector 42. Figure 12 shows an example in which three transmission openings 41, three missing portions 39, and three missing regions 44 are provided. The three missing regions 44 are provided at the same positions as the three transmission openings 41.

The shield 40 has a slit S1 cut in the front-to-rear direction as a transparent opening 41, and the edge of the shield 40 that forms the slit S1 (the edge of the slit S1) has a U-shaped edge shape. The U-shaped edge shape is a U-shaped trajectory that runs along the front surface toward the slit S1, runs toward the rear along the end face that forms the slit S1, and runs along the rear surface away from the slit S1.

The front shield 43 has a front slit S2 cut in the front-rear direction as a missing region 44, similar to the transparent opening 41 in the shield 40. A plurality of missing regions 44 are provided at positions corresponding to the plurality of transparent openings 41. In other words, a plurality of missing regions 44 are provided at the same positions of the plurality of transparent openings 41. The edge of the front shield 43 that forms the front slit S2 (the edge of the front slit S2) has a U-shaped edge shape, similar to the edge of the slit S1 in the shield 40.

In this embodiment, a plurality of missing regions 44 are provided in the front shield 43, a plurality of transmission openings 41 are provided in the shield 40, and a plurality of missing portions 39 are provided in the shielding detector 42. This increases the number of radiation events detected by the gamma ray detector 30 compared to when one each of the missing region 44, transmission opening 41, and missing portion 39 is provided. This allows the arrival position to be determined from a greater number of events, and the arrival position can be determined more accurately.

Figure 13 shows a particle beam therapy system 106 as a modified example of the particle beam therapy system 104 according to the second embodiment. The shield 40 provided in the particle beam arrival position monitor 5 has a slit S3 cut in the front-rear direction as a transmission opening 41, and the edge of the shield 40 that forms the slit S3 (the edge of the slit S3) has a V-edge shape. The V-edge shape is such that at least one of the front and rear surfaces is inclined so that the thickness becomes thinner toward the slit S3.

The front shield 43 has a front slit S4 cut in the front-rear direction as a missing region 44, similar to the transparent opening 41 in the shield 40. A plurality of missing regions 44 are provided at positions corresponding to the plurality of transparent openings 41. The edge of the front shield 43 that forms the front slit S4 (the edge of the front slit S4) has a slope that expands continuously with respect to the slope of the edge of the slit S3 of the shield 40.

As shown in FIG. 13, the edge of the front slit S4 of the front shield 43 has a slope with the same inclination as the slope of the edge of the slit S3 of the shield 40. The slope of the edge of the front slit S4 of the front shield 43 and the slope of the edge of the slit S3 of the shield 40 form a continuously expanding slope of the same plane. With this structure, the number of prompt gamma rays 13 that collide with the edges of the front slit S4 and slit S3 and are not counted by the gamma ray detector 30 is reduced.

Figure 14 shows a particle therapy system 108 as a modified example of the particle therapy system 106. In the modified example shown in Figure 13, if the number of transmission openings 41 is increased, the intervals at which the transmission openings 41 are arranged become narrower, and the area in which the front shield 43 is arranged becomes narrower. In this case, it may be difficult to arrange the front shield 43. Therefore, the front shield 43 may be configured as in the modified example shown in Figure 14. In the front shield 43 of the particle beam arrival position monitor 6 shown in Figure 14, an enlarged defect area 80 is formed that extends above the area in which the multiple transmission openings 41 are provided in the shield 40. The enlarged defect area 80 is an area corresponding to the area covered by the multiple transmission openings 41.

In the particle beam arrival position monitor 6, furthermore, within the area of the shielding detector 42, a detection mitigation area 81 corresponding to the enlarged defect area 80 formed in the front shield 43 is formed thinner than other areas.

In this configuration, the effect of the front shield 43 is not obtained in the enlarged defect region 80. In other words, the effect of decelerating or absorbing radiation particles generated in the irradiated body 20 is not obtained in the enlarged defect region 80. Therefore, in this modified example, a detection mitigation region 81 is provided in the shielding detector 42. The detection mitigation region 81 has a lower sensitivity for detecting radiation than other regions. Therefore, compared to the shielding detector 42 shown in FIG. 13, the number of events detected from the same number of multiple events is smaller. Therefore, the same effect is obtained as when the enlarged defect region 80 is not provided.

Figure 15 shows a particle beam therapy system 110 as a modified example of the particle beam therapy system 108. Like the particle beam arrival position monitor 7 provided in the particle beam therapy system 110, the detection relaxation region 81 in the shielding detector 42 does not need to have a missing portion 39 for all of the transmission openings 41. In other words, the rear of the transmission openings 41 may be blocked by the material that forms the shielding detector 42.

In the particle beam arrival position monitors 3 to 7 using the front shield 43, a particle beam arrival position monitoring method including the following steps (a) to (g) is executed. The particle beam arrival position monitoring method is a method for determining the arrival position of the particle beam 11 incident on the irradiated object 20.

(a) Arrange a gamma ray detector 30 having a plurality of detection elements so that the arrangement direction of the detection elements is aligned with the irradiation axis z direction of the particle beam 11. (b) Arrange a shielding body 40 having a transmission opening 41 through which the prompt gamma ray 13 passes in a direction intersecting the irradiation axis z between the irradiation axis z and the gamma ray detector 30. (c) Arrange a front shielding body 43 between the irradiation axis z and the shielding body 40. (d) Arrange a shielding detector 42 for detecting noise transmitted through the shielding body 40 between the shielding body 40 and the gamma ray detector 30. (e) Invalidate the main detection value output from the detection element when the reference detection value is output from the shielding detector 42. (f) Calculate the dose distribution of the gamma ray along the irradiation axis z direction based on the main detection value that was not invalidated among the main detection values output from each detection element and the positional relationship between each detection element and the transmission opening 41. (g) Calculate the arrival position of the particle beam based on the measured dose distribution.

Although several embodiments of the present invention have been described above, these embodiments are presented as examples and do not limit the scope of the invention. These novel embodiments can be embodied in various other forms. Various omissions, substitutions, modifications, etc. may be made to these embodiments without departing from the gist of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are included in the scope of the invention and its equivalents as set forth in the claims.

1 Particle beam irradiation device, 2 to 7 Particle beam arrival position monitor, 10 Particle beam control device, 11 Particle beam, 13 Prompt gamma ray, 20 Irradiated object, 21 Target, 25 Detection unit, 30 Gamma ray detector, 39 Defective portion, 40 Shielding body, 41 Transmission opening, 42, 60 Shielding detector, 50 Calculation device, 52 Signal analysis unit, 53 Reconstruction calculation unit, 54 Arrival position calculation unit, 55 Display unit, 62 Light emission unit, 64 Light propagation path, 66 Photomultiplier tube, 80 Enlarged defect area, 81 Detection relaxation area, 100, 102, 104, 106, 108, 110 Particle beam therapy system, 200 Dose distribution, S1, S3 Slit, S2, S4 Front slit.

Claims (13)

In a particle beam arrival position monitor for determining the arrival position of a particle beam incident on an irradiated object,
a shield having a transmission opening;
a forward shield disposed in front of the shield and configured to slow down or absorb radiation, or to both slow down and absorb radiation;
a gamma ray detector provided behind the front shield, the gamma ray detector including a plurality of detection elements arranged in an irradiation axis direction of the particle beam;
a shielding detector disposed behind the front shield and in front of the gamma ray detector . 2. The particle beam arrival position monitor according to claim 1 ,
1. A computing device, comprising:
When a reference detection value is output from the shielding detector, a main detection value output from the detection element is invalidated;
A particle beam arrival position monitor comprising: a calculation device that determines the arrival position based on the main detection values that have not been invalidated among the main detection values output from each of the detection elements. 3. The particle beam arrival position monitor according to claim 1,
The front shield is
A particle beam arrival position monitor comprising a plurality of layers formed of different materials. 4. The particle beam arrival position monitor according to claim 3 ,
The front shield is
4. A particle beam arrival position monitor, comprising: a first layer of said plurality of layers being made of a heavy metal; and a second layer being made of a light element. 4. The particle beam arrival position monitor according to claim 3 ,
The front shield is
The method includes the steps of:
2. A particle beam arrival position monitor, comprising: a first layer of said two layers being made of a material containing iron, and a second layer being made of boron. 4. The particle beam arrival position monitor according to claim 3 ,
The front shield is
It includes three layers formed of different materials,
A particle beam arrival position monitor, characterized in that the frontmost layer of the three layers is formed of a material containing iron, the rearmost layer is formed of boron, and the remaining intermediate layers are formed of a material containing hydrogen. 3. The particle beam arrival position monitor according to claim 1,
4. A particle beam arrival position monitor according to claim 1, wherein the shielding detector has a cutout portion at a position corresponding to the transmission opening portion. 3. The particle beam arrival position monitor according to claim 1,
The shield has a plurality of the transmission openings,
the front shield has an enlarged defect area corresponding to an area covered by the plurality of transmission openings;
2. A particle beam arrival position monitor according to claim 1, wherein the shielding detector is formed so that a region corresponding to the enlarged defective region is thinner than other regions. In a particle beam arrival position monitor for determining the arrival position of a particle beam incident on an irradiated object,
a shield having a transmission opening ;
a forward shield disposed in front of the shield and configured to slow down or absorb radiation, or to both slow down and absorb radiation;
a gamma ray detector provided behind the front shield, the gamma ray detector including a plurality of detection elements arranged in an irradiation axis direction of the particle beam;
The transparent opening is a slit formed by cutting the shielding body in the front-rear direction,
A particle beam arrival position monitor characterized in that the edge of the shielding that forms the slit has a V-edge shape with at least one of the front and rear surfaces being inclined so that the thickness becomes thinner toward the slit. 3. The particle beam arrival position monitor according to claim 1,
The transparent opening is a slit formed by cutting the shielding body in the front-rear direction,
A particle beam arrival position monitor characterized in that the edge of the shielding that forms the slit has a V-edge shape with at least one of the front and rear surfaces being inclined so that the thickness becomes thinner toward the slit. 11. The particle beam arrival position monitor according to claim 9 ,
the front shield has a missing region at a position corresponding to the slit,
A particle beam arrival position monitor, comprising: an inclined surface formed on the edge of the defect region, the inclined surface extending continuously from the inclined surface of the edge of the slit. A particle beam arrival position monitor according to claim 1 or 2 ;
a particle beam irradiation device that irradiates the particle beam to the irradiation target;
A particle beam therapy system comprising: 1. A particle beam arrival position monitoring method for determining an arrival position of a particle beam incident on an irradiation target, comprising:
a gamma ray detector including a plurality of detection elements is arranged such that the array direction of the detection elements is aligned with the irradiation axis direction of the particle beam;
A shield having a transparent opening through which gamma rays pass in a direction intersecting the irradiation axis is disposed between the irradiation axis and the gamma ray detector;
A front shield is disposed between the irradiation axis and the shield;
a shielding detector is disposed between the shielding body and the gamma ray detector, the shielding detector detecting noise transmitted through the shielding body;
When a reference detection value is output from the shielding detector, a main detection value output from the detection element is invalidated;
determining a dose distribution of gamma rays along the irradiation axis direction based on the main detection values that have not been invalidated among the main detection values output from each of the detection elements and a positional relationship between each of the detection elements and the transmission opening;
determining the arrival position based on the dose distribution;
A particle beam arrival position monitoring method comprising: