JP2010012056A - Charged particle beam irradiation system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve measuring accuracy of an irradiation field position when measuring the irradiation field position by the measurement with prompt gamma radiation by irradiating an irradiation object with an ion beam. <P>SOLUTION: The charged particle beam irradiation system includes a charged particle beam generating device generating a charged particle beam, an irradiating device emitting the charged particle beam to the irradiation object, a Compton camera detecting the prompt gamma radiation generated from the irradiation object based on the charged particle beam and outputting gamma radiation detecting signals, an absorber arranged between the irradiation object and the Compton camera and absorbing the prompt gamma radiation with energy less than 1 MeV, and an irradiation field confirming device obtaining the irradiation field by the gamma radiation detecting signals from the Compton camera; thereby the position measuring accuracy of the irradiation field can be enhanced. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a particle beam irradiation system, and more particularly to a charged particle beam irradiation system that treats a diseased part such as a tumor by irradiating it with a charged particle beam.

  A method of irradiating a patient with cancer or the like with a charged particle beam such as a proton beam is known. The system used for this irradiation includes a charged particle beam generator, a beam transport system, and a treatment room. The charged particle beam accelerated by the charged particle beam generating apparatus reaches the irradiation apparatus in the treatment room through the beam transport system, and the distribution is expanded by the irradiation apparatus to form an irradiation field suitable for the shape of the affected part in the body of the patient. At that time, as a means for confirming that the irradiation field has been formed at a desired position, a method of photographing by positron tomography (PET) after irradiating annihilation gamma rays from positron emitting nuclides generated in the body by irradiation is used. (Patent Document 1).

  In addition, a method for observing gamma rays (prompt gamma) generated from nuclei such as excited carbon, oxygen, and nitrogen generated in an irradiation field during treatment has been proposed (Non-patent Document 1).

JP-A-9-189769 JP 2001-305233 A “Prompt gamma measurements for locating the dose falloff region in the proton therapy” APPLIED PHYSICS LETTERS 89,183517 (2006) “Review of Scientific Instruments Vol.64 No.8” (August 1993) (p.2055-2122)

  The inventors of the present invention have studied to accurately confirm the irradiation field of the charged particle beam in the charged particle beam irradiation system. As a result, it was found that a method of confirming the position of the irradiation field by measuring prompt gamma rays using a Compton camera as a gamma ray detector was found to be effective.

  Here, when the irradiation field is confirmed by measuring prompt gamma rays, the energy of the gamma rays generated from the irradiation field varies from several hundred keV to several MeV. Of these, gamma rays of less than 1 MeV are likely to scatter after they are generated in the irradiation field and reach the gamma ray detector. On the other hand, since a gamma ray having an energy of 1 MeV or more has a low possibility of being scattered before reaching the gamma ray detector, it is preferable to treat a gamma ray having an energy of 1 MeV or more as an effective detection signal. In such a case, a gamma ray (low energy gamma ray) of several hundreds KeV becomes a background for a gamma ray (high energy gamma ray) having energy of several MeV generated in the irradiation field. The Compton camera has a fixed number of gamma rays that can be detected per unit time, so if many low-energy gamma rays are incident on the gamma-ray detector, the high-energy gamma rays that become signals cannot be detected relatively. Position measurement accuracy decreases.

  An object of the present invention is to provide a charged particle beam irradiation system capable of accurately measuring the position of an irradiation field irradiated with a charged particle beam.

  The features of the present invention that achieve the above object are a charged particle beam generating device that generates a charged particle beam, an irradiation device that emits the charged particle beam to the irradiation target, and a Compton camera that detects prompt gamma rays generated from the irradiation target. And an absorber that absorbs prompt gamma rays of energy less than 1 MeV is disposed between the irradiation object and the Compton camera. Patent Document 2 describes the installation of an absorber as a method for selecting the energy of gamma rays incident on the gamma ray detector. As described above, a feature of the present invention is that a gamma ray detector for prompt gamma rays is used as a gamma ray detector, and an absorber that absorbs prompt gamma rays with energy less than 1 MeV is disposed between the irradiation target and the Compton camera. It is in.

  According to the present invention, low energy gamma rays of several hundred keV are absorbed by the absorber, and high energy gamma rays of 1 MeV or more pass through the absorber and enter the gamma ray detector, thereby improving the ratio of the signal to the background of the gamma rays. The position of the irradiation field can be measured with high accuracy.

  Hereinafter, a charged particle beam irradiation system 10 according to a preferred embodiment of the present invention will be described with reference to FIG.

  A charged particle beam irradiation system 10 according to the present embodiment includes a charged particle beam generator 1, a beam transport system 2, a radiation treatment room 17, and a control system 7.

  The charged particle beam generator 1 includes an ion source (not shown), a linac 3 (previous charged particle beam accelerator), and a synchrotron 4. The synchrotron 4 includes a high-frequency application device 5 and an acceleration device 6. The high-frequency application device 5 includes a high-frequency application electrode (not shown) and a high-frequency application power source (not shown) disposed on the orbit of the synchrotron 4. The high frequency application electrode and the high frequency application power source are connected by a switch (not shown). The acceleration device 6 includes a high-frequency accelerating cavity (not shown) disposed in the orbit of the ion beam and a high-frequency power source (not shown) that applies high-frequency power to the high-frequency accelerating cavity. An exit deflector 11 connects the synchrotron 4 and the beam transport system 2.

  The beam transport system 2 includes a beam path 12, a quadrupole electromagnet (not shown), a deflection electromagnet 14, a deflection electromagnet 15, and a U-shaped deflection electromagnet 16. The beam path 12 includes a quadrupole electromagnet, a deflection electromagnet 14 and a deflection electromagnet 15 from the upstream side in the beam traveling direction. The beam path 12 is connected to an irradiation device 21 installed in the treatment room 17.

  The configuration in the treatment room 17 will be described with reference to FIG. A substantially cylindrical gantry 18 is installed in the treatment room 17. The gantry 18 is provided with a U-shaped deflecting electromagnet 16 that is a part of the beam transport system 2 and an irradiation device 21. Further, the first X-ray imaging apparatus 22 a, the second X-ray imaging apparatus 22 b, and the gamma ray detector 23 are installed inside the rotating drum of the gantry 18. A treatment bed called a couch 24 is installed inside the gantry 18.

  The first X-ray imaging device 22a includes a first X-ray generator 35 (FIG. 3) and a first X-ray detector (in this embodiment, a flat panel display, FPD) installed in the irradiation device 21. 26. The first X-ray imaging device 22a captures a fluoroscopic image in the ion beam irradiation direction (Z-axis direction). The second X-ray imaging device 22b includes a second X-ray generator 27 and a second X-ray detector (in this embodiment, a flat panel display, FPD) 28. The second X-ray imaging device 22b captures a fluoroscopic image in a direction (X-axis direction) perpendicular to the beam axis of the ion beam emitted from the irradiation device 21. In order to confirm that the irradiation object is arranged at a desired position, a fluoroscopic image is taken from two orthogonal directions using the first X-ray imaging device 22a and the second X-ray imaging device 22b. .

  The gamma ray detector 23 detects prompt gamma rays generated from the irradiation field at the time of ion beam irradiation in order to confirm that the irradiation field is formed at a desired position. The gamma ray detector 23 is installed on an axis perpendicular to the beam axis and the rotation axis of the gantry 18 in order to measure the position where the ion beam incident on the irradiation target 25 from the irradiation device 21 has arrived.

  The gantry 18 has a structure that can be rotated by a motor. As the gantry 18 rotates, the U-shaped deflection electromagnet 16 and the irradiation device 21 rotate. By this rotation, the irradiation target 25 can be irradiated from any direction within a plane perpendicular to the rotation axis of the gantry 18. Here we define the coordinate system. The direction that coincides with the direction of the beam incident on the irradiation device 21 in the radial direction of the gantry 18 is defined as the Z axis, the direction of the rotation axis of the gantry 18 is defined as the Y direction, and the direction orthogonal to the Z axis and the Y axis is defined as the X axis. When the gantry 18 rotates, the X-axis and the Z-axis also rotate with the rotation of the gantry, but the Y-axis is a rotation axis and does not change its direction. The first X-ray imaging device 22 a, the second X-ray imaging device 22 b and the gamma ray detector 23 installed in the gantry 18 also rotate with the rotation of the gantry 18. The gamma ray detector 23, the second X-ray generator 27, the FPD 26, and the FPD 28 have a structure that can be stored in a wall surface in the gantry 18 in order to avoid collision with the couch 24 while the gantry 18 is rotating. It has become.

  The structure of the irradiation apparatus 21 is demonstrated using FIG. The irradiation device 21 includes a scanning electromagnet 31, a scanning electromagnet 32, a beam position detector 33, a dose monitor 34, and a first X-ray generator 35. In the charged particle beam irradiation system 10 of the present embodiment, the irradiation device 21 includes two scanning electromagnets 31 and 32, and ion beams are respectively emitted in two directions (X direction and Y direction) in a plane perpendicular to the beam traveling direction. Deflection and change the irradiation position. The beam position detector 33 measures the position of the ion beam and the spread of the ion beam. The dose monitor 34 measures the amount of the irradiated ion beam. In the beam position detector 33, wires are stretched in parallel in the X direction and the Y direction at regular intervals. A high voltage is applied to the wire, and when the ion beam passes, the air in the detector is ionized, and the ionized charged particles are collected on the nearest wire. Measure the amount of charged particles collected. Since the distribution of the beam can be obtained by extending the wire at an interval sufficiently smaller than the spread of the ion beam, the beam position (distribution center of gravity) and the beam width (standard deviation of the distribution) can be calculated. In the dose monitor 34, two electrodes have a parallel plate structure, and a voltage is applied between the electrodes. When the ion beam passes through the dose monitor (dose detector) 34, the air in the detector is ionized by the ion beam, and the ionized charged particles are accumulated on the electrode by the electric field in the detector and read out as a signal. . Here, since the amount of ion beam and the charge accumulated on the electrode are proportional, the amount of ion beam that has passed can be measured. The X-ray generator 35 is arranged on the beam line only during X-ray imaging by moving on the X-ray generator rail 36. There is an irradiation target 37 in the irradiation target 25, and a dose distribution that covers the irradiation target is formed in the irradiation target 25 by irradiating the ion beam. Here, in the case of treatment of cancer or the like, the irradiation target is a person and the irradiation target is a tumor (affected area).

  A control system 7 provided in the particle beam irradiation system of the present embodiment will be described with reference to FIG. The control system 7 includes a database (storage device) 42, a device control system 43, a positioning system 44, and an irradiation field confirmation system 45.

  The device control system 43 includes a central controller 46, an accelerator controller 47, and an irradiation device controller 48. The central controller 46 is connected to an accelerator controller 47 and an irradiation device controller 48. The central controller 46 is connected to the database 42. The central controller 46 receives data from the database 42 and transmits necessary information to the accelerator controller 47 and the irradiation device controller 48 to control it. The accelerator controller 47 is connected to the charged particle beam generator 1, the beam transport system 2, and the gantry 18 and controls them. The irradiation device control unit 48 controls the amount of excitation current flowing through the scanning electromagnets 31 and 32 and processes each monitor signal in the irradiation device 21.

  The positioning system 44 includes a positioning device 49, an X-ray device control unit 50, and a couch control unit 51. The positioning device 49 is connected to the X-ray device control unit 50 and the couch control unit 51. The positioning device 49 is connected to the database 42. The X-ray apparatus control unit 50 is connected to the first X-ray imaging apparatus 22a and the second X-ray imaging apparatus 22b installed in the gantry 18 and controls them. The couch control unit 51 is connected to the couch 24 and controls the couch 24. The positioning device 49 calculates the amount of movement of the couch 24 based on information from the database 42 and data acquired by the first X-ray imaging device 22a and the second X-ray imaging device 22b, and moves to the couch control unit 51. Send quantity.

  The irradiation field confirmation system 45 includes an irradiation field confirmation device 52 and a gamma ray detector control unit 53. The gamma ray detector control unit 53 is connected to the gamma ray detector control unit 53, the irradiation field confirmation device 52, and the irradiation device control unit 48. The irradiation field confirmation device 52 is connected to the database 42. The irradiation field confirmation device 52 receives gamma ray detector data (gamma ray detection signal) sent from the gamma ray detector controller 53 and confirms the irradiation field position based on the gamma ray detection signal and information from the database 42. The gamma ray detector control unit 53 processes movement control of the gamma ray detector 23 and gamma ray detection data output from the gamma ray detector 23. Further, the gamma ray detector control unit 53 receives irradiation spot information and irradiation start / end timing from the irradiation device control unit 48.

  The relationship between the target depth and the ion beam energy in the particle beam irradiation system according to the present embodiment will be described with reference to FIG. FIG. 4 is a diagram illustrating the relationship between the depth of the target and the energy of the ion beam.

  FIG. 4 (a) shows the dose distribution that a single energy ion beam forms in the irradiation object as a function of depth. The peak in FIG. 4A is called a Bragg peak. Since the position of the Bragg peak depends on energy, the irradiation target can be irradiated at the position of the Bragg peak by adjusting the energy of the ion beam according to the depth of the irradiation target. Although the irradiation target has a thickness in the depth direction, the Bragg peak is a sharp peak. Therefore, as shown in FIG. 4B, an ion beam of several energies is irradiated at an appropriate intensity ratio, and the Bragg peak By overlapping the peaks, a uniform high dose region (SOBP) having the same thickness as the irradiation target is formed in the depth direction.

  The relationship between the lateral spread of the irradiation target in the direction perpendicular to the beam axis (the direction of the XY plane) and the ion beam will be described with reference to FIG. The direction perpendicular to the beam axis is called the transverse direction. After reaching the irradiation device 21, the ion beam reaches a desired position in the lateral direction by the two scanning electromagnets 31 and 32 installed vertically. The lateral spread of the ion beam can be approximated by a Gaussian distribution shape. By arranging Gaussian distributions at equal intervals and setting the distance between them to the standard deviation of the Gaussian distribution, the added distribution has a uniform region. The Gaussian distribution dose distribution arranged in this way is called a spot. By scanning the ion beam and arranging a plurality of spots at equal intervals, a uniform dose region can be formed in the lateral direction.

  As described above, a uniform irradiation field can be formed by beam scanning in the horizontal direction by the scanning electromagnet and movement of the Bragg peak in the depth direction by changing the beam energy. A unit of an irradiation field that is irradiated with the same energy and spreads laterally by ion beam scanning with a scanning electromagnet is called a slice.

  Before irradiating the irradiation target 37 with the ion beam, the irradiation planning system 41 determines each parameter required for irradiation. A method for determining parameters by the irradiation planning system 41 will be described.

  The irradiation object 25 is imaged in advance by the CT apparatus 40. The CT apparatus 40 creates image data of the irradiation target 25 based on the acquired imaging data, and transmits the image data to the irradiation planning system 41. The irradiation planning system 41 displays the received image data on the screen of a display device (not shown). When the operator designates an area to be irradiated on the image, the irradiation planning system 41 creates data necessary for irradiation and obtains a dose distribution when irradiation is performed using the data. The irradiation planning system 41 displays the obtained dose distribution on the display device. The area to be irradiated is specified so as to cover the irradiation target 37. The irradiation planning system 41 obtains and determines the installation position of the irradiation target, the gantry angle, the irradiation parameters, and the installation position of the gamma ray detector 23 so that the dose distribution can be formed in the designated area. In addition, the irradiation planning system 41 determines the initial position of the couch 24.

  The irradiation parameters required by the irradiation planning system 41 include ion beam energy, position information in a plane perpendicular to the beam axis (X coordinate, Y coordinate), and a target dose of the ion beam irradiated to each position (FIG. 6). ). That is, the irradiation planning system 41 divides the irradiation target (affected part) 37 into a plurality of slices in the depth direction based on the patient information input by the operator, and determines the required number N of slices. Moreover, the irradiation planning system 41 calculates | requires the energy Ei of the ion beam suitable for irradiation according to the depth of each slice (slice number i). The irradiation planning system 41 further determines the number of irradiation spots Ni, the spot number j, the irradiation position of each spot, and the target dose (target irradiation amount) Dij of each spot according to the shape of each slice. To do. The spot irradiation position (Xij, Yij) is a target irradiation position on a plane perpendicular to the beam traveling direction (depth direction) of the irradiation target.

  The installation position of the gamma ray detector 23 will be described. FIG. 7 shows how spots are formed in the irradiation target. The gamma ray detector 23 measures the arrival position of the ion beam in the beam axis direction (Z direction). The gamma ray detector 23 has the highest detection sensitivity at the center Zc of the detector. For this reason, the irradiation planning system 41 sets the installation position of the gamma ray detector 23 so that the center Zc of the gamma ray detector 23 is positioned on the perpendicular drawn perpendicularly to the beam axis from the calculated depth end of the irradiation field on the beam axis. Is preferably determined. Further, it is preferable to determine the installation position of the gamma ray detector 23 so that the gamma ray detector 23 is as close as possible to the irradiation target (L in FIG. 7 is reduced).

  The irradiation planning system 41 transmits the determined information to the database 42. The database 42 stores data output from the irradiation planning system 41. Of each data registered in the database 42, the data structure of irradiation parameters is shown in FIG. The irradiation parameter has the number of slices N and data of each slice. The data of each slice is composed of slice number i, energy Ei, number of spots Ni, and data of each spot. The spot data further includes a spot number j, an irradiation position (Xij, Yij), and a target dose Dij.

  An operation method of the charged particle beam irradiation system 10 of the present embodiment will be described. Based on the information stored in the database 42, the charged particle beam irradiation system 10 prepares for irradiation as follows.

  The irradiation object 25 is placed on the couch 24. The positioning device 49 acquires the initial position information of the couch 24 from the database 42. The couch control unit 51 that has received the initial position information from the positioning device 49 moves the couch 24 to the initial position. Further, the X-ray device control unit 50 starts moving the first X-ray generator 35 based on a command signal from the positioning device 49. The first X-ray generator 35 moves on the X-ray generator rail 36 and stops on the beam axis. When the second X-ray generation device 27 and the FPDs 26 and 28 are stored in the wall surface in the gantry 18, the X-ray device control unit 50 takes them out from the wall surface of the gantry 18 and arranges them at a predetermined position. After the arrangement at the predetermined position, imaging of the irradiation target 25 is started by the first X-ray imaging device 22a and the second X-ray imaging device 22b. X-rays are generated by the first X-ray generator 35 and the second X-ray generator 27 and emitted to the irradiation target. The FPD 26 and the FPD 28 detect X-rays from each and output an X-ray detection signal. The positioning device 49 creates image data based on the X-ray detection signals acquired by the first X-ray imaging device 22a and the second X-ray imaging device 22b. The positioning device 49 collates the image data obtained from the X-ray detection signal with the image data output from the irradiation planning system 41, calculates the difference between the two pieces of information, and moves the irradiation target to a desired position. Is calculated. The positioning device 49 transmits the calculated value to the couch control unit 51, the couch control unit 51 moves the couch 24 by the specified value, and the irradiation target 25 moves to a desired position.

  The irradiation field confirmation device 52 transmits the installation position information of the gamma ray detector 23 read from the database 42 to the gamma ray detector control unit 53. The gamma ray detector control unit 53 controls the movement of the gamma ray detector 23 based on the received installation position information. The gamma ray detector 23 can move on a rail (not shown) in the X direction and the Z direction, and the amount of movement can be detected by a potentiometer. The gamma ray detector control unit 53 moves the gamma ray detector 23 to a desired position, so that gamma rays generated from the irradiation field at the time of ion beam irradiation can be detected.

  The central controller 46 receives gantry angle information and irradiation parameter information from the database 42. The central controller 46 transmits gantry angle information to the accelerator controller 47. The accelerator controller 47 moves the gantry 18 to a desired gantry angle based on the received gantry angle information. Further, the central controller 46 determines the excitation current amount for exciting the synchrotron 4 and the electromagnet of the beam transport system 2 corresponding to the energy Ei of each slice, and the high frequency applied by the high frequency application device 5 based on the received irradiation parameters. The value and the value of the high frequency applied to the accelerator 6 are referred to from a memory (not shown) of the central controller 46 and transmitted to the accelerator controller 47. Further, the central controller 46 calculates the amount of current for exciting the scanning electromagnets 31 and 32 from the irradiation position and its energy for each spot, and transmits it to the irradiation device controller 48 together with the spot data.

  The ion beam irradiation procedure by the charged particle beam irradiation system 10 will be described with reference to FIG. The central controller 46 outputs the slice number i, spot number j, and energy information Ei to the accelerator controller 47 together with the irradiation start signal. When the first irradiation start is signaled, irradiation is started from slice number i = 1 and spot number j = 1. The accelerator controller 47 that has received the irradiation start signal activates the ion source. Ions (for example, protons (or carbon ions)) generated in the ion source are incident on the linac 3. The linac 3 accelerates and emits ions. The ion beam from the linac 3 is incident on the synchrotron 4. In step 101, the accelerator controller 47 controls the electromagnet of the synchrotron 4 and the acceleration device 6 to accelerate the ion beam incident from the linac 3 to the energy E 1 of slice number 1. That is, the accelerator control unit 47 controls the charged particle beam generator and accelerates the ion beam to a desired energy. This acceleration is performed by applying a high frequency from a high frequency power source to the high frequency acceleration cavity (giving energy to the ion beam that circulates the synchrotron 4 with high frequency power). Further, the accelerator controller 47 controls the excitation amount of the electromagnet of the beam transport system 2 so that the ion beam having the accelerated energy can be transported to the irradiation device 21.

  When the acceleration of the ion beam is completed in step 102 and the beam transport system 2 is ready, the accelerator controller 47 transmits an extraction preparation completion signal to the irradiation device controller 48.

  In step 103, the irradiation device controller 48 that has received the extraction preparation completion signal excites the scanning electromagnet 31 and the scanning electromagnet 32 with the amount of excitation current calculated by the central controller 46 corresponding to slice 1 and spot 1. Further, the irradiation device control unit 48 outputs the slice number and the spot number to the gamma ray detector control unit 53. In this embodiment, the slice number is target irradiation position information of the ion beam in the depth direction of the irradiation target. The spot number is target irradiation position information of the ion beam on a plane perpendicular to the depth direction of the irradiation target.

  In step 104, the irradiation device control unit 48 confirms that the current flowing through the scanning electromagnets 31 and 32 has reached a desired value, and transmits an emission signal to the accelerator control unit 47 and the gamma ray detector 23.

  The accelerator controller 47 that has received the extraction signal in Step 105 starts the extraction of the ion beam from the synchrotron 4 by controlling the high-frequency application device. That is, a switch is connected and a high frequency is applied to the ion beam by the high frequency application device 5. In addition, the gamma ray detector 23 that has received the emission signal starts measuring gamma rays. The ion beam that orbits the synchrotron 4 within the stability limit moves outside the stability limit and is emitted from the synchrotron 4 through the extraction deflector 11. The emitted ion beam passes through the beam transport system 2 and enters the irradiation device 21 and is scanned by the scanning electromagnets 31 and 32. Then, the ion beam passes through the beam position detector 33 and the dose monitor 34 and reaches the irradiation target. To stop. Gamma rays (prompt gamma rays) are emitted from the irradiation target irradiated with the ion beam. The gamma ray detector 23 detects gamma rays and outputs gamma ray detection data to the gamma ray detector control unit 53. The gamma ray detector control unit 53 outputs the received gamma ray detection data to the irradiation field confirmation device 52 together with the slice number and the spot number. That is, each time gamma ray detection data is received, the gamma ray detector control unit 53 assigns a slice number and a spot number corresponding to the received gamma ray detection data, and outputs them to the irradiation field confirmation device 52 as a set of data. The irradiation field confirmation device 52 stores data in a storage device (not shown). The data may be recorded in a storage device (not shown) of the gamma ray detector control unit 53 and output to the irradiation field confirmation device 52 as batch data.

  During emission in step 106, the irradiation device control unit 48 counts the beam dose signal received from the dose monitor 34 to obtain the irradiation dose at each spot, and whether or not the counted irradiation dose has reached the target dose of the spot data. Determine whether.

  When the irradiation device controller 48 determines that the irradiation dose has reached the target dose, it outputs an emission stop signal to the accelerator controller 47 and the gamma ray detector controller 53 in step 107. During the ion beam irradiation, the beam position detector 33 outputs a position detection signal to the irradiation device controller 48. The irradiation device control unit 48 calculates the beam position based on the position detection signal, and confirms that the difference between the calculated beam position and the irradiation position of the spot data is equal to or less than a predetermined threshold value.

  The accelerator controller 47 that has received the extraction stop signal in step 108 controls the high-frequency application device to stop the extraction. The ion beam emission from the synchrotron 4 is stopped by turning off the switch connecting the high-frequency application electrode and the high-frequency application power source to stop the application of the high frequency. Further, the gamma ray detector control unit 53 stops the gamma ray measurement when receiving the emission stop signal. In this way, irradiation of slice number 1 and spot number 1 is completed.

  In step 109, the irradiation apparatus control unit 48 compares the spot number N1 of the slice number 1 with the spot number j. If the spot number j does not reach the number of spots, the operation of step 103 is started to start irradiation of the next spot number j + 1. When the spot number j reaches the number of spots N1, the irradiation device control unit 48 outputs a deceleration signal to the accelerator control unit 47. In step 110, the accelerator control unit 47 controls the electromagnet of the synchrotron 4 to control the inside of the synchrotron 4. The remaining ion beam is decelerated.

  In step 111, the slice number i is compared with the slice number N. If the slice number i does not reach the slice number N, the process proceeds to step 101, and preparation for irradiation of the next slice i + 1 is started. When the slice number i reaches the slice number N, the irradiation is completed.

  As described above, when the ion beam forms an irradiation field, gamma rays are generated from the irradiation field. By detecting the gamma rays and specifying the generation position of the gamma rays, the position of the irradiation field can be confirmed. During irradiation with an ion beam, an emission signal and an extraction stop signal are transmitted together with a slice number and a spot number to the gamma ray detector control unit 53, and when the gamma ray is detected, the spot being irradiated is associated with the detected gamma ray. That is, the irradiation field confirmation device 52 receives a gamma ray detection signal associated with information that can identify an ion beam such as a slice number and a spot number, and based on the slice number and the spot number (identification information of the ion beam), a gamma ray Identify the detection signal.

  The irradiation field confirmation device 52 specifies the irradiation position of the ion beam in the direction perpendicular to the beam traveling direction based on the slice number and the spot number, and specifies the position where the prompt gamma ray is generated from the irradiation position information and the gamma ray detection signal. Find the irradiation field.

  The process of generating gamma rays from the irradiation field will be described. Gamma rays are generated mainly from two processes. One is due to bremsstrahlung and the other is due to nuclear reactions. The bremsstrahlung process is a phenomenon in which charged particles are orbited by the influence of an electromagnetic field to emit gamma rays, and gamma rays with relatively low energy (<1 MeV) are generated. In the nuclear reaction process, the ion beam causes a nuclear reaction with the nuclei that make up the irradiation object. The atom to be irradiated is changed to an excited state by the nuclear reaction, and then gamma rays (prompt gamma rays) are emitted to change to the ground state. The energy of the gamma rays emitted at this time is relatively high at about 1 MeV to 10 MeV. Hereinafter, gamma rays with energy less than 1 MeV are referred to as low energy gamma rays, and gamma rays with energy greater than 1 MeV are referred to as high energy gamma rays.

  Gamma rays react with matter in three processes: photoelectric absorption, Compton scattering, and pair production. The three reactions have different reaction probabilities depending on the energy of gamma rays. When gamma rays of several MeV interact with a substance, the Compton scattering process is the most dominant reaction process. For this reason, a Compton camera using the Compton scattering reaction is preferable as the gamma ray detector 23 in order to measure gamma rays generated from the irradiation field.

  In the present embodiment, the Compton camera shown in FIG. 9 is used as an example of the gamma ray detector 23. FIG. 9 shows a conceptual diagram of the Compton camera.

  The Compton camera 60 includes a first detection unit 61 and a second detection unit 62. Moreover, the absorber 64 is attached between the 1st detection part 61 of a Compton camera, and irradiation object. The gamma ray detection surface 63 is a surface far from the second detection unit 62, and it is assumed that gamma rays are incident from the detection surface 63. The first detector 61 is filled with an inert gas such as xenon, and a high voltage is applied to the detection surface 63. Charge detectors (not shown) are arranged in a lattice pattern near the boundary surface between the first detector 61 and the second detector 62. In the second detector 62, heavy scintillators such as GSO crystals are arranged in a lattice pattern. The absorber 64 has a plate shape of several mm and is in contact with the detection surface 63. The absorber 64 absorbs low energy gamma rays and prevents them from entering the first detection unit 61. The absorber 64 is preferably made of a material having a large atomic number, particularly lead or tungsten. A metal having an atomic number of 72 or more is stable, and a metal that is solid at normal temperature is preferable, and lead having the largest atomic number is particularly preferable. In addition, when lead cannot be used, tungsten that is available at a relatively low cost may be used. The feature of this embodiment is that the absorber 64 is provided between the gamma ray detector (Compton camera) and the irradiation target. The absorber 64 only needs to be disposed between the irradiation target and the gamma ray detector, and does not need to be in contact with the gamma ray detector.

  When gamma rays enter the Compton camera 60, Compton scattering occurs in the first detection unit 61. The solid line in FIG. 9 represents the locus of gamma rays, and the dotted line represents the locus of recoil electrons. The gamma rays are scattered by the first detection unit 61 and lose energy, and after reaching the second detection unit 62, all energy is lost and detected. Recoil electrons scattered by Compton scattering travel while ionizing the gas in the first detection unit 61. Since a voltage is applied in the first detection unit 61, the ionized electrons are directed toward the second detection unit 62 and detected by the charge detector.

  By measuring the time from when the gamma rays are detected by the second detector 62 until the electrons are detected by the charge detector, the position where the inert gas is ionized by recoil electrons can be specified. The position of Compton scattering and the recoil direction of the recoil electrons can be found from the recoil electron trajectory obtained as described above, and the energy of the recoil electrons can be obtained from the total charge detected by the charge detector. Further, the position and energy of the scattered gamma rays are known from the data of the second detection unit 62. By calculating using the law of conservation of momentum and energy conservation, the incident direction, position and energy of incident gamma rays can be determined.

  The incident gamma rays need to reach the second detector 62 after being Compton scattered by the first detector 61. When the light enters from the end of the detection surface 63, there is a high possibility that the light will come out of the detector before reaching the second detection unit 62, and the detection surface center has the highest detection efficiency. For the same reason as for the incident angle, the detection efficiency is best when it is incident perpendicularly to the detection surface 63.

  Note that a semiconductor detector such as Si or CdTe can be used for both the first detector 61 and the second detector 62. By using a semiconductor detector for the first detector 61, the reaction probability of the incident gamma ray at the first detector 61 can be improved. Further, the detection position accuracy can be improved by using a semiconductor detector for the second detector 62.

  Gamma rays generated from the irradiation field pass through the irradiation object and the atmosphere, and reach the absorber 64.

  Since gamma rays generated from the irradiation field are generated by bremsstrahlung and nuclear reaction, the energy distribution of the gamma rays extends from a low energy of 100 keV or less to a high energy of about 10 MeV. The probability that gamma rays react with matter increases with lower energy. For this reason, low-energy gamma rays are likely to cause a Compton scattering reaction or the like in the irradiation target and the like to be bent after they are generated in the irradiation field and reach the gamma ray detector. Even if a gamma ray whose trajectory is bent is detected by a gamma ray detector, information on the correct generation position of the gamma ray cannot be obtained from the detected data. As described above, the low energy gamma ray becomes noise to confirm the irradiation field. Therefore, it is preferable to reconstruct the irradiation field using high energy gamma rays.

  Here, the reaction between the absorber and gamma rays will be described. By installing the absorber 64, most of the low energy gamma rays are absorbed by the absorber and do not reach the gamma ray detector 23. On the other hand, most of the high-energy gamma rays reach the gamma ray detector without being absorbed by the absorber. Low energy gamma rays have a high probability of causing a photoelectric absorption reaction with a substance, and high energy gamma rays have a high probability of causing Compton scattering. In the photoelectric absorption reaction, a substance having a larger atomic number is more likely to react, and Compton scattering has a property that the reaction probability is constant regardless of the kind of the substance. From the above, this effect is further increased by adopting a material having a large atomic number as the material.

  Originally, it is preferable that high energy gamma rays do not react with the absorber, but react at a certain rate. By using a material having a large atomic number as an absorber, more low energy gamma rays can be absorbed with less loss of high energy gamma rays.

  As described above, by installing the absorber, it is possible to prevent many of the low energy gamma rays that are noise components from entering the gamma ray detector. The effect obtained by this will be described.

  When the gamma ray detector is the Compton camera, the energy of incident gamma rays can be measured. After data collection, it is possible to calculate the position of the irradiation field by removing the information of low energy gamma rays. However, when a Compton scattering reaction occurs in the Compton camera, ionized electrons travel from the recoil electron trajectory toward the charge detector. The gamma rays incident on the detector while the ionized electrons move to the charge detector cannot be detected. For example, if a low energy gamma ray reacts in the gamma ray detector and a high energy gamma ray is incident immediately thereafter, the high energy gamma ray cannot be detected. Thus, the incidence of low energy gamma rays on the gamma ray detector reduces the number of high energy gamma rays that are signals. By providing an absorber to prevent low energy gamma rays from entering the gamma ray detector, it is possible to prevent a decrease in the number of detected gamma rays that are effective signals. Irradiation field measurement accuracy can be increased by increasing the number of signals.

  The thickness of the absorber varies depending on the material. Further, the ratio of the low energy gamma rays and the high energy gamma rays emitted from the irradiation target depends on the distance from the irradiation field to the irradiation target surface. Therefore, by referring to the irradiation plan and setting the thickness of the absorber according to the distance from the irradiation field to the irradiation target surface, the ratio of signal to noise can be improved and the position measurement accuracy of the irradiation field can be improved. can do. For this reason, it is preferable that the absorber has a structure in which the thickness can be changed by overlapping the plate-like structures. The absorber 64 is preferably arranged so as to cover the entire detection surface 63 (first detection unit 61) of the gamma ray detector 23. With such a configuration, it is possible to further prevent low energy gamma rays that become noise from entering the gamma ray detector.

  In addition, when using a gamma ray detector that cannot measure energy, the ratio of noise to the signal collected by the gamma ray detector can be reduced. The detection accuracy of the irradiation field position can be increased by reducing the noise ratio.

  A method for reconstructing the irradiation field from the gamma ray detection data detected by the gamma ray detector as described above will be described.

  During irradiation with the ion beam, the irradiation apparatus control unit 48 transmits the slice number i and the spot number j to the gamma ray detector control unit 53. Further, the irradiation device control unit 48 transmits the emission signal and the emission stop signal to the gamma ray detector control unit 53. When receiving the emission signal, the gamma ray detector control unit 53 starts detecting gamma rays and collects gamma ray detection data. The gamma ray detection data obtained here is output to the irradiation field confirmation device 52 together with the slice number i and the spot number j. When the extraction stop signal is received, the data collection is stopped.

  The irradiation field confirmation device 52 obtains the incident position, incident direction, and energy of the gamma ray to the gamma ray detector 23 based on the gamma ray detection data. In the present embodiment, gamma ray detection data having energy of 1 MeV to 10 MeV among the gamma rays obtained by the irradiation field confirmation device 52 is regarded as effective data. The irradiation field confirmation device 52 reads the irradiation position (Xij, Yij) corresponding to the slice number i and the spot number j from the database 42. FIG. 10 shows how spots are formed in the irradiation target. The Z coordinate of the incident position of the gamma ray is Z1, the angle of the incident angle projected onto the XZ plane is θ, and the distance between the gamma ray detector and the irradiation position is L. L is calculated from the position of the gamma ray detector 23 and Xij. By calculating Zij = Z1 + Lcos θ, the Z coordinate of the generation position of the gamma ray can be specified. As described above, the generation position of gamma rays is set to (Xij, Yij, Zij).

  Here, the irradiation position recorded in the database 42 indicates the center position of the spot, but the spot has a finite extent. The position of the gamma ray generation position in the X direction cannot be specified by the extent of the spot spread, but the spot spread is sufficiently small compared to the distance between the gamma ray detector and the irradiation position, so gamma rays are generated from the spot center position. It can be calculated assuming that In this manner, the irradiation field confirmation device 52 obtains the generation position (Xij, Yij, Zij) in the irradiation target where the gamma rays are generated based on the acquired gamma ray detection data, and creates the distribution to thereby generate the irradiation field. The shape can be reconstructed. The irradiation field confirmation device 52 obtains the irradiation field by the ion beam emitted from the irradiation device 21 based on the gamma ray detection signal from the gamma ray detector 23 (Compton camera).

  The irradiation field confirmation device 52 receives from the database 42 the CT image photographed by the CT device 40 before irradiation and the dose distribution calculated by the irradiation planning system 41. Thereafter, the dose distribution obtained from the gamma ray detection data and the dose distribution calculated by the irradiation planning system 41 are displayed on a CT image of a display device (not shown). An operator or the like can confirm the difference between the planned dose distribution and the dose distribution shape formed by actual irradiation. The data generated by the irradiation field confirmation device 52 is not limited to the dose distribution shape, and may be data relating to the irradiation position at the deepest portion in the depth direction (beam traveling direction). In this case, the irradiation position of the deepest part obtained based on the gamma ray detection data and the irradiation position calculated by the irradiation planning system 41 are displayed on the CT image of the display device. Thereby, the operator etc. can confirm the difference between the dose position planned in advance and the irradiation deepest part by actual irradiation.

  According to this embodiment, since the absorber 64 is disposed between the first detection unit 61 of the gamma ray detector and the irradiation target, it is possible to prevent low energy gamma rays of less than 1 MeV from entering the gamma ray detector 23. Further, high energy gamma rays of 1 MeV or more effective for confirming the irradiation field pass through the absorber and enter the gamma ray detector 23. For this reason, it is possible to detect more high-energy gamma rays of 1 MeV or more that are effective in confirming the irradiation field, and to accurately determine the position of the irradiation field. Further, the ratio of the effective signal to the background (noise) of gamma rays can be increased, and the position of the irradiation field can be measured with high accuracy.

  In the present embodiment, since the absorber 64 is disposed so as to contact the gamma ray detector, the absorber can be fixed to the gamma ray detector and can be easily installed. According to the present embodiment, since the thickness of the absorber 64 can be made variable, an absorber suitable for each irradiation target can be installed, and the ratio of noise to an effective signal can be further reduced. Thereby, a more accurate irradiation field can be measured.

  In addition, although the present Example described about the case where an irradiation field was formed by the spot scanning method, it can apply similarly also about another irradiation field formation method. Other irradiation field forming methods are described in Non-Patent Document 2.

  In the present embodiment, the case where the synchrotron is used as the charged particle beam generator has been described. However, the present invention can be similarly applied to the cyclotron.

It is a figure showing the whole schematic structure of the charged particle beam irradiation system which is one suitable embodiment of this invention. It is a figure showing the conceptual structure which shows the structure of the treatment room with which the charged particle beam irradiation system shown in FIG. 1 is equipped. 3 is a cross-sectional view illustrating a configuration of an irradiation apparatus provided in the charged particle beam irradiation system illustrated in FIG. 2. It is a figure which shows dose distribution of the depth direction obtained when the irradiation object shown in FIG. 2 is irradiated with an ion beam. It is a figure which shows the dose distribution of the horizontal direction obtained when the irradiation object shown in FIG. 2 is irradiated with an ion beam. It is a conceptual diagram which shows the irradiation parameter memorize | stored in the database shown in FIG. It is the conceptual diagram which showed the mode of the spot obtained when the irradiation object shown in FIG. 2 was irradiated with an ion beam. It is the block block diagram which showed the procedure to irradiate. 3 is a cross-sectional view showing a configuration of the gamma ray detector shown in FIG. 2. It is the conceptual diagram which showed the mode of the spot obtained when the irradiation object shown in FIG. 2 was irradiated with an ion beam.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Charged particle beam generator 2 Beam transport system 3 Linac 4 Synchrotron 5 High-frequency application device 6 Accelerator 7 Control system 11 Deflector for extraction 12 Beam path 14 and 15 Deflection magnet 16 U-shaped deflection electromagnet 17 Treatment room 18 Gantry 21 Irradiation apparatus,
22a First X-ray imaging device 22b Second X-ray imaging device 23 Gamma ray detector 24 Couch 25 Irradiation target 26, 28 FPD
27, 35 X-ray generator 31, 32 Scanning magnet 33 Beam position detector 34 Dose monitor 36 X-ray generator rail 37 Irradiation target 40 CT device 41 Irradiation planning system 42 Database 43 Equipment control system 44 Positioning system 45 Irradiation field confirmation System 46 Central controller 47 Accelerator controller 48 Irradiator controller 49 Positioning device 50 X-ray apparatus controller 51 Couch controller 52 Irradiation field confirmation device 53 Gamma ray detector controller 60 Compton camera 61 First detector 62 Second detection Part 63 detection surface 64 absorber

Claims (6)

A charged particle beam generator for generating a charged particle beam;
An irradiation device for emitting the charged particle beam to an irradiation target;
A Compton camera for detecting prompt gamma rays generated from the irradiation object;
An absorber that is disposed between the irradiation object and the Compton camera and absorbs the prompt gamma rays having an energy of less than 1 MeV;
A charged particle beam irradiation system comprising: an irradiation field confirmation device that obtains an irradiation field shape by irradiation of the charged particle beam based on a gamma ray detection signal from the Compton camera.   The charged particle beam irradiation system according to claim 1, wherein the absorber is made of lead or tungsten.   The charged particle beam irradiation system according to claim 1, wherein the absorber has a plate shape.   The charged particle beam irradiation system according to claim 3, wherein the plate-like absorber can cover at least a detection surface of the Compton camera.   The charged particle beam irradiation system according to claim 1, wherein the absorber is disposed in contact with a detection surface of the Compton camera.   The charged particle beam irradiation system according to claim 3, wherein the absorber has a variable thickness.

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