JP2009183468A - Radiotherapeutic dose distribution measuring device and radiotherapeutic dose distribution measurement program - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To give treatment accurately and safely by preventing an operator from performing under radiation to a treated part or excessive radiation to a normal tissue in a radiation therapy. <P>SOLUTION: A radiation system 2 radiates therapeutic X-ray beams to a subject. A scattering radiation detection system 3 detects scattering radiation occurred from within the subject on the basis of the therapeutic X-ray beams and generates scattering radiation data. A reconstruction processing part 503 reconstructs scattering radiation volume data showing a three-dimensional distribution of the scattering radiation occurrence density within the subject on the basis of the detected scattering radiation data. A conversion processing part 505 converts the scattering volume data into absorption of radiation data showing a three-dimensional distribution of an amount of radiation absorbed by using density corresponding to a composition of each voxel tissue. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a radiotherapy dose distribution measuring apparatus and a radiotherapy dose distribution measuring program having a function of providing information for assisting treatment to an operator during radiotherapy.

  In radiation therapy represented by external X-ray irradiation treatment, an irradiation plan (from which direction and how much dose is irradiated to the treatment site) is planned on the patient image before treatment, and based on this The patient is irradiated. However, at present there is no means to confirm whether the patient is actually receiving the position and dose as planned, and even if there is under-irradiation to the treatment site or over-exposure to normal tissue, it will not be noticed. Is the current situation. It may be confirmed that irradiation can be performed as planned using a phantom and an X-ray detector before irradiation, but unlike a phantom that can be easily carried and adjusted freely, the patient can be placed on the bed. However, it is difficult to place them in the position as planned for irradiation, and these pre-irradiation confirmations do not completely guarantee the irradiation as planned for the patient.

In addition, as a well-known document relevant to this application, there exist the following, for example.
JP, 5-146426, A The art which this patent document discloses detects a scattered X-ray of an X-ray subject, and obtains a tomographic image of a subject. It is characterized by reconstructing and obtaining a three-dimensional scattered radiation image of a subject by scanning with a pencil beam. In other words, this technology assumes only a pencil-shaped beam, and does not obtain a scattered image (spatial distribution of the dose of the treatment beam) of the region through which the beam with a finite width used in X-ray therapy has passed. . In addition, since scattering of high-energy treatment beams (several MeV) within a subject is superior to forward scattering, it is difficult to distinguish scattered and transmitted rays if a detector is placed in the incident X-ray direction, and detection of scattered rays is difficult. Correction processing is essential.

  With increasing social interest in medical malpractice, over-irradiation of patients has also been reported in radiotherapy. It is expected that it will become more and more important in the future to record the “facts” of the medical practice performed, such as how much dose the patient has received. Recently, in the field of external X-ray irradiation therapy, a method of irradiating in sync with and following the movement of the tumor in the patient due to breathing, a method of collimating and irradiating a therapeutic X-ray beam according to the tumor shape, etc. Attempts have been made to irradiate the lesion more precisely. These precision irradiations are intended to concentrate the dose on the lesion. For this reason, in the unlikely event that the therapeutic X-ray beam deviates from the irradiation target, the normal tissue is seriously damaged. For this reason, it is important to confirm whether or not the irradiation is performed as planned as the irradiation becomes more precise.

  The present invention has been made in view of the above circumstances, and the object of the present invention is to prevent the operator from under-irradiating the treatment site and over-irradiating the normal tissue in the radiation treatment, and accurately and safely treat the treatment. It is another object of the present invention to provide a radiotherapy dose distribution measuring apparatus and a radiotherapy dose distribution measuring program capable of performing the above.

  In order to achieve the above object, the present invention takes the following measures.

  The radiation therapy dose distribution measuring apparatus according to the present invention detects an irradiation means for irradiating a subject with a therapeutic radiation beam, and detects scattered radiation from the subject generated based on the therapeutic radiation beam. Detection means for generating scattered radiation data; and image reconstruction means for reconstructing scattered radiation volume data indicating a three-dimensional distribution of scattered radiation generation density in the subject based on the detected scattered radiation data; Conversion means for converting the scattered radiation volume data into absorbed dose volume data indicating a three-dimensional distribution of the absorbed radiation dose using a density corresponding to the composition of each voxel tissue, and using the absorbed dose volume data And image generating means for generating an absorbed dose image in the subject, and display means for displaying the absorbed dose image.

  The radiotherapy support program according to the present invention is a detection that causes a computer to generate scattered radiation data by detecting scattered radiation from within the subject that is generated based on a therapeutic radiation beam applied to the subject. And an image reconstruction function for reconstructing scattered radiation volume data indicating a three-dimensional distribution of scattered radiation generation density in the subject based on the scattered radiation data for the detected plurality of positions, A conversion function for converting scattered radiation volume data into absorbed dose volume data indicating a three-dimensional distribution of the radiation dose absorbed using a density corresponding to the composition of each voxel tissue, and using the absorbed dose volume data, An image generation function for generating an absorbed dose image in a subject and a display function for displaying the absorbed dose image are executed. That.

  As described above, according to the present invention, in radiation irradiation treatment, a radiotherapy dose distribution measuring apparatus that prevents an operator from under-irradiating a treatment site and over-irradiating normal tissue, and enables accurate and safe treatment. And a dose distribution measurement program for radiotherapy can be provided.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following description, components having substantially the same function and configuration are denoted by the same reference numerals, and redundant description will be given only when necessary.

[Principle and method]
The radiotherapy system according to the present embodiment measures the scattered radiation from the subject based on the radiation irradiated to the subject, and based on this, what part of the subject is irradiated with how much dose? The information which objectively shows is acquired. The principle and method are as follows.

  FIG. 1 is a diagram for explaining the principle and method of measuring scattered radiation from a subject based on therapeutic radiation of the present radiation treatment system.

  The therapeutic effect of external X-ray irradiation is mainly brought about by X-ray scattering that occurs in the patient. That is, when the therapeutic X-ray beam is scattered by electrons in the patient, the electrons that have received energy fly through the tissue and then stop. At this time, the electrons radicalize the molecules in the tissue until they stop, damaging the DNA in the cells. Cells that are damaged and cannot be repaired eventually die. This is the therapeutic effect of X-ray irradiation. The more recoiled electrons are generated, the higher the probability that the cells constituting the tissue will die, so the therapeutic effect is proportional to the number of times the scattering reaction occurs.

  From the above, knowing the number of scatterings that have occurred in the tissue, it is possible to know the therapeutic effect (= how much the tissue has been damaged). The number of scattering that has occurred can be determined by measuring the number of scattered rays. Since most of the scattered X-rays are transferred to the outside of the patient after the traveling direction is changed by electrons, they can be measured by an X-ray detector installed outside the patient.

  In the radiation therapy system according to the first example of the present embodiment, a detector having a collimator is installed at a position that makes a specific angle with respect to the treatment X-ray beam, and only scattered rays that come in that direction are selected. Detect. Since it is theoretically possible to know how much X-rays are scattered at which angle by Compton scattering, if the scattered radiation at one angle can be detected, the number of scattered rays at other angles can be estimated. Furthermore, in order to obtain a three-dimensional distribution of the locations where scattering occurs in the patient, the detector is rotated during irradiation, and the scattered radiation is measured from all directions (see, for example, FIG. 4). Thereafter, reconstruction processing is performed, and the generation distribution of scattered radiation inside the subject is imaged three-dimensionally.

  In addition, in the radiotherapy system according to the second example of the present embodiment, a detector having a collimator is installed at a position that forms a predetermined angle (scattering angle) with respect to the treatment X-ray beam and comes in that direction. The detection X-ray beam and the detection surface are detected while maintaining the angle formed by the axis of the therapeutic X-ray beam irradiated from the irradiation unit and the detection surface of the detector. By scanning while moving the 3D region in the subject. Using the obtained three-dimensional scattered radiation data for a given scattering angle, the scattered radiation volume data is reconstructed, and the scattered radiation volume data is converted into absorbed dose volume data indicating a three-dimensional distribution of absorbed radiation. Then, an absorbed dose image is generated.

[Constitution]
FIG. 2 shows a block configuration diagram of the radiation therapy system 1 according to the present embodiment. As shown in the figure, the radiation treatment system 1 includes a radiation irradiation system 2, a scattered radiation detection system 3, a data acquisition control unit 4, a data processing system 5, a display unit 6, a storage unit 7, an operation unit 8, and a network I. / F9. The radiation irradiation system 2 and the scattered radiation detection system 3 are installed on a gantry, and can be arranged at arbitrary positions with respect to the subject by moving and rotating the gantry. Moreover, the data acquisition control part 4, the data processing system 5, the display part 6, the memory | storage part 7, the operation part 8, and network I / F9 are installed in the main body (housing | casing) of the radiotherapy system 1, for example.

[Radiation irradiation system]
The radiation irradiation system 2 includes a power supply unit 201, an irradiation unit 203, a timing control unit 205, and a gantry control unit 207.

  The power supply unit 201 supplies power to the irradiation unit 203 according to the control from the data acquisition control unit 4.

  The irradiation unit 203 is a radiation irradiation apparatus having a mechanism such as a linear accelerator (linac). In the irradiation unit 203, thermoelectrons emitted from the cathode are accelerated to several hundred keV by an electron gun provided at one end of the accelerating tube. Next, the microwave generated by the klystron is guided to the accelerating tube using a waveguide, where the thermoelectrons are accelerated until they reach several MeV energy. The accelerated thermoelectrons are changed in direction by the magnet and collide with the transmission target. At this time, X-rays with energy of several MeV are generated by bremsstrahlung. The irradiation unit 203 shapes the X-rays into a predetermined shape (for example, a conical shape or a thin planar shape) using a collimator, and irradiates the three-dimensional region of the subject placed on the bed.

  The timing control unit 205 controls the power supply unit 201 so that power is supplied to the irradiation unit 203 at a predetermined timing according to the control from the data acquisition control unit 4.

  The gantry control unit 207 controls the movement position / rotation position of the gantry in accordance with, for example, a control instruction from the operation unit 8 or the data acquisition control unit 4.

[Scattered radiation detection system]
The scattered radiation detection system 3 includes a detector 301, a collimator 303, a movement mechanism unit 305, and a position detection unit 307.

  The detector 301 is a semiconductor detector capable of detecting X-rays of several hundred keV, an imaging plate, or the like, and detects scattered rays from the subject based on radiation irradiated to the subject. A preferable size of the detector, an arrangement angle with respect to the irradiation beam axis, the number of pixels, and the like will be described later.

  The collimator 303 is a diaphragm device for selectively detecting only scattered rays coming in a specific direction.

  The moving mechanism unit 305 has an angle of the detection surface of the detector 301 with respect to the irradiation beam axis of the irradiation unit 203 (that is, an angle between the irradiation beam axis and the normal of the detection surface of the detector 301), and a radiation beam axis as a center. It is a moving mechanism unit for moving the position and angle of the detector 301 in order to control the rotation angle of the detector 301, the distance between the subject and the detection surface of the detector 301, and the like.

  The position detection unit 307 is an encoder for detecting the position of the detector 301.

[Data acquisition control unit]
The data acquisition control unit 4 performs comprehensive control relating to scattered radiation measurement during radiation therapy. For example, the data acquisition control unit 4 obtains a signal from the timing control unit 205 of the radiation irradiation system 2 and transmits a scattered radiation measurement start trigger or a detection data transmission trigger to the scattered radiation detection system 3. The radiation therapy system 1 is controlled statically or dynamically for irradiation, scattered radiation measurement, data processing, image display, network communication, and the like. In addition, the data acquisition control unit 4 optimizes the scan time according to the irradiation time of each irradiation based on the treatment plan received from the radiation treatment planning apparatus via the network as necessary.

  The data processing system 5 includes a correction processing unit 501, a reconstruction processing unit 503, a conversion processing unit 505, and an image processing unit 507.

  The correction processing unit 501 performs data calibration processing, correction processing for removing noise, and the like as necessary. The details of the correction process executed by the correction process 501 will be described in detail later.

  The reconstruction processing unit 503 executes image reconstruction processing using the scattered radiation image data detected in the scattered radiation detection system 3 and position information indicating the position where each scattered radiation image data is detected, and the number of scattering events ( Scattered ray volume data showing a three-dimensional distribution of the number of scattering occurrences) is acquired. As the reconstruction method, for example, a computerized tomography (CT) reconstruction method is used if the direction of the collimator is orthogonal to the scan axis, and if it is not orthogonal, a tomographic reconstruction method is used. .

  The conversion processing unit 505 converts the three-dimensional image data obtained by the image reconstruction process into absorbed dose volume data indicating a three-dimensional distribution of absorbed radiation dose (absorbed dose).

  The image processing unit 507 generates absorbed dose image data indicating the distribution of the absorbed radiation dose (absorbed dose) regarding a predetermined part of the subject using the absorbed dose volume data or the like. When the absorbed dose image is displayed by fusion, the image processing unit 507 performs image composition processing using the absorbed dose volume data or the like.

[Display unit, storage unit, operation unit, network I / F]
The display unit 6 displays the absorbed dose image in a predetermined form using the absorbed dose image data. For example, the display unit 6 displays the absorbed dose image by fusion with a plan image or an image obtained immediately before or during irradiation as necessary.

  The storage unit 7 has a predetermined scan sequence, correction processing, image reconstruction processing, conversion processing, and display processing for acquiring (scanning) scattered radiation data while rotating the detector 301 around the axis of the radiation beam to be irradiated. Control program for executing the above, a dedicated program for displaying and editing a treatment plan in the system, scattered radiation volume data, absorbed dose volume data, absorbed dose acquired by the radiation treatment dose distribution measuring apparatus 1 Image data, image data acquired by other modalities such as an X-ray computed tomography apparatus, and the like are stored. The storage unit 7 stores a CT value-density conversion table 701. The CT value-density conversion table 701 is used for conversion processing, and details of the contents will be described later. Data stored in the storage unit 7 can also be transferred to an external device via the network I / F 90.

  The operation unit 8 includes various switches, buttons, a trackball 13s, a mouse 13c, and a keyboard for incorporating various instructions, conditions, a region of interest (ROI) setting instruction, various image quality condition setting instructions, and the like from the operator into the apparatus main body 11. 13d and the like.

  The network I / F 9 transfers the absorbed dose image data obtained by the radiation treatment dose distribution measuring device 1 to another device via the network, and also, for example, the treatment plan created in the radiation treatment planning device. Obtain via the network.

(Method of generating absorbed dose image data)
(First embodiment)
Next, a method for generating absorbed dose image data using the radiotherapy system 1 according to the first embodiment will be described. In the radiotherapy system according to the present embodiment, a detector having a collimator is installed at a position that forms a specific angle with respect to the therapeutic X-ray beam, and only scattered rays that come in that direction are selectively detected. Furthermore, in order to obtain a three-dimensional distribution of the locations where scattering occurs in the patient, the detector is rotated during irradiation, and the scattered radiation is measured from all directions (see, for example, FIG. 4). Thereafter, reconstruction processing is performed, and the generation distribution of scattered radiation inside the subject is imaged three-dimensionally.

  FIG. 3 is a flowchart showing the flow of processing at the time of radiation therapy including generation processing of absorbed dose image data according to the present embodiment. Hereinafter, the processing content of each step will be described.

[Subject placement, etc .: Step S1a]
First, the data acquisition control unit 4 acquires treatment plan information related to the subject via a network, for example, and displays it on the display unit 6. The surgeon places the subject on the bed according to the displayed treatment plan, sets the radiation irradiation time via the operation unit 8, and sets the number of times of performing scattered radiation measurement and the angle to be measured in one rotation. For example, a scan sequence is selected (step S1a). Note that the setting of the radiation irradiation time and the like may be automatically performed based on the acquired treatment plan information.

[Radiation irradiation / Acquisition of scattered radiation image data in multiple directions: Step S2a]
FIG. 4 is a view showing a measurement form of scattered radiation of the radiation therapy system 1. As shown in the figure, the radiation irradiation system 2 generates therapeutic radiation for irradiating a subject with a three-dimensional region at a predetermined timing. Further, the scattered radiation detection system 3 detects the scattered radiation coming out of the subject based on the irradiation radiation at a plurality of rotation angles around the axis of the radiation beam irradiated (step S2a). For example, when irradiation can be performed for 3 minutes from one direction, data in 18 directions is collected for 10 seconds per direction. At this time, the 18 directions are preferably equiangular intervals with the beam axis as the center. The count number of scattered rays detected by the detector 301 in each direction and the position information of the detector 301 at the time of detecting scattered rays measured by the position detector 307 are transmitted to the data processing system 5.

  In the present embodiment, the detector 301 is arranged such that the backscattered rays having a scattering angle θ within a range of 120 ° ≦ θ ≦ 165 ° (for example, 155 °) are detected. It is assumed that an arrangement angle 301 is set.

In the above example, for example, when 2 Gy irradiation is performed from three directions, the number of counts per direction is 1.24 × 10 ⁵ × 1 / 3≈4 × 10 ⁴ [counts / cm ² ]. If it is irradiated for 180 seconds per direction and measured for 10 seconds, 4 × 10 ⁴ × 10/180 = 2 × 10 ³ [counts / cm ² ] is obtained, but there is no problem in the S / N ratio.

  In addition, the detection of scattered radiation requires at least two directions, but it is preferable to detect in as many directions as possible in practice. Moreover, it is preferable that the detection positions are arranged at equiangular intervals around the axis of the irradiation beam.

[Preprocessing (correction processing, etc.): Step S3a]
In the collected data, only X-rays scattered in the detector installation angle direction are counted. In reality, however, X-rays are scattered in all directions. The correction processing unit 501 of the data processing system 5 corrects the count value of the detector, and acquires the number of scattering in all directions according to a predetermined calculation threshold (step S3a).

[Image reconstruction processing: Step S4a]
The reconstruction processing unit 503 of the data processing system 5 executes image reconstruction processing using multi-directional projection data and acquires scattered radiation volume data (step S4a). At this time, when the rotation axis of the detector 301 and the direction of the collimator are orthogonal and an image is captured in an angle range of 180 degrees (+ α) or more, a CT reconstruction method may be used. A tomographic reconstruction method is used. As a tomographic technique, for example, a filtered backprojection method in which a back projection process is performed after a filter process is applied to a projection image is used. As a filter configuration method, a classic Shepp-Logan filter or a filter disclosed in Japanese Patent Application Nos. 2006-284325 and 2007-269447 is used. In particular, if a method described in Japanese Patent Application Nos. 2006-284325 and 2007-269447 is used, a scatter source distribution image with a clear physical meaning can be generated.

  An image obtained by subjecting the detector image to filter processing and performing back projection has a scattered radiation generation density per unit volume (the number of scatterings per unit volume). Three-dimensional distribution of scattered radiation generation density in the vicinity where therapeutic radiation passes through the subject through all steps of the reconstruction process (various correction processes, filter processes, and back projection processes) (scattered volume data) Can be obtained.

[Conversion processing: Step S5a]
The conversion processing unit 505 of the data processing system 5 converts the number n of scattering per unit volume calculated for each voxel into an absorbed dose, thereby absorbing the radiation dose absorbed by the scattered radiation volume data (absorbed dose). ) Is converted into absorbed dose volume data indicating a three-dimensional distribution (step S5a). Details of this processing will be described later.

[Generation of Absorbed Dose Image Data / Display of Image Data: Steps S6a, S7a]
The image processing unit 507 generates absorbed dose image data indicating the distribution of absorbed radiation dose (absorbed dose) related to a predetermined part of the subject CT image, and synthesizes it with, for example, a CT image (step S6a). The display unit 6 displays the absorbed dose image in a predetermined form (step S7a).

(Second embodiment)
Next, a method for generating absorbed dose image data using the radiation therapy system 1 according to the second embodiment will be described. In the radiotherapy system according to the present embodiment, a detector equipped with a collimator is installed at a position that forms a predetermined angle (scattering angle) with respect to the therapeutic X-ray beam, and only scattered rays that come in that direction are selectively selected. By detecting and performing this detection while moving the therapeutic X-ray beam and the detection surface while maintaining the angle formed by the axis of the therapeutic X-ray beam irradiated from the irradiation unit and the detection surface of the detector Scan a 3D area within the subject. Using the obtained three-dimensional scattered radiation data for a given scattering angle, the scattered radiation volume data is reconstructed, and the scattered radiation volume data is converted into absorbed dose volume data indicating a three-dimensional distribution of absorbed radiation. Then, an absorbed dose image is generated.

  FIG. 5 is a flowchart showing a flow of processing at the time of radiation therapy including processing for generating absorbed dose image data according to the present embodiment. Hereinafter, the processing content of each step will be described.

[Subject placement, etc .: Step S1b]
First, similar to the first embodiment, the placement of the subject is executed (step S1b).

[Radiation irradiation (acquired scattered radiation data): Step S2b]
FIG. 6 is a diagram showing an example of a measurement form of scattered radiation of the radiation therapy system 1. As shown in the figure, the radiation irradiation system 2 irradiates a subject with an X-ray beam B2 shaped into a thin flat surface at a predetermined timing, and the scattered radiation detection system 3 is based on the irradiation radiation. Scattered rays having a predetermined scattering angle coming out of the subject are detected. In addition, the data acquisition control unit 4 moves the excitation cross section of the X-ray beam B2 while maintaining the angle formed by the axis of the therapeutic X-ray beam B2 irradiated from the irradiation unit 203 and the line-of-sight direction of the detector 301. Then, the gantry control unit 207 or the movement mechanism unit 305 is controlled so as to scan the three-dimensional region in the subject (step S2b). By scanning the three-dimensional region using the therapeutic X-ray beam B2, three-dimensional scattered radiation data including a plurality of two-dimensional scattered radiation data corresponding to the plane of the X-ray beam B2 is acquired.

  FIG. 6 is an example of a measurement form of scattered radiation. Therefore, the scattered radiation measurement mode according to the present embodiment is not limited to this example. For example, as shown in FIG. 7, the axis of the therapeutic radiation beam is maintained while maintaining the detection surface of the detector 301 (and the opening surface of the collimator 303) at a constant angle between the detection surface and the irradiation direction of the therapeutic radiation beam. The three-dimensional scattered radiation data consisting of a plurality of two-dimensional scattered radiation data can also be acquired by moving the movement in conjunction with the movement of the position.

[Preprocessing (correction processing, etc.): Step S3b]
The correction processing unit 501 of the data processing system 5 performs preprocessing including attenuation correction and acquires projection data (step S3b). Here, the attenuation correction is a correction process related to signal attenuation caused by propagation of therapeutic radiation or scattered radiation in the subject.

[Image reconstruction processing: Step S4b]
The reconstruction processing unit 503 of the data processing system 5 executes image reconstruction processing using the acquired projection data, and acquires scattered radiation volume data (step S4b).

[Conversion process: Step S5b]
The conversion processing unit 505 of the data processing system 5 converts the scattered radiation volume data into absorbed dose volume data indicating a three-dimensional distribution of absorbed radiation dose (absorbed dose), as in the first embodiment (step S5b). ). Details of this processing will be described later.

[Generation of Absorbed Dose Image Data / Display of Image Data: Steps S6b, S7b]
The image processing unit 507 generates absorbed dose image data indicating the distribution of absorbed radiation dose (absorbed dose) regarding a predetermined part of the subject using the absorbed dose volume data and the like, for example, a CT image for fusion display (Step S6b). The display unit 6 displays the absorbed dose image in a predetermined form (step S7b).

  Here, the details of the conversion processing in step S5a and step S5b will be described.

  As shown in FIG. 8, when it is assumed that the human tissue is composed of homogeneous water, there is a possibility that the absorbed dose is underestimated when substances having a large density difference are close to each other. That is, it is not possible to correctly calculate the absorbed dose in tissues having a density different from that of homogeneous water such as air-containing lungs, luminal organs, and bones. Therefore, in the present embodiment, the density corresponding to the composition of the tissue (component and the ratio of the amount contained in the component) is calculated for each voxel using the CT image obtained by imaging the subject, and more accurately. Try to find the correct absorbed dose.

FIG. 9 is a flowchart showing the procedure of the conversion process. Hereinafter, the processing content of each step will be described.
[CT image reading: step S1c]
The data processing system 5 reads a CT image of the patient. This CT image is a CT image that is finally displayed in comparison with the absorbed dose distribution, and an image that correctly represents the state of the patient being treated is desirable. For example, a CT image (for example, can be taken with a device called Linac-integrated CT) taken in the same posture as during treatment is used immediately before treatment.

[Calculation of Mass Density of CT Image voxel: Step S2c]
The conversion processing unit 505 converts the CT value of each voxel in the CT image into a mass density. This process is performed as follows. First, the CT value is converted into relative electron density. Here, the relative electron density is the ratio of the electron density of a certain substance to the electron density of water. The relationship between the CT value and the relative electron density ρ _e can be measured using a phantom. For example, the following relationship is known (SJ THOMAS, Relative electron density calibration of CT scanners for radiotherapy treatment planning, The British Journal of Radiology, August 1999). Here, HU is a Hounsfield Unit and a CT value.

  By using the above equation, the relative electron density of the CT image voxel is known. In addition, since the above formula is based on data photographed at a tube voltage of 120 to 140 keV, when using CT images photographed at other tube voltages, measurement with a phantom is performed separately and relative to the CT value. It is necessary to obtain an electron density conversion formula.

Furthermore, conversion from relative electron density to mass density is performed. As the constituent elements of the human body, elements with atomic number 20 or less occupy 99% or more. As shown in FIG. 10, in such a relatively small range of atomic numbers, the atomic number, that is, the number of electrons of the atom and the atomic weight are substantially proportional, and the electron density and mass density in the human body have the same relationship. Can be considered. For this reason, voxel having a relative electron density of 2.0 (twice the electron density of water) can be regarded as a mass density of 2.0 [g / cm ³ ] twice that of water.

A substantially correct mass density can be obtained by the above approximate conversion. Here, comparison is made with the following literature values.
Since the relative electron density of fat is 0.96 g / cm ³ (document value), according to the above method, the mass density is converted to 0.96 g / cm ³ (document value of mass density: 0.94 g). / cm ³ ).
Further, since the relative electron density of muscle is 1.05 g / cm ³ (document value), according to the above method, the mass density is converted to 1.05 g / cm ³ (document value of mass density: 1 .06 g / cm ³ ).
Source of relative electron density: Yukihiro Matsuda et al., Survey on CT value-electron density conversion method in radiation therapy planning CT, Journal of Japanese Society of Radiological Technology, 63 (8), 888-, 2007
Source of mass density 1: Japan Swimming Federation, Synchro Committee official site, <URL: http://synchrocafe.ijiss.jp/modules/smartsection/item.php?itemid=43>
Source of Mass Density 2: Japan Swimming Federation, Synchronized Teaching Manual, 2002
If necessary, the correspondence relationship between the obtained CT value and mass density may be tabulated and stored in the storage unit 7 as the CT value-density conversion table 701.

[CT image-dose distribution image alignment: step S3c]
The image processing unit 507 aligns both images in order to associate the CT image with the voxel of the dose distribution image. When using a CT image taken in the same posture as during treatment or a CT image at the time of treatment plan creation immediately before treatment, the position in the irradiation device coordinate system with the irradiation center as the origin is known, so this processing Is easy to implement.

[Calculation of Mass Density of Dose Distribution Image Voxel: Step S4c]
Next, the mass density of each voxel in the dose distribution image is calculated from the mass density value of the corresponding CT image voxel. The voxel sizes of both images are generally different. In many cases, there is a relationship that the voxel size of the dose distribution image is larger than the voxel size of the CT image (CT images can be taken with a voxel size of 1 mm or less, but the dose distribution image can be obtained with a smaller voxel size. If you try to do so, the accuracy will drop, which is actually difficult). In this case, an average value may be obtained. Furthermore, depending on the positional relationship between the two images, there may be a case where the CT image voxel included in a certain dose distribution image voxel is a non-integer number. The j-th voxel in the dose distribution image has 1 voxel in the CT image in which part or all is included, and the volume ratio in which each voxel is included is ν _i (i = 1, 2,..., l) When the mass density is ρ _i (i = 1, 2,..., l), the mass density ρ _j ^Dose of the j-th voxel in the dose distribution image is calculated by the following equation.

[Calculation of absorbed dose [Gy]: Step S5c]
The absorbed dose D _i [Gy] = [J / kg] of the j-th voxel in the dose distribution image is the absorbed energy E _{ab, j} [J], the mass density ρ _j ^Dose [g / cm ³ ], and the volume of the voxel When ν [cm ³ ] is used, it can be calculated by the following formula.

  As described above, in the above-described embodiment, the radiation irradiation system 2 irradiates the subject with the therapeutic radiation beam, and the scattered radiation detection system 3 detects from the inside of the subject generated based on the therapeutic radiation beam. Scattered radiation is detected and scattered radiation data is generated. The reconstruction processing unit 503 reconstructs scattered radiation volume data indicating a three-dimensional distribution of scattered radiation generation density in the subject based on the detected scattered radiation data. The conversion processing unit 505 converts the scattered radiation volume data into absorbed dose volume data indicating a three-dimensional distribution of the absorbed radiation dose using a density corresponding to the composition of each voxel tissue.

  If an absorbed dose distribution image is created by regarding human tissue as homogeneous water, the dose may be underestimated when substances with large density differences are close to each other. On the other hand, according to the present embodiment, there is an advantage that accurate dose evaluation can be performed even in a case where tissues with greatly different densities are adjacent, such as a tumor in a lung field. This makes it possible to accurately determine whether or not radiotherapy is being performed according to the treatment plan, and it is possible to prevent under-irradiation of the radiation treatment site, over-irradiation to the surrounding normal site, and the like. As a result, it is possible to improve the effect of radiation therapy and reduce the amount of extra exposure to the subject, thereby contributing to the improvement of the safety and quality of radiation therapy.

  Note that the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. For example, each function according to the present embodiment can also be realized by installing a program for executing the processing in a computer such as a workstation and developing these on a memory. At this time, a program capable of causing the computer to execute the technique is stored in a recording medium such as a magnetic disk (floppy (registered trademark) disk, hard disk, etc.), an optical disk (CD-ROM, DVD, etc.), or a semiconductor memory. It can also be distributed.

  In addition, various inventions can be formed by appropriately combining a plurality of components disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, constituent elements over different embodiments may be appropriately combined.

  As described above, according to the present invention, in radiation treatment, it is actually measured which part of a patient is irradiated with how much dose, and the result is provided in a predetermined form. It is possible to realize a radiotherapy dose distribution measuring apparatus and a radiotherapy dose distribution measuring program capable of preventing excessive irradiation and excessive irradiation of normal tissues.

FIG. 1 is a diagram for explaining the principle and method of measuring scattered radiation from a subject based on therapeutic radiation of the present radiation treatment system. FIG. 2 shows a block configuration diagram of the radiation therapy system according to the present embodiment. FIG. 3 is a flowchart showing the flow of processing during radiotherapy including the operation of the present radiotherapy system. FIG. 4 is a diagram showing an example of a measurement form of scattered radiation of the present radiation treatment system. FIG. 5 is a flowchart showing the flow of processing during radiotherapy including the operation of the present radiotherapy system. FIG. 6 is a diagram showing an example of a measurement form of scattered radiation of the present radiation treatment system. FIG. 7 is a diagram showing another example of the measurement form of scattered radiation of the present radiotherapy system. FIG. 8 is a diagram showing the concept of tissue density in a voxel. FIG. 9 is a flowchart showing the procedure of the conversion process. FIG. 10 is a diagram showing the relationship between the atomic number and the atomic weight.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Radiation treatment system, 2 ... Radiation irradiation system, 3 ... Scattered ray detection system, 4 ... Data acquisition control part, 5 ... Data processing system, 6 ... Display part, 7 ... Memory | storage part, 8 ... Operation part, 9 ... Network I / F, 201 ... power supply unit, 203 ... irradiation unit, 205 ... timing control unit, 207 ... gantry control unit, 301 ... detector, 303 ... collimator, 305 ... moving mechanism unit, 307 ... position detection unit, 501 ... Correction processing unit 503 ... Reconstruction processing unit 505 ... Conversion processing unit 507 ... Image processing unit 701 ... CT value-density conversion table.

Claims (8)

Irradiation means for irradiating the subject with a therapeutic radiation beam;
Detection means for detecting scattered radiation from within the subject generated based on the therapeutic radiation beam and generating scattered radiation data;
Based on the detected scattered radiation data, image reconstruction means for reconstructing scattered radiation volume data indicating a three-dimensional distribution of scattered radiation generation density in the subject;
Conversion means for converting the scattered radiation volume data into absorbed dose volume data indicating a three-dimensional distribution of the radiation dose absorbed using a density corresponding to the tissue composition of each voxel;
Image generation means for generating an absorbed dose image in the subject using the absorbed dose volume data;
Radiation therapy dose distribution measuring apparatus comprising: display means for displaying the absorbed dose image.   2. The dose distribution measuring apparatus for radiotherapy according to claim 1, wherein the conversion means calculates the density of each voxel based on a morphological image obtained by imaging the subject.   3. The dose distribution measuring apparatus for radiotherapy according to claim 2, wherein the morphological image is a computerized tomography (CT) image, and the density is calculated from a CT value.   3. The radiotherapy dose distribution measurement according to claim 2, wherein the conversion means calculates a density of voxels of the scattered radiation volume data by averaging a density of at least one voxel of the corresponding morphological image. apparatus. On the computer,
A detection function for generating scattered radiation data by detecting scattered radiation from within the subject that is generated based on a therapeutic radiation beam irradiated to the subject;
An image reconstruction function for reconstructing scattered radiation volume data indicating a three-dimensional distribution of scattered radiation generation density in the subject based on the scattered radiation data for the detected plurality of positions;
A conversion function for converting the scattered radiation volume data into absorbed dose volume data indicating a three-dimensional distribution of the radiation dose absorbed using a density corresponding to the tissue composition of each voxel;
Using the absorbed dose volume data, an image generation function for generating an absorbed dose image in the subject,
A radiation therapy dose distribution measuring program for executing a display function for displaying the absorbed dose image.   6. The radiotherapy dose distribution measurement program according to claim 5, wherein the conversion function calculates the density of each voxel based on a morphological image obtained by imaging the subject.   The dose distribution measurement program for radiotherapy according to claim 6, wherein the morphological image is a computerized tomography (CT) image, and the density is calculated from a CT value.   7. The radiotherapy dose distribution measurement according to claim 6, wherein the conversion function calculates a density of voxels of the scattered radiation volume data by averaging a density of at least one voxel of the corresponding morphological image. program.

Images