JP2009189559A - Radiotherapeutic system and radiotherapeutic program - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a raditherapeutic system or the like capable of improving accuracy of obtained scattering radiation source distribution by introducing a condition for a region irradiated with scattering radiation to reconfiguration in reconfiguration processing. <P>SOLUTION: This invention is related to a reconstructing method in the radiotherapeutic system of a type for monitoring the dose of the therapeutic radiation by the scattering radiation. Generally, the therapeutic radiation uses a thin radiation cone, but the conventional tomography does not utilize such information. A repetitive reconstructing method is adopted and it includes a step of reducing the scattering radiation source distribution corresponding to the area of the small value of scattering radiation images on the basis of the scattering radiation images measured during repetition. Also, it includes a step of replacing the scattering radiation source distribution with 0 for the area where the scattering radiation source distribution is a negative value. A condition that the scattering radiation source distribution is a value close to 0 at the position of a subject corresponding to the region where the scattering radiation image indicates the value close to 0 especially is added. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a radiotherapy system having a function of measuring and displaying a dose actually irradiated in the radiotherapy system. In particular, the present invention relates to a reconstruction process for three-dimensionally reconstructing a dose distribution from a scattered radiation image.

  In radiation therapy represented by external X-ray irradiation treatment, an irradiation plan (from which direction and how much dose is irradiated to the lesion) 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 under-irradiation of the lesion or over-exposure to normal tissue occurs 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.

  While irradiating a part of the subject (patient's affected area) with the radiation irradiation apparatus, X-rays scattered in the radiation passing region are measured in multiple directions with a scattered radiation detector, and the distribution of the scattering source is determined by tomography. A method for dimension reconstruction has been proposed.

Radiation therapy usually uses a thin radiation beam that matches the shape of the trunk. Scattered rays are mainly generated in a region irradiated with radiation, but the region is a very part of the subject. In general CT reconstruction or tomography reconstruction processing, reconstruction is performed without assuming the existence area of the solution. However, when applied to monitoring the irradiation area of radiotherapy, only a part of the location of the scattering source is used. It is possible to add a condition that it exists. It is considered that the accuracy of the solution can be improved more than using a general CT reconstruction or tomographic reconstruction processing.
JP, 5-146426, A The technique 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 in that a three-dimensional scattered radiation image of a subject is reconstructed by scanning with a pencil beam. In other words, this technology assumes only a pencil-shaped beam, and does not obtain a scattered image (a spatial distribution of the dose of the treatment beam) of a region through which a beam having a finite width used in X-ray therapy has passed. . In addition, since scattering of high-energy treatment beams (several MeV) within the 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.

  Radiation therapy usually uses a thin radiation beam that matches the shape of the trunk. Scattered rays are mainly generated in a region irradiated with radiation, but the region is a very part of the subject. In general CT reconstruction or tomography reconstruction processing, reconstruction is performed without assuming the existence area of the solution, but when imaging the radiation treatment irradiation area, there is only part of the scattering source area. It is possible to add a condition to do. Therefore, it is considered that the accuracy of image diagnosis can be improved if a reconstruction method specific to imaging of an irradiation region of radiotherapy can be established rather than using general CT reconstruction or tomographic reconstruction processing.

  The present invention has been made in view of the above circumstances, and in the reconstruction process, conditions for the region irradiated with scattered radiation can be introduced into the reconstruction to improve the accuracy of the obtained scatter source distribution. An object of the present invention is to provide a radiotherapy system and a radiotherapy program.

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

  According to the first aspect of the present invention, there is provided irradiation means for irradiating a subject with a therapeutic radiation beam, scattered radiation from the subject generated based on the therapeutic radiation beam, and detecting scattered radiation data. The detection means to be generated, the determination means for determining the scatter source existence area in the subject on each of the detected scattered ray data, and the respective scattered ray data for which the scatter source existence area has been determined are input. A reconstructed image is generated by repeatedly executing a reconstruction process according to a predetermined condition, and using the reconstructed image, scattered ray volume data indicating a three-dimensional distribution of scattered radiation generation density in the subject is generated. Reconstructing means, converting means for converting the scattered radiation volume data into absorbed dose volume data indicating a three-dimensional distribution of absorbed radiation dose, and the absorbed dose volume data Used, the a radiation treatment system characterized by comprising an image generating device which generates an absorbed dose image in inside of the subject, and display means for displaying the absorbed dose image.

  According to a sixth aspect of the present invention, there is provided a detection function that causes a computer to detect scattered radiation from within the subject that is generated based on a therapeutic radiation beam irradiated to the subject, and the detected scattered radiation. A data generation function for generating a plurality of scattered radiation data based on the above, a function for determining and determining a scattering source existence area in the subject on each of the detected scattered radiation data, and a determination of the scattering source existence area The reconstructed image is generated by repeatedly executing the reconstructing process according to a predetermined condition using each of the scattered radiation data thus input, and the reconstructed image is used to generate a three-dimensional scattered radiation generation density in the subject. Reconstructing function to generate scattered radiation volume data showing a spatial distribution, and changing the scattered radiation volume data into absorbed dose volume data showing a three-dimensional distribution of absorbed radiation dose. A radiation function that realizes a conversion function to be performed, an image generation function for generating an absorbed dose image in the subject using the absorbed dose volume data, and a display function for displaying the absorbed dose image It is a treatment program.

  As described above, according to the present invention, in the reconstruction process, the condition for the region irradiated with the scattered radiation is introduced into the reconstruction, and the radiation treatment system and the radiation treatment program can improve the accuracy of the obtained scatter source distribution. Can be realized.

(First embodiment)
[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 externally irradiated X-ray irradiation is mainly brought about by X-ray scattering occurring in the patient. That is, when the therapeutic X-ray beam is scattered by the electrons in the patient, the electrons that have received the energy fly in 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 radiotherapy 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 rays at a certain angle can be detected, the number of scattered rays at other angles can also be estimated. Furthermore, in order to obtain a three-dimensional distribution of locations where scattering occurs in the patient, the detector is rotated during irradiation, and 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. Only the scattered radiation is selectively detected, and the X-ray beam for detection 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. Is performed while moving, thereby scanning a three-dimensional region in the subject. Using the obtained three-dimensional scattered radiation data relating to the predetermined 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 having energy of several MeV are generated by the 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.

[Data processing system]
The data processing system 5 includes a correction processing unit 501, a scattering source existence region determination unit 502, a reconstruction processing unit 503, a conversion processing unit 505, and a data 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 scatter source existence region determination unit 502 determines a region corresponding to the scatter source that is localized in the subject by back projection processing using measurement results at each position of the scatter detector. The contents of the process for determining the scattering source existence area 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 a reconstruction method, for example, a CT reconstruction method is used if the direction of the collimator is orthogonal to the scan axis, and a tomographic reconstruction method is used if the direction is not orthogonal. In particular, the reconstruction processing unit 503 assumes that the scattering source distributed outside the irradiation region is zero, and targets only the scattering presence region, and corrects the solution while correcting the solution until the projection image matches the measurement image. And the reconstruction process is repeated. The contents of this iterative reconstruction process will be described in detail later.

  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).

[Display unit, storage unit, operation unit, network I / F]
The display unit 6 includes a display such as an LCD. Based on the data output by the various modules of the data processing unit 507, the display unit 6 displays, for example, an absorbed dose image by fusion with a plan image or an image obtained immediately before or during irradiation.

  The storage unit 7 has a predetermined scan sequence 701 for acquiring (scanning) scattered radiation data while rotating the detector 301 around the axis of the radiation beam to be irradiated, correction processing, image reconstruction processing, conversion processing, and display. Control program 702 for displaying and editing a treatment plan and the like in the system, scattered radiation volume data 703, absorbed dose volume data 704, X-ray computed tomography apparatus acquired by the radiotherapy system 1 The morphological image data 705 acquired by other modalities are stored. These 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 radiotherapy system 1 to another apparatus via the network, and acquires, for example, the treatment plan created in the radiotherapy planning apparatus via the network. To do.

(Method of generating scattered radiation volume data, etc.)
(First embodiment)
Next, a method for generating scattered radiation volume data and the like using the radiation therapy 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 treatment 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 scattered radiation is measured from all directions (see, for example, FIG. 6). 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 a flow of processing during 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 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 arranges the subject on the bed according to the displayed treatment plan, and performs setting of the radiation irradiation time, setting of the rotation angle for performing scattered radiation measurement, selection of the scan sequence, etc. via the operation unit 8. (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 2 detects the scattered radiation that comes 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 303 in each direction and the position information of the detector 303 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 so that the arrangement angle of the detector 301 is such that backscattered rays having a scattering angle θ in 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, which is about 4 × 10 ⁴ [counts / cm ² ]. . If it is irradiated for 180 seconds per direction and measured for 10 seconds, it becomes 4 × 10 ⁴ × 10/180 = 2 × 10 ³ [counts / cm ² ], 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 practice, 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]
Next, the image reconstruction processing unit 503 of the data processing system 5 performs image reconstruction processing using multidirectional 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 (scattered radiation volume data) through all steps of the above reconstruction processing (various correction processing, filter processing, back projection processing) Can be obtained.

  In the radiotherapy system according to the present embodiment, a conditional iterative reconstruction process described later is executed in this step.

[Conversion processing: Step S5a]
Next, the conversion processing unit 507 of the data processing system 5 converts the number of scatterings n per unit volume calculated for each voxel into an absorbed dose, thereby absorbing the radiation dose absorbed by the scattered radiation volume data. It is converted into absorbed dose volume data indicating a three-dimensional distribution of (absorbed dose) (step S5a).

[Generation of Absorbed Dose Image Data / Display of Image Data: Steps S6a, S7a]
Next, the image processing unit 507 uses the absorbed dose volume data or the like to generate absorbed dose image data indicating the distribution of the absorbed radiation dose (absorbed dose) for a predetermined part of the subject, for example, for fusion display. Are combined with the 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 scattered radiation volume data and the like 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 having a collimator is installed at a position that forms a predetermined angle (scattering angle) with respect to a 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 three-dimensional region in the subject. Using the obtained three-dimensional scattered radiation data relating to the predetermined 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, as in the first embodiment, the placement of the subject is performed (step S1b).

[Radiation irradiation (acquisition of scattered radiation data): Step S2]
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 the subject with an X-ray beam B2 shaped into a thin flat surface at a predetermined timing, and the radiation detection system 2 performs the irradiation based on the irradiation radiation. Scattered rays with a predetermined scattering angle coming out of the specimen 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. 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 S2). 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 this 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. 3D scattered ray data consisting of a plurality of two-dimensional scattered ray data can also be acquired by moving in conjunction with the movement of the position.

[Preprocessing (correction processing, etc.): Step S3b]
Next, the correction processing unit 501 of the data processing system 5 executes preprocessing including attenuation correction, and acquires projection data (step S3). 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]
Next, the image reconstruction processing unit 503 of the data processing system 5 performs image reconstruction processing using the acquired projection data, and acquires scattered radiation volume data (step S4). In the radiotherapy system according to the present embodiment, a conditional iterative reconstruction process described later is executed in this step.

[Conversion process: Step S5b]
Next, the conversion processing unit 507 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 S5).

[Generation of Absorbed Dose Image Data / Display of Image Data: Steps S6b, S7b]
Next, the image processing unit 507 uses the absorbed dose volume data or the like to generate absorbed dose image data indicating the distribution of the absorbed radiation dose (absorbed dose) for a predetermined part of the subject, for example, for fusion display. Are combined with the CT image (step S6b). The display unit 6 displays the absorbed dose image in a predetermined form (step S7b).

(Conditional iterative reconstruction function)
Next, the conditional iterative reconstruction function of the present radiotherapy system 1 will be described. Unlike general X-ray tomography and X-ray CT, when imaging an X-ray irradiation region (scattering source distribution) of a radiotherapy system, it is known that the region irradiated with radiation is limited. In the conditional iterative reconstruction function, first, as shown in FIG. 8A, the existing region of the scattered radiation source is determined by back projection processing using the measurement results at each position of the scattered radiation detector. Next, as shown in FIG. 8B, in the reconstruction process, assuming that the scattering source distributed outside the irradiation region is 0, only the scattering presence region is targeted, and the projection image matches the measurement image. Reconstruction processing that repeats back-projection and projection while correcting the solution to a suitable position, and appropriately displays the spatial distribution of the scattering source (or the spatial distribution of the absorbed dose) in the irradiated region of the radiation localized in the subject It is to become. This conditional iterative reconstruction function is executed in, for example, step S4a in FIG. 3 and step S4b in FIG.

  FIG. 9 is a flowchart showing a flow of processing according to the conditional iterative reconstruction function (conditional iterative reconstruction processing). As shown in the figure, this conditional iterative reconstruction process mainly comprises two steps. Hereinafter, the contents of processing in each step will be described.

[Determination Processing of Scattering Source Possible Area: Step S41]
FIG. 10 is a flowchart showing a flow of determination processing of a scatter source existence possible region.

  First, the scattering presence region determining unit 502 divides each of the scattered radiation images of Np projections into a region where the measured value is close to 0 and other regions as shown in FIG. 11A (step S411). . A threshold method may be used for region division, but for example, (peak value of histogram + constant) can be used to determine the threshold. After binarization using a threshold, a process for removing an isolated small foreground region is performed in order to remove the influence of noise. At this point, a binarized projection image is obtained for each projection. The binarized projection image has a value of 0 in a region where the scattered radiation image is close to 0, and a value of 1 in other regions.

  Next, the scattering presence region determination unit 502 backprojects the binarized projection image as shown in FIG. 11B (step S412). The scattering presence region determination unit 502 determines a region having a value equal to the number of projections in the image obtained as a result of the back projection as a scattering source existence region (step S413). In the example of the present embodiment, as shown in FIG. 11C, a region having a value of 4 is determined as the scattering source existence region S.

[Iterative reconstruction processing: step S42]
FIG. 12 is a flowchart showing a flow of determination processing of a scatter source existence possible region.

  First, the reconstruction processing unit 503 sets a provisional solution image (step S421). In the present embodiment, the solution obtained in step S41 is used as the initial value of the provisional solution image.

  Next, the reconstruction processing unit 503 replaces the region other than the scatter source existable region of the provisional solution image with 0 (step S422), generates a projection image by projection processing, and outputs the projection image as a measurement value. The difference is obtained (step S423), and the difference is reconstructed using the difference as a projection image (step S424, step S425).

  Next, the reconstruction processing unit 503 subtracts the difference reconstructed image from the first provisional solution image (replaced by 0) (step S426), and the number of iterations of the reconstruction process reaches a predetermined number. Determine whether or not. If it is determined that the predetermined number of times has not been reached, the reconstruction processing unit 503 repeats the process between steps S422 to S426 using the latest projection image obtained in step S425 as a provisional solution image. That temporary solution image is iteratively corrected and converged toward the final result. On the other hand, if it is determined that the predetermined number of times has been reached, the reconstruction processing unit 503 ends the iterative reconstruction process.

(effect)
According to the configuration described above, the following effects can be realized.

  In this radiotherapy system, the scattered radiation source existing area in the subject is determined by back projection processing using the measurement results at each position of the scattered radiation detector, and the provisional solution outside the scatter source existing possible area is set to 0. Replace with and perform the first reconfiguration. As a result, most of the projected image corresponding to the error is reconstructed into the scattering source existable region by the first iteration of reconstruction, and the value of the provisional solution image in the scattering source existable region is greatly reduced. In the next reconstruction process, the provisional solution image obtained in the previous reconstruction process is used as an input image, the provisional solution outside the scatterable source existence region is replaced with 0, and the reconstruction process is executed again. Repeat this. Such an iterative reconstruction process can faithfully visualize the signal corresponding to the region where the scattering source can exist, and can exclude the signal that does not correspond to the region where the scattering source can exist. Can be reduced and the image quality can be improved.

  This conditional iterative reconstruction process has a great effect for improving the reconstruction error even when at least two solutions are repeated. It is more effective to increase the number of iterations, but it is sufficient to repeat 10 times at the maximum.

(Second Embodiment)
In the first embodiment, the accuracy of the solution is improved by introducing the condition that the distribution of the scattering source is 0 for the region where the scattering source should not exist. On the other hand, in the present embodiment, a method that generalizes the condition that the distribution of the scattering sources is 0 will be described. The introduction of the condition that the distribution of the scattering source is zero for the region where the scattering source should not exist has two effects.

In the conventional reconstruction method, a solution larger than 0 may be obtained in a region where the scattering source is supposed to be zero. Since the region should originally be 0, the accuracy of the solution can be improved by adding the condition that the scattering source = 0 for this region.

In the conventional reconstruction method, a negative value solution may be obtained in a region where the scattering source has a small value. In particular, it is easy to obtain a negative solution in a region where the scattering source should be zero. In principle, since the value of the scattering source does not become a negative value, the accuracy of the solution can be improved by adding the condition that the scattering source = 0 for this region.

  In this embodiment, the above two replacements are generalized as follows, and an iterative reconstruction process is performed according to the conditions for the upper and lower limits of the solution.

The image obtained by projecting the solution does not exceed the projection image (reconstructed input projection image) that is input in the reconstruction process.

• The solution will not be negative (non-negative condition).

  In consideration of the above conditions, the policy is to select a solution so that “the image onto which the solution is projected is as close as possible to the reconstructed input projection image”. In the first embodiment, the condition can be added only for the region where the scattering source is supposed to be 0, but in the second embodiment, the upper limit and the lower limit of the solution are set under the above two conditions. Therefore, it is possible to add conditions for the entire area.

  Specific processing contents executed in the reconstruction processing unit 503 are as follows.

[Basic method]
The projection processing of the image f in the reconstruction space is denoted as Af. The result of this process is a vector of projection space. Further, the back projection processing of the projection image g is denoted as A ^T g. The result of this processing is a vector of reconstruction space.

First, the conventional method that is the basis of this method will be described. Consider the case of obtaining f satisfying the following expression (1).

Pf is a penalty vector of the solution f, and f ^T Pf has a large value with respect to undesired f. Always f ^T Pf ≧ 0. From the above equation (1), the following equation (2) is obtained.

Further, a function q (f) having the right side of Expression (2) as a derivative is defined by the following Expression (3).

That is, to solve equation (1), equation (3) may be minimized as an objective function. In order to minimize the equation (3), a minimization method such as a conjugate gradient method can be used. Many minimization methods including the conjugate gradient method require the calculation of the evaluation function q and the calculation of the derivative thereof, but the processing elements Af and A ^T g are projection processing and back projection processing as described above. It is necessary to implement these projection processing and back projection processing. For reference, the derivative of q (f) is shown in the following equation (4).

When trying to correct in a certain direction h (specifically, adding λh to the solution f), it is necessary to determine how much distance is advanced in the direction h to be minimized. This is to determine λ that minimizes q (f + λh), and can be obtained by solving the following equation (5).

The calculation for solving equation (5) is shown below.

  FIG. 13 shows a processing procedure of the conventional method in which the above is specifically implemented. An example of the steepest descent method is shown as an illustrative example, but in practice, a better method such as a conjugate gradient method should be used.

[Introduction of non-negative conditions]
In the present embodiment, in order to introduce a condition that the solution does not become a negative value, the processing procedure shown in FIG. 13 is changed to the processing procedure shown in FIG. 14 (step 9b is a newly added step). .

[Introduction of a condition that the image projected with the solution does not exceed the reconstructed input projected image]
Instead of the equation (3), the objective function shown in the following equation (6) is used.

Here, Positive (v) is a function in which the value of the i-th element is ν _i when the value of the element ν _i is a positive value, and the value of the i-th element is 0 when the value is 0 or less. Conversely, Negative (v) is a function in which the value of the i-th element is ν _i when the value of the element ν _i is 0 or less, and the value of the i-th element is 0 when the value is positive. Equation (6) is equivalent to Equation (3) when α _g = α _s = 1. Typically, a value that satisfies α _g > α _s is used.

The following equation (7) is used as a derivative of equation (6).

Af−g in Equation (6) is the difference between the projection of the provisional solution and the input projection image. When a value satisfying α _g > α _s is used, if the provisional solution projection exceeds the input projection image, a larger coefficient is applied, so the objective function (6) has a larger value.

FIG. 15 is an example of α _g = 2 and α _s = 1. In the optimization process, a solution that reduces the objective function is selected. As a result, a solution whose projection of the provisional solution does not greatly exceed the input projection image is selected.

  The processing procedure is as shown in FIG. The characteristic point of this processing procedure is that in calculating the derivative in step 6b, the projection of the provisional solution hi is compared with the input projection image, and the positive value is more in accordance with the sign of each element of Af-g. The point which adds the process which multiplies each factor of Af-g by a big coefficient is a point. Since λ calculated in step 8 becomes inaccurate due to the change in step 6b, it is desirable to add step 8b to correct λ to a more accurate value.

(effect)
According to the conditional iterative reconstruction process according to the present embodiment described above, the condition regarding the lower limit that the solution is non-negative and the condition regarding the upper limit that the image onto which the solution is projected do not exceed the reconstructed input projection image are introduced. Is possible, and the solution can be limited. For example, a condition that the scatter source distribution has a value close to 0 is added particularly at the position of the subject corresponding to the region where the scattered radiation image shows a value close to 0. Therefore, the second embodiment has the same effect as the first embodiment. Further, even in a region where the scattered radiation image shows a certain non-zero value, a condition is added that the value obtained by projecting the scattering source distribution becomes a smaller value. This imposes stronger restrictions on the reconstruction in the second embodiment than in the first embodiment. Therefore, according to the second embodiment, there is an effect that the accuracy of reconstruction can be further improved as compared with the first embodiment.

(Third embodiment)
In the second embodiment, a non-negative condition can be imposed on the entire solution. However, in such a method, the non-negative condition may not function effectively. For example, when the scatter source has only a large value as shown in FIG. 17 (a), an erroneous solution having a negative value is likely to be obtained around it as shown in FIG. 17 (b). In this case, the solution can be improved by the non-negative condition. However, when a uniform scatter source distribution is added to FIG. 17A as shown in FIG. 17C, the reconstruction method similar to FIG. 17B is as shown in FIG. 17D. The solution does not become negative. That is, even if the solution includes the same error as in FIG. 17B, the uniform component is added, so the solution does not become a negative value and the error cannot be corrected.

  Therefore, in the present embodiment, a solution (scattering source distribution) is expressed by combining a plurality of modeled distributions. That is, a solution that is originally one is considered to be decomposed into a plurality of modeled solutions. In addition, by imposing the conditions of the second embodiment for each of the decomposed solutions, a suitable image reconstruction can be performed even when a uniform scattering distribution as shown in FIG. It is realized.

The synthesis of the solution can be expressed by the following equation (8).

In the above equation (8), the solution is represented by the synthesis of M functions. Of these, a synthesis equation approximated by only the first R is made to be calculated by the following equation (9).

The distribution corresponding to the small j among the decomposed solution distributions f ^j is a wide-area distribution, and f ^j is determined so as to become a local distribution as j increases. In particular, ^{f 0} is a uniform distribution. It is preferable to set each element of f ^j to have a value of 0 or more.

The objective function becomes the following equation (10) by substituting equation (8) into equation (6).

Further, the following equation (11) is used as a derivative.

Since the value of F _R m _R is a composition of R solution distributions f ^j , the objective function and the derivative are calculated differently depending on the value of R. Be determined to R, seek m _R by the method of the second embodiment, it can be solved distribution by calculating the F _R m _R. At that time, the A in the description of the second embodiment it is necessary to replace the AF _R of the present embodiment.

In this embodiment, starting from R = 1, the reconstruction process is repeatedly executed as shown in FIG. 19 while gradually increasing R. Projection image g is setting the value of the first reconstructed input image, continue to subtract the projection Af _k solution f _k for each iteration. In step k during the iteration, reconstruction is performed from the projection image subtracted in all previous steps. Since f ^j has a wider distribution with respect to small j and R increases as the iteration proceeds, a fine structure is reconstructed as the step proceeds. As an example, a uniform distribution is first estimated, and then a local distribution is estimated. Since the projection image corresponding to the uniform distribution is removed from the projection image after the second step, the non-negative condition is effective when estimating the local distribution in each subsequent step as described in the conceptual diagram shown first. To work.

  According to the reconfiguration processing according to the present embodiment described above, in the case of the example of FIG. 18C (same as the example of FIG. 17C), the solution is represented by the uniform component shown in FIG. It decomposes into local components shown in 18 (c2), and reconstructs them by imposing a non-negative condition on each. At this time, since a non-negative condition is added, errors included in the solution are reduced as shown in FIGS. 18 (e1) and 18 (e2), respectively. The finally obtained solutions are synthesized, but of course, the error included in the final solution is also reduced as shown in FIG.

  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. Specific examples of modifications are as follows.

  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 the reconstruction process, the condition for the region irradiated with the scattered radiation is introduced into the reconstruction, and the radiation treatment system and the radiation treatment program can improve the accuracy of the obtained scatter source distribution. Can be realized.

FIG. 1 is a diagram for explaining the principle and method of measuring scattered radiation from a subject based on therapeutic radiation in the radiation therapy dose distribution measuring apparatus. FIG. 2 shows a block diagram of the radiation therapy dose distribution measuring apparatus 1 according to the present embodiment. FIG. 3 is a flowchart showing the flow of processing at the time of radiotherapy including generation processing of absorbed dose image data. FIG. 4 is a view showing a measurement form of scattered radiation of the radiation therapy system 1. 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. FIG. 6 is a diagram showing an example of a measurement form of scattered radiation of the radiation therapy system 1. FIG. 7 is a view showing another example of the measurement form of scattered radiation of the present radiation therapy system 1. FIGS. 8A and 8B are diagrams for explaining the concept of the conditional iterative reconstruction function. FIG. 9 is a flowchart showing a flow of processing according to the conditional iterative reconstruction function. FIG. 10 is a flowchart showing a flow of determination processing of a scatter source existence possible region. FIGS. 11A, 11 </ b> B, and 11 </ b> C are diagrams for explaining the determination process of the scattering source existence possible region. FIG. 12 is a flowchart showing a flow of determination processing of a scatter source existence possible region. FIG. 13 is a flowchart showing the processing procedure of the steepest descent method. FIG. 14 is a flowchart showing a processing procedure of the steepest descent method in which a non-negative condition is introduced. FIG. 15 is a diagram illustrating an example of an input projection image and a provisional solution projection image when α _g = 2 and α _s = 1. FIG. 16 is a flowchart showing the processing procedure of the steepest descent method when the condition that the image on which the solution is projected does not exceed the reconstructed input projection image is introduced. FIG. 17 is a diagram for explaining the concept of the reconstruction process according to the third embodiment. FIG. 18 is a diagram for explaining the concept of the reconstruction process according to the third embodiment. FIG. 19 is a flowchart showing the flow of the reconstruction process according to the third embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Radiation therapy dose distribution measuring apparatus, 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 ... Movement mechanism unit, 307 ... Position detection 501 ... correction processing unit 502 ... scattering source existence region determination unit 503 ... reconstruction processing unit 505 ... conversion processing unit 507 ... image processing unit

Claims (10)

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;
Determination means for determining a scatter source existing area in the subject on each detected scattered ray data;
A reconstructed image is generated by repeatedly executing a reconstruction process according to a predetermined condition using each scattered radiation data in which the scattering source existence area is determined, and the reconstructed image is used to generate a reconstructed image in the subject. A reconstruction means for generating scattered radiation volume data indicating a three-dimensional distribution of scattered radiation generation density;
Converting means for converting the scattered radiation volume data into absorbed dose volume data indicating a three-dimensional distribution of absorbed radiation dose;
Image generation means for generating an absorbed dose image in the subject using the absorbed dose volume data;
Display means for displaying the absorbed dose image;
A radiation therapy system comprising:   The reconstruction unit repeats the reconstruction process according to a condition that the reconstructed image generated in each reconstruction process does not exceed each scattered radiation data input in each reconstruction process. The radiation therapy system according to claim 1, wherein the radiation therapy system is executed.   The reconstruction means repeatedly executes the reconstruction process according to a condition in which, for each of the scattered radiation data that is input, an area where the scattered radiation has a negative value is 0. The radiation therapy system according to claim 1 or 2.   The reconstruction means repeatedly executes the reconstruction process for each of the scattered radiation data that is input according to a condition that the scattered radiation is 0 for a region that does not correspond to the scattering source existence region. The radiation therapy system according to claim 1 or 2.   The radiotherapy system according to claim 3 or 4, wherein the reconstruction unit decomposes each input scattered radiation data into a plurality of model solutions, and executes the reconstruction processing for each model solution. . On the computer,
A detection function for detecting scattered radiation from within the subject generated based on a therapeutic radiation beam irradiated to the subject; and
A data generation function for generating a plurality of scattered radiation data based on the detected scattered radiation;
A function for determining and determining a scatter source existing area in the subject on each of the detected scattered ray data,
The scattered radiation data in which the scattering source existence area is determined is input to generate a reconstructed image by repeatedly executing a reconstruction process according to a predetermined condition, and using the reconstructed image, A reconstruction function for generating scattered radiation volume data showing a three-dimensional distribution of scattered radiation generation density;
A conversion function for converting the scattered radiation volume data into absorbed dose volume data indicating a three-dimensional distribution of absorbed radiation;
Using the absorbed dose volume data, an image generation function for generating an absorbed dose image in the subject,
A display function for displaying the absorbed dose image;
A radiation therapy program characterized by realizing the above.   In the reconstruction function, the reconstruction process is performed according to a condition that the reconstructed image generated in each reconstruction process does not exceed each scattered radiation data input in each reconstruction process. The radiotherapy program according to claim 6, which is repeatedly executed.   In the reconstruction function, for each of the scattered radiation data that is input, the reconstruction process is repeatedly executed according to a condition that the value of the scattered radiation area is a negative value of 0. The radiotherapy program according to claim 6 or 7.   In the reconstruction function, for each of the scattered radiation data that is input, the reconstruction processing is repeatedly executed according to a condition that the scattered radiation is 0 for a region that does not correspond to the scattering source existence region. The radiotherapy program according to claim 6 or 7.   10. The radiotherapy according to claim 8, wherein in the reconstruction function, each of the scattered radiation data that is input is decomposed into a plurality of model solutions, and the reconstruction processing is executed for each model solution. program.

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