Water phantom structure for multi-point dose measurement and its segmentation measurement method

Abstract

A water phantom structure for multi-point dose measurement and its segmentation measurement method, the structure includes a hemispherical head, a cylindrical middle section, a U-shaped water tank tail and an ionization chamber drive assembly. The ionization chamber drive assembly includes a drive body, a buckle, a folding arm and a fixing frame; the hemispherical head, the cylindrical middle section and the U-shaped water tank tail are fixedly sealed and connected in sequence or integrally formed, the thickness of the hemispherical head and the cylindrical middle section are the same, the inner and outer surfaces of the hemispherical head and the cylindrical middle section are smoothly connected to the plane where the diameter of the hemispherical head is located, and the buckle of the ionization chamber drive assembly cooperates with the bayonet of the U-shaped water tank tail. The segmentation measurement method includes the installation and fixation of the water phantom structure and the filling of water, the horizontal adjustment of the water phantom structure and the ionization chamber, the execution of the radiotherapy plan, the acquisition and correction of the point dose measurement readings and the calculation of the point dose pass rate. The present invention has a simple structure and accurate measurement, and provides a reliable reference for the execution of the actual phantom plan.

Description

Technical Field

[0001] The invention relates to the field of radiation measurement instruments and equipment, in particular to a water model structure for multi-point dose measurement and a segmentation measurement method thereof. Background Art

[0002] Currently, when verifying a patient's radiotherapy plan, it is necessary to verify the absolute dose at a specific point during the radiotherapy process to ensure the quality of the radiotherapy plan and the safety of the patient's treatment. To this end, the physicist needs to map the actual treatment plan onto the phantom to produce a phantom plan, which is then actually measured by a third-party phantom. In actual application scenarios, for multiple metastatic lesions in the brain, neck, etc., measuring and analyzing the doses projected to multiple lesions separately is helpful in evaluating the quality of the radiotherapy plan.

[0003] The phantoms currently used in radiotherapy plan verification are mostly fixed shapes such as solid water, small water tanks, etc.: cubes, cylinders or spheres. However, the isocenter or ionization chamber channel of fixed-shaped phantoms such as small water tanks is fixed. When measuring the phantom plan, the dose at the center of multiple lesions in multiple tumors cannot be measured one by one; the cubic-shaped water phantom is quite different from the human head and neck structure. When using this shape of water phantom instead of human CT images, it cannot well reflect the dose deposition in the structure close to the patient, and therefore cannot well simulate the dose distribution in the phantom plan of patients with head and neck tumors; even if the relative position of the ionization chamber is adjusted in the vertical direction by adjusting the water volume in the small water tank and raising and lowering the treatment bed, its application range is limited to the vertical downward irradiation field, and it cannot accurately simulate the dose received by the patient during actual treatment; the cylindrical phantom only measures the dose at the isocenter, which is of little significance for evaluating the quality of the plan; the position of the ionization chamber channel of the spherical phantom is basically fixed, and only point dose measurements can be performed on the central axis of the phantom and the position at a fixed distance from the phantom surface. When used in plan verification, it cannot well meet the situation where the measurement point position is random and variable. Summary of the invention

[0004] The object of the present invention is to overcome the above-mentioned deficiencies in the prior art and to provide a water phantom structure for multi-point dose measurement and a segmentation measurement method thereof.

[0005] The technical solution of the present invention is: a water model structure for multi-point dose measurement, comprising a hemispherical head, a cylindrical middle section, a U-shaped water tank tail and an ionization chamber driving component.

[0006] The upper surface of the cylindrical middle section is provided with a first level parallel to its axis, the upper surface of the U-shaped water tank tail is provided with a second level perpendicular to the first level, and the inner wall of the U-shaped water tank tail is provided with a bayonet.

[0007] The ionization chamber drive assembly includes a drive body, buckles and a rotating shaft respectively fixed to opposite sides of the drive body, a folding arm connected to the rotating shaft, and a fixing frame located at the top of the folding arm; the ionization chamber drive assembly is made of a lightweight material with a density equal to or close to that of water.

[0008] The hemispherical head, the cylindrical middle section and the U-shaped water tank tail are fixedly sealed and connected in sequence or integrally formed. The thickness of the hemispherical head and the cylindrical middle section are the same. The inner and outer surfaces of the hemispherical head and the cylindrical middle section are smoothly connected to the plane where the diameter of the hemispherical head is located. The buckle of the ionization chamber drive assembly cooperates with the bayonet of the U-shaped water tank tail, so that the ionization chamber drive assembly is fixed on the U-shaped water tank tail.

[0009] A further technical solution of the present invention is: 3 to 6 cylindrical gold labels with a length of 4 to 6 mm and a diameter of 0.3 to 1.0 mm are inlaid on the inner surface of the hemispherical head and the cylindrical middle section, and the material of the hemispherical head, the cylindrical middle section and the U-shaped water tank tail is a transparent organic glass material, and the density and relative electron density are the same or close to those of water.

[0010] A further technical solution of the present invention is: the ionization chamber drive assembly is replaced by a drive body, a buckle and a rotating shaft respectively fixed to opposite sides of the drive body, a first lead screw connected to the rotating shaft, a second lead screw mounted on the first lead screw and capable of reciprocating movement, and a fixed frame capable of reciprocating movement on the second lead screw.

[0011] A further technical solution of the present invention is: the ionization chamber drive assembly is replaced by a first bracket, a buckle and a second bracket respectively fixed to the opposite sides of the first bracket, a third bracket connected to a fixing ring and capable of sliding / rotating on the second bracket, a fixing bracket located on the third bracket and capable of sliding along the third bracket, and a horizontal adjustment nut located on the top of the first bracket; the surface of the first bracket is provided with an angle scale, the surface of the second bracket is provided with a length scale, and the third bracket is provided with a marking line and a depth scale respectively.

[0012] A further technical solution of the present invention is that the number of the third brackets is more than two.

[0013] Another technical solution provided by the present invention is: a method for multi-point dose measurement based on the aforementioned water phantom structure for multi-point dose measurement, comprising the following steps:

[0014] Step 1: Install and fix the water model structure and fill it with water: Place the water model structure for multi-point dose measurement at the measurement position, add water to submerge the hemispherical head and the cylindrical middle section, so that there are no bubbles in the hemispherical head and the cylindrical middle section.

[0015] Step 2: Adjust the level of the water model structure: Observe the second level of the first level to check whether the water model structure is level, and adjust the placement position of the water model structure to ensure that the water model structure is level.

[0016] Step 3. Fix and level the ionization chamber drive assembly: Fix the measuring ionization chamber on the fixing bracket of the ionization chamber drive assembly, place the ionization chamber drive assembly into the tail of the U-shaped water tank, fix the buckle of the ionization chamber drive assembly with the bayonet at the tail of the U-shaped water tank, and adjust the level adjustment nut of the ionization chamber drive assembly to ensure that the ionization chamber drive assembly is level.

[0017] Step 4: Execution of the radiotherapy plan and acquisition of point dose measurement readings: Start the ionization chamber and its supporting electrometer, and start the radiotherapy equipment, execute the phantom plan corresponding to the verification radiotherapy plan, and record the reading results on the ionization chamber electrometer.

[0018] Step 5. Correction of the point dose measurement readings at the position to be measured: Correct the reading results according to the temperature, air pressure and ionization chamber calibration standard factors to obtain the absolute dose Mi of the point dose at each position to be measured.

[0019] A further technical solution of the present invention is: it also includes step 6, calculation of the point dose passing rate of the position to be measured: comparing the absolute dose Mi of the point dose of each position to be measured with the point dose Di of the position to be measured in the phantom plan; using the following passing rate formula to calculate the point dose passing rate of each position to be measured:

[0020]

[0021] Where: γ is the pass rate, n is the number of positions to be tested, i is the number of the positions to be tested, and 1≤i≤n.

[0022] The point dose Di of the position to be measured in the phantom plan is obtained by the dose segmentation method of the phantom plan, as follows:

[0023] S01, judging the plan execution system, judging whether to adopt the CyberKnife radiotherapy plan or the conventional accelerator radiotherapy plan, if the CyberKnife radiotherapy plan is adopted, executing S02; if the conventional accelerator radiotherapy plan is adopted, executing S03.

[0024] S02, determining the number of planning centers in the CyberKnife radiotherapy plan, if the number of planning centers is one, proceeding to S021; if the number of planning centers is more than one, proceeding to S022.

[0025] S021, the CyberKnife robot arm is divided into segments with the large-scale transfer as the boundary, and the MUs assigned to each point to be tested are sorted in each segment, and the dose Di of each point to be tested is calculated in the planning system or by using the particle transport software with the assigned field parameters and MUs.

[0026] S022, calculating the dose Di of each point to be measured based on multiple centers in the plan.

[0027] S03, judging whether the conventional accelerator radiotherapy plan is to execute a fixed field intensity modulation plan or a volumetric rotation intensity modulation plan. If the fixed field intensity modulation plan is to be executed, go to S031; if the volumetric rotation intensity modulation plan is to be executed, go to S032.

[0028] S031, sort the points to be tested according to the field MU, compare the number of fields with the number of points to be tested, if the number of fields is less than the number of points to be tested, exclude the points to be tested that are not assigned to the fields, and calculate the dose Di of each point to be tested with the assigned field MU; if the number of fields is greater than the number of points to be tested, allocate all fields to all points to be tested, and calculate the dose Di of each point to be tested with the assigned field MU.

[0029] S032, split the complete arc into half arcs, consider the incomplete arc as a half arc, compare the number of half arcs with the number of points to be tested, if the number of half arcs is less than the number of points to be tested, exclude the points to be tested that are not assigned to half arcs, and calculate the dose Di of each point to be tested with the assigned half arc MU; if the number of half arcs is greater than the number of points to be tested, assign all half arcs to all points to be tested, and calculate the dose Di of each point to be tested with the assigned half arc MU.

[0030] Compared with the prior art, the present invention has the following characteristics:

[0031] (1) The water phantom structure of the present invention can be used to measure the point dose in the verification of radiotherapy plans output by conventional or non-coplanar, non-isocentric radiotherapy equipment, and can measure the point dose at any position to be measured on the phantom.

[0032] (2) The water model structure space of the present invention uses water as the medium, which is stable and convenient for obtaining multi-point doses; the water fills the effective measurement volume, effectively avoiding the liquid surface disturbance in the effective measurement volume that may be caused by the movement of the robotic arm, thereby improving the measurement accuracy; the robotic arm in the water model structure adopts a lightweight material with a density equal to or close to that of water, effectively avoiding the interference of side scatter changes on dose measurement.

[0033] (3) The water phantom structure segmentation measurement method of the present invention can segment the dose of the phantom plan for plan verification, which is convenient for comparison with the point dose in the actual measurement of the phantom, and can be used to compare the point dose pass rates of multiple locations to be measured in a single plan.

[0034] The detailed structure of the present invention is further described below in conjunction with the accompanying drawings and specific implementation methods. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Attached Figure 1 This is a schematic diagram of the structure of Embodiment 1 of the present invention;

[0036] Attached Figure 2 For attachment Figure 1 Left view of

[0037] Attached Figure 3 It is a structural schematic diagram of an ionization chamber driving assembly of Example 1;

[0038] Attached Figure 4 It is a structural schematic diagram of the ionization chamber driving assembly of the second embodiment;

[0039] Attached Figure 5 This is a schematic diagram of the structure of Embodiment 3 of the present invention;

[0040] Attached Figure 6 This is a schematic diagram of the structure of the ionization chamber drive assembly of Example 3;

[0041] Attached Figure 7 For attachment Figure 6 Left view of

[0042] Attached Figure 8 This is a flow chart of the method for multi-point dose segmentation measurement of the present invention;

[0043] Attached Fig. 9 This is a flow chart of the dose segmentation method for the phantom plan of the present invention. DETAILED DESCRIPTION

[0044] Embodiment 1, as attached Figure 1-3 As shown, the water model structure used for multi-point dose measurement includes a hemispherical head 1, a cylindrical middle section 2, a U-shaped water trough tail 3 and an ionization chamber drive assembly 4.

[0045] The upper surface of the cylindrical middle section 2 is provided with a first level 2-1 parallel to its axis, and the upper surface of the U-shaped water tank tail 3 is provided with a second level 3-1 perpendicular to the first level 2-1, which are respectively used to confirm when the hemispherical head 1, the cylindrical middle section 2 and the U-shaped water tank tail 3 are adjusted horizontally before measurement. The inner surfaces of the hemispherical head 1 and the cylindrical middle section 2 are inlaid with 3 to 6 cylindrical gold marks (not shown in the figure) with a length of 4 to 6 mm and a diameter of 0.3 to 1.0 mm, so that the imaging device on the radiotherapy equipment can identify and verify the position of the hemispherical head 1 and the cylindrical middle section 2 through the gold marks. A bayonet 3-2 is provided on the inner wall of the U-shaped water trough tail 3. The materials of the hemispherical head 1, the cylindrical middle section 2 and the U-shaped water trough tail 3 are transparent organic glass materials, and the density and relative electron density are the same as or close to those of water. Therefore, in the measurement, the influence of the difference between the materials of the hemispherical head 1 and the cylindrical middle section 2 and water on the different X-ray energy deposition can be ignored, thereby improving the accuracy of the measurement.

[0046] The ionization chamber driving assembly 4 includes a driving body 4-1, buckles 4-2 and a rotating shaft 4-3 respectively fixed to the opposite sides of the driving body 4-1, a folding arm 4-4 connected to the rotating shaft 4-3, and a fixing frame 4-5 located at the top of the folding arm 4-4. The fixing frame 4-5 is used to fix the ionization chamber 5, and is a clamp or other fixing device adapted to the ionization chamber, which matches the shape specifications of commonly used small-volume ionization chambers and can be replaced according to different selected ionization chambers.

[0047] The ionization chamber drive assembly 4 is made of lightweight material with a density equal to or close to that of water, so that gravity and buoyancy cancel each other out during measurement, thereby minimizing the external interference to the movement of the ionization chamber drive assembly 4 and minimizing the interference to side scattered electrons.

[0048] The hemispherical head 1, the cylindrical middle section 2 and the U-shaped water tank tail 3 are fixedly sealed and connected in sequence or integrally formed. The hemispherical head 1 and the cylindrical middle section 2 have the same thickness. The inner and outer surfaces of the hemispherical head 1 and the cylindrical middle section 2 are smoothly connected to the plane where the diameter of the hemispherical head 1 is located. The buckle 4-2 of the ionization chamber drive assembly 4 cooperates with the bayonet 3-2 of the U-shaped water tank tail 3, so that the ionization chamber drive assembly 4 is fixed on the U-shaped water tank tail 3.

[0049] The effective measurement volume of the water model structure used for multi-point dose measurement is composed of the space formed by the hemispherical head 1 and the cylindrical middle section 2, wherein the area connecting the cylindrical middle section 2 and the U-shaped water trough tail 3 may not be used as the effective measurement volume.

[0050] The radius of the hemispherical structure 1 and the cylindrical middle section 2 is 10 cm to 15 cm, which is close to the radius of an average human head and is convenient for measurement.

[0051] The buckle 4-2 of the ionization chamber drive assembly 4 and the bayonet 3-2 of the U-shaped water tank tail 3 can be selected according to actual needs, and other fixing devices such as screws and nuts, screw guides, and tenons can be selected to cooperate with each other to fix the ionization chamber drive assembly 4 to the U-shaped water tank tail 3.

[0052] Embodiment 2, as attached Figure 4 As shown, the structure of the second embodiment is basically similar to that of the first embodiment, and the difference is that the ionization chamber driving assembly 6 includes a driving body 6-1, a buckle 6-2 and a rotating shaft 6-3 respectively fixed to the opposite sides of the driving body 6-1, a first lead screw 6-4 connected to the rotating shaft 6-3, a second lead screw 6-5 mounted on the first lead screw 6-4 and capable of reciprocating movement, and a fixing frame 6-6 capable of reciprocating movement on the second lead screw 6-5. When the ionization chamber 5 is fixed to the fixing frame 6-6, the first lead screw 6-4 and the second lead screw 6-5 are driven to move by the rotating shaft 6-3.

[0053] Embodiment 3, as attached Figure 5-7 As shown, the structure of the third embodiment is basically similar to that of the second embodiment, and the difference is that the ionization chamber drive assembly 7 includes a first bracket 7-1, a buckle 7-2 and a second bracket 7-3 respectively fixed to the opposite sides of the first bracket 7-1, a third bracket 7-5 connected to the fixing ring 7-4 and rotatable / slidable on the second bracket 7-3, a fixing frame 7-6 located on the third bracket 7-5 and slidable along the third bracket 7-5, and a horizontal adjustment nut 7-7 located on the top of the first bracket 7-1.

[0054] The surface of the first bracket 7-1 is provided with an angle scale 7-1-1, the surface of the second bracket 7-3 is provided with a length scale 7-3-1, and the third bracket 7-5 is provided with a marking line 7-5-1 and a depth scale 7-5-2 respectively. By comparing the angle scale 7-1-1 on the surface of the first bracket 7-1 with the marking line 7-5-1 on the third bracket 7-5, the rotation angle of the third bracket 7-5 can be determined, and the depth of the third bracket 7-5 in the water model structure can be checked through the length scale 7-3-1 on the surface of the second bracket 7-3; the position of the ionization chamber 5 on the fixed bracket 7-6 can be checked through the depth scale 7-5-2 on the third bracket 7-5.

[0055] In order to simultaneously measure the point doses between multiple different positions, the number of the third brackets 7 - 6 is set to be more than two.

[0056] Embodiment 4, as attached Figure 8 As shown, the method for performing multi-point dose measurement based on the water phantom structure for multi-point dose measurement in the above embodiment includes the following steps:

[0057] Step 1: Install and fix the water model structure and fill it with water: Place the water model structure for multi-point dose measurement at the measurement position, add water to submerge the hemispherical head 1 and the cylindrical middle section 2, so that there are no bubbles in the hemispherical head 1 and the cylindrical middle section 2.

[0058] Step 2: Horizontal adjustment of the water model structure: Observe the second level 3-1 of the first level 2-1 to check whether the water model structure is horizontal, and adjust the placement position of the water model structure to ensure that the water model structure is horizontal.

[0059] Step 3, fixation and horizontal adjustment of the ionization chamber drive assembly 4: fix the measuring ionization chamber 5 on the fixing frame 4-5 of the ionization chamber drive assembly 4, put the ionization chamber drive assembly 4 into the U-shaped water tank tail 3, fix the buckle 4-2 of the ionization chamber drive assembly 4 with the bayonet 3-2 of the U-shaped water tank tail 3, and adjust the horizontal adjustment nut 4-1-1 of the ionization chamber drive assembly 4 to ensure that the ionization chamber drive assembly reaches the level.

[0060] Step 4: Execution of radiotherapy plan and acquisition of point dose measurement readings: Start the ionization chamber 5 and its supporting electrometer, and start the radiotherapy equipment, execute the phantom plan corresponding to the verification radiotherapy plan, and record the reading results on the electrometer of the ionization chamber 5.

[0061] Step 5. Correction of the point dose measurement readings at the position to be measured: Correct the reading results according to the temperature, air pressure and ionization chamber calibration standard factors to obtain the absolute dose Mi of the point dose at each position to be measured.

[0062] Furthermore, in order to know the accuracy of the absolute dose Mi measured in step 5, step 6 is also included, which is to calculate the passing rate of the point dose at the position to be measured: the absolute dose Mi of each point dose at the position to be measured is compared with the point dose Di at the position to be measured in the phantom plan; the passing rate of the point dose at each position to be measured is calculated using the following passing rate formula:

[0063]

[0064] Where: γ is the pass rate, n is the number of positions to be tested, i is the number of the positions to be tested, and 1≤i≤n.

[0065] For radiotherapy plans that target multiple lesions, it is necessary to use typical locations of different lesions as multiple test locations and perform dose measurements on these multiple test locations. These typical locations usually include the isocenter of the lesion, the maximum dose point of the radiotherapy plan, the dose drop area around the lesion, the location in normal tissue, and the location in the water tank corresponding to the area where the medium density in the human body changes dramatically.

[0066] When measuring the dose at each position to be measured, multiple ionization chambers can be used to measure each position to be measured simultaneously, or a single ionization chamber can be used to measure multiple positions to be measured in segments. Using multiple ionization chambers for fixed-position measurement can directly calculate the point dose of each position to be measured, while using a single ionization chamber for mobile measurement requires segmenting the dose distributed at each position to be measured in the phantom plan during the measurement time, which requires the dose segmentation method of the phantom plan.

[0067] The point dose Di of the position to be measured in the phantom plan is obtained by the dose segmentation method of the phantom plan, as shown in the attached Fig. 9 As shown, the details are as follows:

[0068] S01, judging the plan execution system, judging whether to adopt Cyber ​​Knife radiotherapy plan or conventional accelerator radiotherapy plan, if Cyber ​​Knife radiotherapy plan is adopted, executing S02; if conventional accelerator radiotherapy plan is adopted, executing S03.

[0069] S02, determining the number of planned centers in the Cyber ​​Knife radiotherapy plan, if the number of planned centers is one, proceeding to S021; if the number of planned centers is more than one, proceeding to S022.

[0070] S021, the CyberKnife robot arm is divided into segments with the large-scale transfer as the boundary, and the MUs assigned to each point to be tested are sorted in each segment, and the dose Di of each point to be tested is calculated in the planning system or by using the particle transport software with the assigned field parameters and MUs.

[0071] S022, calculating the dose Di of each point to be measured based on multiple centers in the plan.

[0072] S03, determining whether the conventional accelerator radiotherapy plan is to implement a fixed-field intensity modulated radiation therapy plan IMRT or a volumetric rotation intensity modulated radiation therapy plan IMAT / VMAT / ARC. If the fixed-field intensity modulated radiation therapy plan IMRT is to be implemented, then go to S031; if the volumetric rotation intensity modulated radiation therapy plan IMAT / VMAT / ARC is to be implemented, then go to S032;

[0073] S031, sort the points to be tested according to the field MU, compare the number of fields with the number of points to be tested, if the number of fields is less than the number of points to be tested, exclude the points to be tested that are not assigned to the fields, and calculate the dose Di of each point to be tested with the assigned field MU; if the number of fields is greater than the number of points to be tested, allocate all fields to all points to be tested, and calculate the dose Di of each point to be tested with the assigned field MU.

[0074] S032, split the complete arc into half arcs, consider the incomplete arc as a half arc, compare the number of half arcs with the number of points to be tested, if the number of half arcs is less than the number of points to be tested, exclude the points to be tested that are not assigned to half arcs, and calculate the dose Di of each point to be tested with the assigned half arc MU; if the number of half arcs is greater than the number of points to be tested, assign all half arcs to all points to be tested, and calculate the dose Di of each point to be tested with the assigned half arc MU.

[0075] Among them, MU (Machine Unit) is the dose unit output by the accelerator, which is usually defined as the output dose of the central axis of the ray at a certain depth of the phantom under a certain specification of the radiation field, 1MU=1cGy.

[0076] In actual clinical practice, a phantom plan was selected for verification. The phantom plan was generated based on a radiotherapy plan for a patient with three brain metastases. In the phantom plan, nine positions, including the centers of three lesions, 5 mm to the left of lesion 1, 5 mm to the left of lesion 2, the planned maximum dose point, the center of the brainstem, the center of the left eyeball, and the center of the optic chiasm, were selected as test points. The position of the ionization chamber 5 was adjusted for each test point in a conventional accelerator room to obtain the value of the absolute dose Mi of each test point. The phantom plan was executed using a conventional accelerator to calculate the value of the point dose Di of each test point. The absolute dose Mi, point dose Di, and pass rate γ of each test point are listed in Table 1 below.

[0077] serial number Location of the test position The point dose calculated by the plan is cGy The absolute dose measured is cGy Pass rate% 1 Lesion 1 center 770.6 703.1 0.912406 2 Lesion 2 Center 736.3 671.3 0.911721 3 Lesion 3 Center 753.9 681.6 0.904099 4 5 mm to the left of lesion 1 787.1 720.3 0.915131 5 5 mm to the left of lesion 2 761.2 711.0 0.934051 6 Planned maximum dose point 793.4 765.2 0.964457 7 Brainstem Center 20.4 23.9 0.828431 8 Left eye center 16.7 12.9 0.772455 9 Chiasm Center 23.7 18.1 0.763713

[0078] It can be seen from Table 1 that the overall pass rate of the multi-point dose of the phantom plan is 87.85%, and the pass rate of the point dose of each position to be tested is shown in the last column of the above table, so the actual output dose of each position to be tested in the phantom plan can be evaluated. In this example, the pass rate of the point dose with the center of the lesion as the position to be tested is relatively stable and reaches 90%, which can meet the goal of evaluating the actual irradiation dose of the lesion as the main purpose; when the critical organ far away from the lesion is used as the position to be tested, the pass rate is low, which is related to the large low-dose deviation caused by the method of normalizing the dose calculation to 95% of the prescription dose or the maximum dose in the planning system. The measurement results can be used as a reference for the actual execution of the phantom plan, which can not only ensure the accuracy of the actual execution of the phantom plan, but also ensure that the critical organs are protected from receiving large doses of irradiation.

Claims

1. Water phantom structure for multi-point dose measurement, Its characteristics are: It includes a hemispherical head, a cylindrical middle section, a U-shaped water tank tail section and an ionization chamber drive assembly; The upper surface of the cylindrical middle section is provided with a first level parallel to its axis, the upper surface of the U-shaped water tank tail is provided with a second level perpendicular to the first level, and the inner wall of the U-shaped water tank tail is provided with a bayonet; The ionization chamber drive assembly comprises a drive body, buckles and a rotating shaft respectively fixed to opposite sides of the drive body, a folding arm connected to the rotating shaft, and a fixing frame located at the top of the folding arm; the ionization chamber drive assembly is made of a lightweight material having a density equal to or close to that of water; The hemispherical head, the cylindrical middle section and the U-shaped water tank tail are fixedly sealed and connected in sequence or integrally formed. The thickness of the hemispherical head and the cylindrical middle section are the same. The inner and outer surfaces of the hemispherical head and the cylindrical middle section are smoothly connected to the plane where the diameter of the hemispherical head is located. The buckle of the ionization chamber drive assembly cooperates with the bayonet of the U-shaped water tank tail, so that the ionization chamber drive assembly is fixed on the U-shaped water tank tail.

2. The water phantom structure for multi-point dose measurement according to claim 1, Its characteristics are: The inner surfaces of the hemispherical head and the cylindrical middle section are inlaid with 3 to 6 cylindrical gold labels with a length of 4 to 6 mm and a diameter of 0.3 to 1.0 mm. The hemispherical head, the cylindrical middle section and the U-shaped water tank tail are made of transparent organic glass material, and the density and relative electron density are the same or close to those of water.

3. The water phantom structure for multi-point dose measurement according to claim 1, Its characteristics are: The ionization chamber drive assembly is replaced by a drive body, a buckle and a rotating shaft respectively fixed to opposite sides of the drive body, a first lead screw connected to the rotating shaft, a second lead screw mounted on the first lead screw and capable of reciprocating movement, and a fixed frame capable of reciprocating movement on the second lead screw.

4. The water phantom structure for multi-point dose measurement according to claim 1, Its characteristics are: The ionization chamber drive assembly is replaced by a first bracket, a buckle and a second bracket respectively fixed to the opposite sides of the first bracket, a third bracket connected to a fixing ring and capable of sliding / rotating on the second bracket, a fixing bracket located on the third bracket and capable of sliding along the third bracket, and a horizontal adjustment nut located on the top of the first bracket; the surface of the first bracket is provided with an angle scale, the surface of the second bracket is provided with a length scale, and the third bracket is provided with a marking line and a depth scale respectively.

5. The water phantom structure for multi-point dose measurement according to claim 4, Its characteristics are: The number of the third brackets is more than two.

6. A method for multi-point dose measurement based on the water phantom structure for multi-point dose measurement according to any one of claims 1, 3 to 5, Its characteristics are: The following steps are included: Step 1: Install and fix the water model structure and fill it with water: Place the water model structure for multi-point dose measurement at the measurement position, add water to submerge the hemispherical head and the cylindrical middle section, so that there are no bubbles in the hemispherical head and the cylindrical middle section; Step 2: Horizontal adjustment of the water model structure: Observe the second level of the first level to check whether the water model structure is horizontal, and adjust the placement position of the water model structure to ensure that the water model structure is horizontal; Step 3: Fixing and leveling the ionization chamber drive assembly: fix the ionization chamber for measurement on the fixing bracket of the ionization chamber drive assembly, put the ionization chamber drive assembly into the tail of the U-shaped water tank, fix the buckle of the ionization chamber drive assembly with the bayonet at the tail of the U-shaped water tank, and adjust the level adjustment nut of the ionization chamber drive assembly to ensure that the ionization chamber drive assembly reaches the level; Step 4: Execution of the radiotherapy plan and acquisition of point dose measurement readings: Start the ionization chamber and its supporting electrometer, and start the radiotherapy equipment, execute the phantom plan corresponding to the verification radiotherapy plan, and record the reading results on the ionization chamber electrometer; Step 5. Correction of the point dose measurement readings at the position to be measured: Correct the reading results according to the temperature, air pressure and ionization chamber calibration standard factors to obtain the absolute dose Mi of the point dose at each position to be measured.

7. The method for performing multi-point dose measurement using the water phantom structure for multi-point dose measurement according to claim 6, Its characteristics are: The method also includes step 6, calculating the passing rate of the point dose at the position to be measured: comparing the absolute dose Mi of the point dose at each position to be measured with the point dose Di at the position to be measured in the phantom plan; and calculating the passing rate of the point dose at each position to be measured using the following passing rate formula: Where: γ is the pass rate, n is the number of positions to be tested, i is the number of the positions to be tested, where 1≤i≤n; The point dose Di of the position to be measured in the phantom plan is obtained by the dose segmentation method of the phantom plan, as follows: S01, judging the plan execution system, judging whether to adopt the CyberKnife radiotherapy plan or the conventional accelerator radiotherapy plan, if the CyberKnife radiotherapy plan is adopted, executing S02; if the conventional accelerator radiotherapy plan is adopted, executing S03; S02, determining the number of planning centers in the CyberKnife radiotherapy plan, if the number of planning centers is one, proceeding to S021; if the number of planning centers is more than one, proceeding to S022; S021, dividing the large-scale transfer of the CyberKnife robot arm into segments, sorting the MUs assigned to each test point in each segment, and calculating the dose Di of each test point in the planning system or using the particle transport software based on the assigned field parameters and MUs; S022, calculate the dose Di of each test point using multiple centers in the plan; S03, determining whether the conventional accelerator radiotherapy plan is to execute a fixed field intensity modulation plan or a volumetric rotation intensity modulation plan. If the fixed field intensity modulation plan is to be executed, then go to S031; if the volumetric rotation intensity modulation plan is to be executed, then go to S032; S031, sorting the test points according to the field MU, comparing the field number with the number of test points, if the field number is less than the test points, excluding the test points that are not assigned to the field, and calculating the dose Di of each test point with the assigned field MU; if the field number is greater than the test points, all the fields are assigned to all the test points, and calculating the dose Di of each test point with the assigned field MU; S032, split the complete arc into half arcs, consider the incomplete arc as a half arc, compare the number of half arcs with the number of points to be tested, if the number of half arcs is less than the number of points to be tested, exclude the points to be tested that are not assigned to half arcs, and calculate the dose Di of each point to be tested with the assigned half arc MU; if the number of half arcs is greater than the number of points to be tested, assign all half arcs to all points to be tested, and calculate the dose Di of each point to be tested with the assigned half arc MU.

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