Rapid sclera external applicator dose calculation method based on Monte Carlo simulation

Abstract

The invention discloses a method for rapidly calculating the dose of an external sclera applicator based on Monte Carlo simulation, which aims at the external sclera applicator loaded with 125I particles, adopts a high-resolution and high-precision Monte Carlo simulation strategy, quantitatively studies the influence of interded effect on the dose, and provides a method for rapidly calculating the dose of the external sclera applicator loaded with 125I particles. The rationality and reliability of the method for obtaining the dose of any particle number and activity combination based on single-particle high-precision dose data linear operation are verified. Compared with the limitations that a traditional TG-43 dose calculation formula is insufficient in calculation precision in a non-aqueous medium and a Monte Carlo method is high in precision but long in consumed time, the method has the advantages that the dose calculation time is greatly shortened to about 100ms, and meanwhile, the clinical precision is ensured. A clinician can quickly obtain a high-precision estimated treatment dose field result according to the actual size and position of a tumor of a patient, and then adjust parameters such as particle activity and applicator position, so that optimized iteration of a treatment plan is realized, and the safety and accuracy of intraocular tumor close-range radiotherapy are remarkably improved.

Description

Technical Field

[0001] The present invention belongs to the field of multidisciplinary cross-technology, involving computer science and technology, nuclear physics, medical imaging and radiotherapy, and specifically relates to a Monte Carlo simulation-based method for rapid calculation of the dose of an episcleral applicator for brachytherapy of intraocular tumors. Background Art

[0002] Melanoma, the most common primary intraocular malignancy, originates in the uveal layer, which is composed of the choroid, ciliary body, and iris from front to back. Although melanoma commonly occurs on areas of skin exposed to sunlight (such as the arms, back, face, and legs), the uveal layer of the eye contains melanin-producing cells and is therefore also at risk for this cancer. Therefore, it is also called uveal melanoma or choroidal melanoma. These tumors often occur in areas such as the back of the eye that are difficult to see with a regular mirror, making early detection more difficult.

[0003] Historically, enucleation was the main treatment. With the development of medical technology, a variety of treatment options have been formed, including photon-based external beam radiotherapy (EBRT, such as megavoltage linear accelerator therapy, gamma knife therapy), proton or helium ion-based charged particle external beam radiotherapy, and intraocular brachytherapy. Studies have shown that proton therapy performs poorly in clinical efficacy compared to brachytherapy; the 5-year survival rate of patients receiving applicator radiotherapy or enucleation is similar (82% vs. 81%), but applicator brachytherapy can preserve the eyeball and may also preserve vision. Brachytherapy for intraocular tumors was first used in the 1930s using the radioactive element radon. Applicator particle brachytherapy is an important method for treating eye cancer, using low-dose rate (LDR) photon-emitting radionuclides such as 125 I. 103 Pd, 131 Cs or use beta-emitting radionuclides such as 106 Ru / 106 Rh, 90 Sr / 90 Y's ocular applicator brachytherapy has been established as the standard treatment for intraocular tumors. Typically, a dose of 85 to 100 Gy is delivered to the tumor tip while maintaining a uniform tumor dose and sparing organs at risk (OARs).

[0004] Due to the special anatomical structure of the eyeball, its outer diameter is approximately 25 mm, which makes the dose distribution extremely sensitive to the basic assumptions of dose calculation. Therefore, accurate eye applicator dose calculation is crucial for predicting the location and probability of potential side effects. However, currently, neither the U.S. Food and Drug Administration (FDA) nor the European Conformity Certification (CE) has approved a commercial software package that can be used to calculate the dose of photon and beta-emitting applicators taking into account the characteristics of the applicator and the patient's non-aqueous tissue. This fully reflects the complexity of small area dose calculation in real brachytherapy scenarios. In the implementation of applicator particle brachytherapy, how to accurately control the duration of radiotherapy so that the radiation dose can effectively inhibit tumor growth without causing excessive radiation damage to key structures around the eyeball (such as the optic nerve) remains an important problem that needs to be solved.

[0005] The dose calculation formula of the American Association of Physicists in Medicine (AAPM) Task Group 43 (TG-43) is a fast method commonly used for brachytherapy seed implant dose calculations. However, this formula is only applicable to dose deposition calculations in pure water media, which deviates significantly from actual clinical conditions. Given the high atomic number (high Z) characteristics of the applicator housing material, the Monte Carlo simulation dose determination method has been recognized by the AAPM and the American Brachytherapy Society as the most accurate method for calculating applicator dose distributions. However, the Monte Carlo method has the inherent disadvantage of low computational efficiency. Acquiring data that meets clinical accuracy requirements typically takes several days, which severely limits its application in the rapid development of clinical radiotherapy plans. Summary of the Invention

[0006] In view of the problems existing in the prior art, the present invention aims to propose a method for rapid calculation of the dose of an episcleral applicator based on the Monte Carlo method, so as to help clinicians efficiently formulate radiotherapy plans and improve the safety and effectiveness of treatment, thereby overcoming the technical bottlenecks of the long calculation time of the high-precision Monte Carlo method and the insufficient calculation accuracy of the TG-43 formula.

[0007] Aspect 1: Quantitative analysis of inter-particle effects

[0008] The present invention first quantitatively studies the impact of the interseed effect on the total dose field of the scleral applicator: three different sizes of scleral applicators are taken, namely, diameter 17mm (18 slots), diameter 19mm (20 slots), and diameter 21mm (24 slots). Through Monte Carlo simulation, the following two situations are simulated respectively: ① the dose field distribution when each slot is individually loaded with particles; ② the dose field distribution when all slots are fully loaded with particles (in this case, there is an attenuation effect of the interseed effect on the dose field). The simulation data of situation ① are linearly superimposed to obtain the dose field distribution when all slots are fully loaded with particles when there is no interseed effect (referred to as situation ③). By comparing the dose field data of situation ② and situation ③, the results show that the degree of attenuation of the total dose field of the scleral applicator by the interseed effect is less than or equal to 1%. In clinical applications, the error control of less than or equal to 2% is generally considered to be within the acceptable range. The quantitative analysis results provide a solid theoretical basis for the subsequent proposed rapid dose calculation method.

[0009] The second aspect: dose calculation process

[0010] Based on the above quantitative analysis conclusions, the present invention firstly compares the three sizes of scleral episcleral applicators, 125 I particles and the simulation phantom are physically modeled. The Monte Carlo dose element di is then obtained: the Monte Carlo simulation dose field of each slot of the applicator loaded with particles in sequence, where i is the slot number. The eye phantom coordinate system (X, Y, Z) and the applicator coordinate system (x, y, z) are then determined to facilitate the determination of the radiotherapy position of the applicator within the eye phantom. Based on the treatment plan generated by the clinician, the size of the applicator to be used, the relative position of the applicator and the eye phantom, the activity of the particles loaded in each slot of the applicator of this size (the activity of the slot without particles is considered to be 0), and the position of the highest point of the tumor, APEX (X, Y, Z), are determined. According to the recommendations of the COMS (Collaborative Ocular Melanoma Study), a dose of 85 Gy needs to be deposited at the highest point of the tumor. The above data are used to quickly calculate the total dose field D and treatment time T under the clinician's treatment plan. Finally, the clinician can iteratively optimize the treatment plan based on the dose field D and treatment time T to improve the safety and efficiency of treatment.

[0011] The third aspect: dose calculation tools

[0012] The present invention provides a dose calculation tool for a Monte Carlo simulation-based rapid dose calculation method for episcleral applicators. The tool comprises a radiotherapy plan input module, which allows clinicians to select episcleral applicators of different sizes based on actual treatment needs and accurately input key parameters such as the activity of particles loaded in each slot, providing accurate baseline data for subsequent dose calculations. A rapid dose calculation module, based on the input clinical treatment plan, calculates the dose field D for the entire eye phantom and the required treatment time T for an 85 Gy dose deposited at the highest point of the tumor in a very short time (approximately 100 milliseconds). This module significantly improves computational efficiency while maintaining accuracy and precision comparable to traditional Monte Carlo methods (which require over 36 hours of computation time). A dose curve module, based on the calculated total dose field D, visually displays isodose curves for the applicator's central axis section (y = 0 mm) and the dose distribution along the central axis. By observing these curves, clinicians can quickly assess the safety of the treatment plan and optimize and adjust potential risk points, further improving the safety and effectiveness of treatment.

[0013] The beneficial effects of the present invention are as follows:

[0014] Through in-depth research on interparticle effects and innovative computational method design, this invention significantly reduces dose calculation time from over 36 hours to approximately 100 milliseconds, while retaining the high precision and accuracy of Monte Carlo simulation. This breakthrough enables clinicians to adjust treatment plans in real time based on dose calculation results, achieving precise control of radiotherapy duration. This effectively inhibits tumor growth while minimizing radiation damage to critical organs around the eye. This significantly improves the safety, precision, and efficacy of episcleral applicator brachytherapy, providing strong technical support for brachytherapy of eye cancer.

[0015] It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for the specific embodiments or the description of the prior art. Obviously, the drawings in the following description are Figure 1 This is an implementation mode of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0017] Figure 1 This is a flow chart of a Monte Carlo-based method for rapidly calculating the dose of an intrascleral applicator with high precision and accuracy according to an embodiment of the present invention;

[0018] Figure 2 This is the parameter input interface for the dose calculation method for the 19mm diameter applicator of the present invention. On the left, enter the activity value for each particle individually. On the right, select the dose required to achieve 85Gy at the highest point of the tumor. Click Calculate Dose to start the rapid dose calculation for the treatment plan.

[0019] Figure 3 The dose calculation results for a treatment plan of the 19 mm diameter applicator of the present invention are shown in the right figure, where the isodose curves at the central axis section / y=0 mm plane are shown;

[0020] Figure 4 The dose calculation results for a treatment plan using a 19 mm diameter applicator of the present invention are shown in the figure on the right, where the dose distribution on the central axis is shown.

[0021] Figure 5 This is the treatment time calculation result of a treatment plan for the 19 mm diameter applicator of the present invention.

[0022] Figure 6 This is a schematic diagram of the actual size and shape of the 19mm diameter applicator of the present invention. The silver-white center is the outer lining of the applicator, which is used to shield the dose of particles emitted toward the convex surface; the right side is the silicone inner support, and the convex surface has grooves for loading radioactive particles. DETAILED DESCRIPTION

[0023] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings. The described embodiments are only part of the embodiments of the present invention, rather than all of the embodiments.

[0024] First embodiment:

[0025] Figure 1 FIG. 1 is a flow chart of a Monte Carlo-based method for rapidly calculating the dose of an intrascleral applicator according to an embodiment of the present invention. Figure 1 As shown, the method includes the following 5 steps:

[0026] Step 1: Accurate 3D geometric modeling of the three sizes of intrascleral applicators: 17 mm (18 slots), 19 mm (20 slots), and 21 mm (24 slots); 125 I particles were physically modeled using the most commonly used Amersham Health 6711 phantom; the eye phantom, region of interest (ROI), and head phantom were modeled to simulate the radiation physics properties of human tissue.

[0027] Step 2: Load each slot of the applicator individually 125I particles were used for Monte Carlo simulation. In the simulation process, to ensure the accuracy of calculation, the voxel size of the eyeball phantom and the region of interest (ROI) was set to 0.3mm. 3 , the voxel size of the head model is 0.5mm 3 To meet the requirements of high-precision spatial resolution; by setting the number of simulated photons to N = 10 12 , effectively controlling the statistical error of each voxel to below 0.02%, meeting the high accuracy requirement. In this way, high-precision and high-accuracy Monte Carlo dose metadata d i , i is the slot number of the applicator.

[0028] Step 3: Determine the eye phantom coordinate system (X, Y, Z) and the applicator coordinate system (x, y, z) to facilitate the subsequent determination of the applicator radiotherapy position. The eye phantom coordinate system uses the center of the eye as the coordinate origin. The X-axis passes through the fovea and the center of the lens, with the positive direction pointing toward the lens; the Z-axis points vertically to the top of the eye; the Y-axis follows the right-hand rule, pointing from the left to the right of the observer and is independent of the left or right side of the eye. The origin of the applicator coordinate system is set at the intersection of the applicator's central axis and the inner sclera (i.e., the point where the deepest part of the applicator contacts the eye), the z-axis runs along the applicator's central axis, from the particle collection to the center of the eye; the x-axis points away from the suture lug; and the y-axis is determined by the cross product of the z-axis and the x-axis. A three-dimensional rotation matrix is ​​used to transform the coordinates of the eye (which encompasses structures such as the lens, retina, lacrimal gland, fovea, and optic disc) to the applicator coordinate system. This matrix is ​​constructed by aligning the eye's x-axis with the applicator's z-axis. Based on the specific location of the tumor, clinicians determine the applicator's placement coordinates (X, Y, Z) within the eye phantom coordinate system, typically aligning the applicator's innermost point closely with the eye's surface.

[0029] Step 4: Based on the treatment plan developed by the clinician, determine the size of the applicator used, the relative position of the applicator to the eye phantom, the activity of the particles loaded in each slot (the activity of the slot without particles is set to 0), and the location of the highest point of the tumor, APEX (X, Y, Z). According to the recommendations of the COMS (Collaborative Ocular Melanoma Study), a dose of 85 Gy needs to be deposited at the highest point of the tumor. Based on this information, the total dose field D under this treatment plan can be quickly calculated using the following formula:

[0030]

[0031] a i =A i / A total ,

[0032] A iis the activity of each particle in the treatment plan;

[0033] n is the total number of particles loaded as planned by the doctor;

[0034] di is the Monte Carlo dose element obtained in step 1;

[0035] D target It is the dose value prescribed by the doctor at the highest point of the tumor APEX (X, Y, Z) in the treatment plan (usually 85Gy). is the dose value at this location obtained by linear calculation.

[0036] The total dose field D can accurately reflect the dose distribution of the entire eyeball phantom, the region of interest phantom, and the head phantom when the treatment plan deposits a dose of 85 Gy at the highest point of the tumor, which are recorded as D 眼球 、D ROI 、D 头 The computation speed of this method is much faster than that of the Monte Carlo method. The linear computation process in this method only takes about 100ms, while the Monte Carlo method requires N=10 simulations. 9 When measuring particles, it takes up to 36 hours to achieve higher accuracy requirements.

[0037] When calculating the treatment time T, first calculate the total number of photons N required for the corresponding treatment plan based on the total dose field D. total :

[0038] N total =N*RATIO

[0039] The dose unit di obtained in step 1 is the Monte Carlo simulation setting 10 12 The number of photons is I 125 The particle source decays and emits a total of N = 10 12 The dose field deposited by photons is

[0040] The time required to calculate particle decay is calculated using the formula given by AAPM Task Group 43 (TG-43):

[0041]

[0042] λ is I 125 The decay constant of

[0043] 3.7×10 7 It is the conversion factor that converts the activity from mCi to Bq units.

[0044] The required radiotherapy time T can be accurately calculated using the above parameters.

[0045] Step 5: Clinicians can iteratively optimize the treatment plan based on the calculated dose field D and treatment time T. By analyzing the dose distribution of the eye, region of interest, and head phantoms, combined with the isodose curves of the applicator's central axis section (y = 0 mm plane) and the dose distribution along the central axis, they can adjust parameters such as particle activity and applicator position to develop a safer and more effective treatment plan.

[0046] Finally, it should be noted that the above-described embodiments are only specific implementation methods of the present invention, which are used to illustrate the technical solutions of the present invention, rather than to limit them. The scope of protection of the present invention is not limited thereto. Although the present invention has been described in detail with reference to the above-described embodiments, those skilled in the art should understand that any person skilled in the art can modify or easily conceive of changes to the technical solutions described in the above-described embodiments within the technical scope disclosed by the present invention, or replace some of the technical features therein with equivalents. Such modifications, changes, or replacements do not deviate from the spirit and scope of the technical solutions of the embodiments of the present invention, and should be included in the scope of protection of the present invention. Therefore, the scope of protection of the present invention shall be subject to the scope of protection of the claims.

Claims

1. A method for rapid calculation of episcleral applicator dose based on Monte Carlo simulation, characterized in that: The following steps are involved: Three different sizes of scleral applicators (diameter 17mm, 19mm, 21mm, corresponding to 18 slots, 20 slots, 24 slots respectively), 125 I particles and simulation phantoms are physically modeled; each slot of the applicator is loaded separately 125 Monte Carlo simulation is performed using I particles to obtain high-precision and high-accuracy Monte Carlo dose metadata. The eye phantom coordinate system (X, Y, Z) and the applicator coordinate system (x, y, z) are established separately, and the coordinate conversion between the two is achieved using a three-dimensional rotation matrix. Based on the treatment plan formulated by the clinician, the applicator size, its relative position with the eye phantom, the activity of the particles loaded in each slot, and the location of the highest point of the tumor are determined, and the total dose field D and treatment time T are quickly calculated. Clinicians iteratively optimize the treatment plan based on the calculated dose field D and treatment time T.

2. The method according to claim 1, characterized in that The physical modeling includes: 1:1 high-precision modeling of the inner lining and outer support of the scleral episcleral applicator, accurately setting the material density and composition to be consistent with actual physical properties; 125 I particles use the real physical model of Amersham OncoSeed 6711, with modeling accuracy reaching micron level; the relative position of the center of the inner support notch is determined by the inner support model and design drawings. 125 The I seed model is precisely placed in the inner bracket; at the same time, the eye phantom, region of interest (ROI) phantom and head phantom are modeled to simulate the radiation physical properties of human tissue.

3. The method according to claim 1, characterized in that The Monte Carlo simulation to obtain dose metadata includes: setting the voxel size of the eyeball phantom and the ROI phantom to 0.3mm 3 , the voxel size of the head model is 0.5mm 3 ; Set the number of simulated photons to 10 12 , so that the uncertainty of the dose of each voxel in the eye and ROI in the obtained dose unit is less than 0.02%.

4. The method according to claim 1, wherein The eyeball phantom coordinate system has the center of the eyeball as its origin, the axis passes through the fovea and the center of the lens with the positive direction pointing toward the lens, the axis points vertically to the top of the eyeball, and the axis points from left to right of the observer following the right-hand rule; the origin of the applicator coordinate system is located at the intersection of the applicator central axis and the inner sclera, the axis points from the particle collection to the center of the eyeball along the applicator central axis, the axis points away from the suture lug, and the y-axis is determined by the cross product of the z-axis and the x-axis.

5. The method according to claim 1, wherein The total dose field D is given by the formula Calculate: a i =A i / A total , A i is the activity of each particle in the treatment plan; n is the total number of particles planned by the doctor; di is the Monte Carlo dose element obtained in step 1; D target It is the dose value prescribed by the doctor at the highest point of the tumor APEX (X, Y, Z) in the treatment plan (usually 85Gy). is the dose value at this location obtained by linear calculation.

6. The method according to claim 1, wherein The treatment time T is calculated according to the formula Calculated: N total =N*RATIO, N is the calculated dose element d i The total number of photons set when N = 10 12 ; λ is I 125 The decay constant is 3.7×10 7 It is the conversion factor that converts the activity from mCi to Bq units.

7. The method according to claim 1, characterized in that The iterative optimization includes: analyzing the dose distribution of the eyeball phantom, ROI phantom and head phantom, combining the dose curve of the applicator central axis section (y=0mm plane) and the central axis dose distribution, and adjusting the treatment plan parameters such as particle activity and applicator position.

8. A device for rapid calculation of episcleral applicator dose based on Monte Carlo simulation, characterized in that: include: Physical modeling module for episcleral applicators, 125 I particles and simulation models for physical modeling; Simulation calculation module for loading each slot of the applicator individually 125 I particle conducts Monte Carlo simulation to obtain Monte Carlo dose metadata; A coordinate system establishment module is used to establish the eye phantom coordinate system and the applicator coordinate system respectively, and realize the coordinate conversion between the two; The dose calculation module is used to calculate the total dose field D and treatment time T according to the treatment plan; the optimization module is used to iteratively optimize the treatment plan based on the dose field D and treatment time T.

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