A hybrid dose calculation method based on electron energy deposition kernels

Abstract

The application discloses a mixed dose calculation method based on an electron energy deposition kernel, Monte Carlo simulation is adopted for photons, the collision and transportation process is simulated in detail, different methods are adopted according to the energy of electrons, the dose distribution of low-energy and medium-energy electrons with energy lower than a threshold value in space is calculated by using a pre-calculated energy deposition kernel, and Monte Carlo transportation is carried out for high-energy electrons, and the energy deposition kernel is used when the energy of the high-energy electrons is reduced to the threshold value. The setting of the energy threshold value in the application influences the precision and speed, the precision is high and the simulation speed is slow when the threshold value is low, the speed is fast and the precision is lower when the threshold value is high, a low energy threshold value is set in a region with relatively high precision requirement, and a high energy threshold value is set in a region with less importance, so that the Monte Carlo-analytical coupling method of the region mixing is realized, and the calculation efficiency is improved while the precision is ensured.

Description

TECHNICAL FIELD

[0001] The present application relates to dose calculation of photon and electron radiotherapy, in particular to a hybrid dose calculation method based on electron energy deposition kernel. BACKGROUND

[0002] Dose calculation is a fundamental key in radiotherapy, and accurate dose distribution in human body is needed for radiotherapy planning. The dose calculation method in clinic can be divided into two categories, one is analytical method, including pencil beam algorithm, point kernel method, etc., and the other is Monte Carlo (MC) method. The analytical method decomposes the treatment radiation into multiple pencil beams or differential beams (point kernel method), and the dose distribution of each beam is calculated in advance, and the total dose distribution is obtained by superimposing the dose distribution of each beam; the MC method tracks the transport process of multiple radiation particles in detail, and obtains the dose distribution by using statistical method. For the most common X-ray radiotherapy in radiotherapy, MC dose calculation needs to simulate the transport and energy deposition process of source photons, contaminant electrons and secondary electrons and photons in the phantom. Compared with the analytical method, the MC method is more accurate, but in order to obtain a result with low statistical error, a large number of particles need to be simulated, which leads to slow calculation and greatly restricts its application.

[0003] X-ray belongs to indirect ionizing radiation, and needs to transfer energy to the medium through interaction with the medium to produce secondary electrons, so the electron transport calculation must also be carried out in photon radiation dose calculation. However, electrons are charged particles, and the Coulomb effect leads to frequent collisions, so the transport process is complex, and even if the process is simplified, the MC simulation is still time-consuming. In contrast, photons have fewer collisions and simpler transport process, so the simulation efficiency is high, and the transport simulation of electrons in photoelectron MC dose calculation often consumes most of the time. Although the transport process of electrons is complex, its range is limited, and its penetration ability is weaker than that of photons with the same energy, i.e. the dose distribution range is limited, for example, the CSDA range of 1 MeV electron in water is only 4.367 mm, and its energy deposition is also basically within this range, and the energy of secondary electrons produced in 6MV X-ray radiotherapy is also basically lower than this energy, so for such low-energy electrons, the detailed transport process can not be simulated in dose calculation, and the energy deposition distribution of each energy electron is obtained by pre-calculation, which can greatly improve the simulation efficiency. For high-energy electrons such as 10 MeV or more, the efficiency improvement is limited due to the large dose range, and the non-uniformity effect is more significant, so MC simulation is still used, and only when the energy is reduced to below the threshold value, the energy deposition kernel is used. SUMMARY

[0004] Invention purposes: The purpose of the present application is to provide a mixed dose calculation method based on electron energy deposition kernel, so as to realize the regionally mixed Monte Carlo-analytical coupling method, and improve the calculation efficiency while ensuring the accuracy.

[0005] Technical scheme: The mixed dose calculation method based on electron energy deposition kernel comprises the following steps:

[0006] (1) establishing a database;

[0007] (1.1) dividing the energy range of electrons into low and high energy according to the electron range;

[0008] (1.2) pre-calculating the energy deposition distribution of multiple energy electrons in water in the low and medium energy section, i.e. energy deposition kernel;

[0009] (2) determining the energy threshold of each calculation region according to the energy of the ray and the dose calculation requirement;

[0010] (3) performing dose calculation;

[0011] (3.1) for the source particles, if they are photons, the conventional Monte Carlo simulation is performed, i.e. sampling the collision distance, performing transport and then collision, sampling the reaction type and secondary particles according to the reaction cross section;

[0012] (3.2) for electrons, whether they are source electrons or secondary electrons, if their energy is higher than the threshold, the Monte Carlo transport is performed until their energy is reduced to below the threshold, and if the energy is lower than the threshold, the pre-calculated energy deposition kernel is used to calculate the dose distribution thereof;

[0013] (4) simulating a large number of source particles, and statistically calculating the dose distribution, if the specified number of simulated particles is reached or the uncertainty of the result is reduced to below the target, the calculation is stopped.

[0014] In the step (1.1), the energy threshold of the electrons is divided, the dose distribution of the low and medium energy electrons is calculated by calling the deposition kernel, and the high energy electrons are subjected to Monte Carlo transport; by controlling the threshold value, the calculation precision and speed can be further balanced, the lower the threshold value, the higher the precision, and the higher the threshold value, the faster the calculation speed.

[0015] In the step (1.2), the pre-calculated energy deposition kernel is the distribution in water, denoted as EK0(E,r), and the energy deposition distribution in other density media in the actual dose calculation can be obtained from the energy deposition kernel in water by using O'Connor theorem; if the electron density of the medium relative to water is ρ, the energy deposition distribution thereof is EK ρ (E,r')=ρ 2 EK0(E,r), wherein

[0016] In the step (2), different energy thresholds are selected for different media, a low threshold is set in a region with high precision requirement, and is mainly obtained by Monte Carlo simulation; a high threshold is set in a region with low precision requirement, and is mainly obtained by an analytical energy deposition kernel, so that the region mixing of Monte Carlo and analytical methods is realized.

[0017] In the step (2), in a low-density medium (such as lung and cavity), because the electron range is longer than that in water, a lower threshold is selected for a low-density region when calculating the dose of the lung, so as to ensure the accuracy of dose calculation in the low-density medium.

[0018] In the step (3.2), a cutoff energy is set, that is, the energy deposition kernel of an electron with a CSDA range less than 1 mm is a delta function, and the energy deposition of such an electron is local, so that the calculation of electron energy deposition can be accelerated.

[0019] A computer storage medium has a computer program stored thereon, and the computer program is executed by a processor to implement the above-mentioned hybrid dose calculation method based on an electron energy deposition kernel.

[0020] A computer device includes a storage, a processor, and a computer program stored on the storage and executable on the processor, and the processor implements the above-mentioned hybrid dose calculation method based on an electron energy deposition kernel when executing the computer program.

[0021] Advantages: Compared with the prior art, the present application has the following advantages: the present application uses Monte Carlo simulation for photons, and simulates the collision and transport process in detail; for electrons, different methods are adopted according to the energy, the dose distribution of low-energy and medium-energy electrons with energy lower than a threshold is calculated by using a pre-calculated energy deposition kernel, and high-energy electrons are transported by Monte Carlo, and the energy deposition kernel is used when the energy is reduced to the threshold, so that a region-mixed Monte Carlo-analytical coupling method is realized, the advantages of the two methods can be combined, the calculation efficiency is improved while the accuracy is ensured. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 is a step flow chart of the method of the present application;

[0023] Figure 2 is a process schematic diagram of the method of the present application; the square block in the figure is a phantom range, each straight line segment is a photon trajectory, the intersection of the broken line segments is a collision of photons, the curved broken line is a high-energy electron trajectory, and the oval range is a simple schematic diagram of an electron energy deposition kernel;

[0024] Figure 3is a schematic diagram of the direction of particles after collision of photons with secondary electrons in a medium, the incident photon is along the z-axis direction, the collision occurs at the origin, and the particle velocities after collision are v1 and v2, respectively. DETAILED DESCRIPTION

[0025] The technical solutions of the application will be further described below with reference to the drawings.

[0026] As Figures 1-2 shown, a hybrid dose calculation method based on electron energy deposition kernels includes the following steps:

[0027] (1) Establish a database;

[0028] (1.1) Divide the energy range of electrons into low and high energy according to the electron range;

[0029] (1.2) Pre-calculate the energy deposition distribution of multiple energy electrons in water in the low and medium energy section, i.e. energy deposition kernels;

[0030] (2) Determine the energy threshold of each calculation region according to the energy of the radiation source and the dose calculation requirements;

[0031] (3) Perform dose calculation;

[0032] (3.1) For the source particles, if they are photons, perform conventional Monte Carlo simulation, i.e. sample the collision distance, perform transport and then collision, and sample the reaction type and secondary particles according to the reaction cross section;

[0033] (3.2) For electrons, whether they are source electrons or secondary electrons, if their energy is higher than the threshold, perform Monte Carlo transport until their energy decreases below the threshold, if their energy is lower than the threshold, use the pre-calculated energy deposition kernels to calculate their dose distribution;

[0034] (4) Simulate a large number of source particles and statistically analyze the dose distribution, if the specified number of simulated particles is reached or the uncertainty of the results is reduced below the target, stop the calculation.

[0035] Embodiment:

[0036] The CSDA range of electrons in water at each energy is shown in Table 1, assuming that this method is used for dose calculation of head and neck or abdomen, the electron threshold can be selected as 4.0 MeV, for chest cases, because the density of lung is lower, the electron range is longer, so a lower threshold of 1.5 MeV is selected for lung and its vicinity.

[0037] Table 1 CSDA range of electrons in water at each energy

[0038]

[0039]

[0040] The transport of electrons with energy less than threshold energy in water is simulated to get the energy deposition distribution, and a library of deposition kernels is made for look-up table. For electron energy less than 0.4 MeV, its range is less than 1.3 mm, so the cut-off energy is defined as 0.4 MeV, and for energy less than this value, the deposition kernel is considered as a delta function, i.e. all the energy is deposited locally.

[0041] When dose calculation is performed, for a source particle, if it is a photon, the conventional Monte Carlo simulation is performed, i.e. sampling the collision distance, transporting until collision, and sampling the reaction type and secondary particles according to the reaction cross section. For an electron, whether it is a source electron or a secondary electron, if its energy is higher than the threshold value, the Monte Carlo transport is performed until its energy is reduced to below the threshold value, and if its energy is below the threshold value, the pre-calculated energy deposition kernel is used to calculate its dose distribution.

[0042] When the electron energy deposition kernel is called, the difference of medium density needs to be considered. The pre-calculated energy deposition kernel in water is denoted as EK0(E,r), and in dose calculation, if the position of the calculation point relative to the electron is r, and the average electron density of the medium on the path from the electron position to the calculation point relative to water is p, then the energy deposition distribution of the electron at the calculation point is EK ρ (E,r') = p 2 EK0(E,r), where

[0043] In the collision simulation process of Monte Carlo transport, secondary particles are produced, Figure 3 is a simple and general example, and the angle θ1 between the secondary particle and the incident particle in these processes is called the scattering angle, which satisfies the distribution of the differential cross section, but the azimuth angle is an average distribution, and if only one secondary particle is sampled, the distribution of the secondary particle has a high degree of directionality, so that multiple original particles need to be simulated to get a more average result. Therefore, for the collision process that produces secondary electrons, such as photoelectric reaction, multiple groups of particles can be produced by splitting, denoted as n groups, and the azimuth angle of each group of particles is The weight of each secondary particle is 1 / n of the original photon. In this way, a single collision process can get a result with lower uncertainty, reducing the number of simulated source particles.

Claims

1. A method of mixed dose calculation based on electron energy deposition kernels, characterized in that, The method comprises the following steps: (1) establishing a database; (1.1) dividing the energy range of electrons into low and high energy according to electron range; (1.2) pre-computing the energy deposition distribution of multiple energy electrons in water, i.e. energy deposition kernel, in the low and medium energy section; (2) determining the energy threshold of each calculation region according to the energy of the rays and the dose calculation requirements; (3) performing dose calculation: (3.1) for radioactive source particles, if they are photons, performing conventional Monte Carlo simulation, i.e. sampling collision distance, performing transport and then collision, sampling reaction type and secondary particles according to reaction cross section; (3.2) for electrons, whether they are source electrons or secondary electrons, if their energy is higher than the threshold, performing Monte Carlo transport until their energy is reduced to below the threshold, and if their energy is lower than the threshold, using the pre-computed energy deposition kernel to calculate their dose distribution; (4) simulating a large number of source particles and statistically calculating the dose distribution, and stopping the calculation when the specified number of simulated particles is reached or the uncertainty of the result is reduced to below the target.

2. A hybrid dose calculation method based on electron energy deposition kernels according to claim 1, wherein, In the step (1.1), the energy threshold is divided for electrons, the deposition kernel is called to calculate the dose distribution for low and medium energy electrons, and Monte Carlo transport is performed for high energy electrons.

3. The hybrid dose calculation method based on electron energy deposition kernel according to claim 1, wherein, In the step (1.2), the pre-calculated energy deposition kernel is denoted as The energy deposition distribution in other density medium in the actual dose calculation can be obtained from the energy deposition kernel in water using O'Connor's theorem; if the electron density of the medium relative to water is p, then its energy deposition distribution is where .

4. The hybrid dose calculation method based on electron energy deposition kernel according to claim 1, wherein, In the step (2), different energy thresholds are selected for different media, a low threshold is set in a region with high accuracy requirement and is obtained by Monte Carlo simulation, and a high threshold is set in a region with low accuracy requirement and is obtained by analytical energy deposition kernel, thereby realizing regional mixing of Monte Carlo and analytical methods.

5. The method of claim 1, wherein the method is based on a hybrid dose calculation method of electron energy deposition kernels, and In the step (2), in a low-density medium, the electron range is longer than in water, and a lower threshold is selected for the low-density region when calculating the dose of the lung, thereby ensuring the accuracy of dose calculation in the low-density medium.

6. The hybrid dose calculation method based on electron energy deposition kernel according to claim 1, wherein, In the step (3.2), a cutoff energy is set, i.e. the energy deposition kernel of an electron with a CSDA range less than 1.3 mm is a delta function, and the energy deposition of such an electron is local, thereby accelerating the calculation of electron energy deposition.

7. A computer storage medium having stored thereon a computer program, characterized in that The computer program is executed by a processor to implement a hybrid dose calculation method based on an electron energy deposition kernel as claimed in any one of claims 1-6.

8. A computer device comprising a memory, a processor, and a computer program stored on the memory and executable on the processor, characterized in that, The processor executes the computer program to implement a hybrid dose calculation method based on an electron energy deposition kernel as claimed in any one of claims 1-6.

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