Research on Monte Carlo calculation method for photon absorbed dose
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摘要: 为了能够通过选择合适的计算方法对不同辐射情况的光子剂量进行准确有效的计算和评估，对比研究了蒙特卡罗模拟软件中常用的四种光子剂量计算方法：剂量转换系数法、发热数法、径迹长度能量沉积法和脉冲高度计数法，从计算原理出发结合仿真结果分析比较了这四种方法。通过模拟不同能量的单能光子束入射不同体积大小的水球，分析了用比释动能近似吸收剂量导致的误差，并且模拟分析了不同含量的高原子序数元素钆对于吸收剂量计算结果差异的影响。由于转换系数是根据参考人模计算得到的，因此剂量转换系数法只能对吸收剂量进行快速的估计计算，很难在特定情况下得到精准剂量；当高能量光子入射小体积物质时，比释动能会大于吸收剂量，使用发热数法和径迹长度能量沉积法会产生误差，采用脉冲高度计数法更加合适；在低能量和大体积时可以根据计算精度和计算机资源选择任意方法；当组成物质的高原子序数元素含量增多时，径迹长度能量沉积法和脉冲高度计数法之间的计算误差会减少。Abstract: To accurately and effectively calculate and evaluate photon doses under different radiation conditions by selecting appropriate calculation methods, this paper compares and studies four commonly used photon dose calculation methods in Monte Carlo simulation software: dose conversion coefficient method, heat number method, track length energy deposition method and pulse height method. Starting from the calculation principle and combining simulation results, these four methods are analyzed and compared. By simulating single energy photon beams with different energies incident on water spheres of different volume sizes, the error caused by approximating absorbed dose with kerma was analyzed, and the influence of different contents of high atomic number element gadolinium on absorbed dose calculation results was simulated and analyzed. Due to the fact that the conversion coefficient is calculated based on the reference human model, the dose conversion coefficient method can only quickly estimate and calculate the absorbed dose, it is difficult to obtain accurate dose in specific situations. When high-energy photons are incident on a small volume of material, kerma will be greater than the absorbed dose, thus the heat number method and the track length energy deposition method will produce errors, and the pulse height counting method is more suitable. At low energy and large volume, any method can be selected based on computational accuracy and computer resources. When the content of high atomic number elements in a substance increases, the calculation error between the track length energy deposition method and the pulse height method will decrease.
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Key words:
- photons /
- absorbed dose /
- Monte Carlo method /
- human phantom simulation
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图 2 X射线照射人体头模的模拟示意图
Figure 2. Simulation diagram of X-ray irradiation on human head model
图 3 四种方法计算大脑吸收剂量对比
Figure 3. Comparison of four methods for calculating brain absorbed dose
图 5 能量30 keV、60 keV和120 keV光子在头模内的运动径迹
Figure 5. 30 keV, 60 keV and 120 keV photon tracks in the head phantom
图 6 径迹长度能量沉积法和脉冲高度计数法在水球中的吸收剂量相对误差变化曲线
Figure 6. Curve of relative error of absorbed dose for track length energy deposition method and pulse height method in water sphere
表 1 钯－100源产生的X射线的能量与相对强度
Table 1. Energy and relative intensity of X-ray generated by palladium 100 source
energy/keV relative intensity energy/keV relative intensity 32.66 4.90 86.37 0.05 42.08 13.50 119.18 0.13 53.52 0.08 126.15 15.00 61.60 0.51 139.92 0.35 72.52 0.15 151.88 0.61 74.78 92.00 154.00 0.06 84.00 100.00 158.87 3.20 
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