医用加速器8MV X射线在水模体中的一阶散射X射线能谱的蒙特卡罗计算

Monte Carlo Calculation of Spectrum of the First-order Scattered X-rays Produced by Linac in Water Phantom

目的:分析医用电子直线加速器的高能X射线与水模体相互作用过程中所产生的一次散射光子的能谱角分布和光子强度角分布。方法:利用蒙特卡罗粒子输运程序Geant4,模拟粒子输运过程,计算加速器8 MeV高能X射线能谱,并根据在水模体中实际测量的PDD吸收曲线为依据,修正蒙特卡洛计算的能谱;并以此能谱为虚拟源能谱,通过对X射线与水模体相互作用后的光子-电子联合输运过程进行蒙特卡罗模拟的方法获取有关散射X线能谱数据。结果:用蒙特卡洛方法得到加速器8 MV初始X射线与水模体作用产生的一次散射光子的散射光子强度和散射光子能量随散射角度变化的规律。结论:根据ICRP85出版物、ICRU44报告给出的数据,可以用组织平均原子序数作为组织等效原子序数;因此,组织密度变化在物理上反映了组织的原子密度的变化,当入射光子注量改变,模体密度变化时,仅引起相互作用的总截面相对于原子微分截面的线性变化,并不影响一阶散射X射线的散射光子的相对强度角分布和散射光子能量角分布,而散射光子发射的绝对量与初始X射线强度、组织的原子密度成正比。因此,一次散射光子的注量角分布、平均能量角分布结果可形成可调用的数据库,对快速蒙特卡洛计算很有意义。

Objective： To analyze spectrum angular distributions and intensity angular distributions of primary scattered photons produced in interactions between Linac X-ray and water. Methods： First, by Monte Carlo program ＂Geant 4＂ to simulate parti- cles transport processes, and calculate the spectrum of 8 MV X-ray, then, it would be used as the linac X-Ray virtual source after to being corrected on the basis of the actual measurements of PDD curves in water phantom for the Monte Carlo simulation to calculate the primary scattered photons. Results： We obtained the datasets about the spectrum angular distribution and intensity angular distribution of the primary scattered photons in water phantom by Monte Carlo method. Conclusions： According to the ICRP No.85 publication and the ICRU No.44 report, the tissue average atomic number can be used as the equivalent atomic number, therefore, changes of tissue density reflect the changes of the atomic density, and the changes of incident photons fluxes and phantom densities will only lead to linear changes of total interaction cross sections to the Atomic differential cross section, it don＇t affect the angular distribution of relative intensity and energy of the first-order X-ray scattered photons; but the absolute amount of the scattered photons emission is proportional with the initial X-ray intensities and the atomic densities of the medium. So, to studying the database about the angel distributions of the first-order scattered photons intensity and average energy is very significant for fast Monte Carlo calculations.