基于蒙特卡罗方法计算电子线在人体的剂量分布

Calculating the Dose Distribution of Electrons in Body Based on Monte Carlo Method

目的基于蒙特卡罗方法深度剖析医用电子直线加速器电子线束流特性,并系统分析该特性对人体剂量分布的影响。方法使用EGSnrc系列程序建立医用电子直线加速器6 MeV与9 MeV机头模型,通过实际测量数据验证模型的可靠性,分析电子束流能谱与能量注量成分差异;对两种射线源在水与软组织百分深度剂量(Percentage Depth Dose,PDD)与Profile的结果进行修正,探索一种快速计算电子线剂量的方法;分析电子线在软组织与肺组织模体下的PDD特性与CT图像上的剂量分布,评估电子线二维照射时对肺的损伤。结果6 MeV电子源产生的电子线平均能量约为4.2 MeV,X线平均能量约为0.8 MV,9 MeV电子源产生的电子线平均能量约为6.9 MeV,X线平均能量约为1.0 MV。在模体入射表面,射野范围外依然具有较强的散射电子与高能X射线。水模体与软组织修正因子在6 MeV与9 MeV电子源照射时最大值达1.15与1.17,且随深度变化较大。肺组织剂量跌落区为软组织的3~4倍,同时交界面会发生轻微的剂量跃变,肺组织会形成高剂量区。结论蒙特卡罗程序进行电子线剂量计算时在个性化分析与精确度方面具有一定优势,在快速估算电子线照射的射野参数时应充分考虑修正因子,降低X线污染对于正常组织的影响,对于大野电子线照射时应当注重肺损伤评估。

Objective To analyze the electron beam characteristics of medical electron linear accelerator in depth based on Monte Carlo method,and to analyze the influence of the characteristics on the dose distribution of human body systematically.Methods The 6 MeV and 9 MeV head models of medical electron linac were established by using EGSnrc series program.The reliability of the models was verified by the actual measurement data,and the differences of electron beam energy spectrum and energy flux components were analyzed.By modifying the two radiation sources in the water and soft tissue percentage depth dose(PDD)and Profile results,a fast method of calculating the dose of the electronic beam was proposed.The PDD characteristics of electron beams under soft tissue and lung tissue phantoms and the dose distribution on CT images were analyzed,and the damage to the lungs during two-dimensional electron beam irradiation was evaluated.Results The average energy of the electron rays produced by 6 MeV electron source was about 4.2 MeV,the average energy of X-ray was about 0.8 MV,the average energy of electron rays produced by 9 MeV electron source was about 6.9 MeV,and the average energy of X-ray was about 1.0 MV.On the incident surface of the phantom,there were still strong scattered electrons and high-energy X-rays outside the range of the radiation field.The maximum value of the water phantom and soft tissue correction factor was 1.15 and 1.17 when irradiated by 6 MeV and 9 MeV electron sources,and it varied greatly with depth.The lung tissue dose drop area was about 3-4 times that of the soft tissue,and there would be a slight dose jump at the interface,and the lung tissue would form a high dose area.Conclusion Monte Carlo program has certain advantages in personalized analysis and accuracy in electron ray dose calculation.Correction factors should be fully considered in rapid estimation of radiation field parameters of electron ray radiation to reduce the impact of X-ray pollution on normal tissues.Lung injury assessment should be emphasized in on electron ray radiation.