医用加速器电子治疗模式的BEAM程序分析

Electron Beam Therapy Mode Analysis of Medical Linac by BEAM Code

医用电子直线加速器XHA9在水模体中的实测曲线显示出其标称能量低、表面剂量偏高等现象。为寻求解决途径,用蒙特卡罗程序BEAM对XHA9 7 MeV电子治疗模式E7建立了初始电子束模型,并计算了水模中的百分深度剂量(PDD)曲线,分析了各部件产生的电子对治疗电子束的贡献,确定限光筒产生的大量低能电子是导致出现标称能量低、表面剂量偏高等现象的主要原因。通过修改限光筒、散射箔等部件,分步优化了治疗头结构。模拟计算结果表明:优化方案提高了加速器的标称能量,降低了表面剂量和轫致辐射剂量,使剂量均匀性得到明显改善。

For the XHA9 medical linear accelerator, the measuremed depth-dose distri- bution in a water phantom shows some defects including less-than-desired nominal electron energy and higher relative surface dose. In order to solve these, the initial electron model of XHA9 was established in model E7 by Monte-Carlo code BEAM. The percentage depth-dose distribution curve of model E7 in water phantom was calculated and proved to be nearly the same as the measured one, which indicates that structural optimization of the treatment head is necessary. Contributions of each component to the therapy electrons determine that the main contribution is the large amount of low-energy electrons scattered by the applicator. Structural optimization of the treatment head was performed by modifying the applicator and the scattering foils. Final simulation results prove that the optimized scheme can enhance the nominal electron energy, decrease the surface dose and the bremsstrahlung dose, and ameliorate the dose uniformity.