基于蒙特卡洛方法研究电子线输出因子影响因素

Analysis of electron beam output factors by Monte Carlo method

目的 使用蒙特卡洛方法计算电子线输出因子,探讨直线加速器各组成部分对输出因子影响.方法 使用蒙特卡洛MCTP软件模拟瓦里安23EX医用直线加速器6、9、18 MeV电子线,源皮距100 cm下不同射野大小（2 cm×2cm～25 cm×25 cm）的输出因子.使用Scanditronix Wellhofer Blue Phantom自动扫描系统三维水箱测量相同条件下的输出因子.观察计算与测量结果差异,要求两者＜2％.再使用蒙特卡洛方法分别计算不同能量下不同限束系统条件下原粒子和散射粒子的输出因子（包括限光筒、铅门、挡铅的输出因子）,并讨论限束系统各个部件对输出因子的影响.结果 MCTP系统计算的6、9、18 MeV电子线输出因子与测量结果间差异＜1％.电子线能量、限光筒—铅门—挡铅组合以复杂的方式影响原粒子和散射粒子,从而影响输出因子.结论 蒙特卡洛MCTP软件能精确计算不同能量下不同限束系统条件下原粒子和散射粒子的输出因子,电子线能量及限束系统以复杂方式影响输出因子.

Objective To investigate the application of the Monte Carlo dose calculation of output factors for electron beams in radiotherapy.Methods The code EGS4/MCTP was used to simulate the head of a medical linear accelerator （Varian 23EX） to calculate the output factors for 6 MeV,9 MeV and 18 MeV electron beams.The source-to-surface distance used was 100 cm.The field sizes ranged from 2 cm × 2 cm to 25 cm × 25 cm.The calculated output factors agreed with the corresponding measured factors which were measured by the IBA Phantom system to within 2%.Then,the output factors of direct articles and indirect articles which were under different energy and various cone-insert combinations were calculated by the code EGS4/MCTP.Results The calculated output factors agreement with the measurements is found to be mostly under the 1% level.The variation of output factors depends on the characteristics of the beams and the modifications that the various cone-insert combinations introduce to these characteristics.Conclusions Monte Carlo dose calculations for electron beams in homogeneous water phantoms have been demonstrated to be accurate under the 1% level in comparison with measurements.The output factors were influenced by energy and cone-insert combinations in complex ways.