6MV医用加速器电子对效应的蒙特卡罗模拟研究

Monte Carlo Simulation Study on Electron-pair Effect of 6 MV Medical Accelerator

以Geant4编程构建6MV医用加速器机头和照射条件,并通过实测数据比对验证模型的正确性,模拟加速器粒子输运过程,获取不同射野和不同层面的粒子相空间文件,以正电子和湮没光子为指标研究射野及各机头部件对电子对效应产生的影响,模拟电子对效应对加速器射束及水模中剂量沉积的影响。结果表明:射野面积越小,电子对效应产生越多;在靶、均整器及次级准直器处均会有较明显的电子对效应发生,其中次级准直器是主要的发生部件;电子对效应使射束中污染电子增加了约4%,降低了射束的平均能量,使射束粒子分布更加分散,增加了模体表面剂量及射野外剂量。

The Geant4 w as used to construct the head and irradiation conditions of 6MV medical accelerator . The measurement data in the water phantom were compared to ensure the constructed model correct .Phase space files of different fields and different levels were obtained by simulating the process of particle transporting of the accelera‐tor .The positron and annihilation photon were as indicator ,and the effect of fields and components of the accelerator head on the production of electron‐pair was studied by analyzing those files .The influence of electron‐pair effect on accelerator beam and ener‐gy deposited in the phantom was simulated .The results show that the smaller the field is ,the more the production of electron‐pair is .The production of electron‐pair is obvi‐ous when beam passes through target ,flatten filter and secondary collimator ,and it is especially obvious to secondary collimator .The electron‐pair effect makes contamination electron increase about 4% ,beam’s average energy lower ,beam’s particle angular dis‐tribution more dispersed and doses of surface of phantom and outside fields increase .