基于Monte Carlo的西门子6MV加速器散射因子的模拟研究

目的：研究探讨西门子（Siemens）6MV加速器总散射因子受射野变化的影响，为临床剂量计算提供参考。方法：利用蒙特卡罗程序BEAMnrc以及DOSXYZnrc对西门子加速器6MV光子束进行了模拟研究，详细分析了总散射因子受射野变化的影响．并结合模拟结果分析不同计算方法计算矩形野总散射因子的精度。结果：方形野总散射因子随着射野射野边长减小而减小。当方野边长小于2cm时，总散射因子减小趋势明显。当矩形野边长都大于2cm时，使用面积周长比方法以及K因子法计算总散射因子有较好的精度，但如果有一边边长小于2cm时，使用B因子法明显降低了误差。结论：在计算一般矩形野总散射因子时面积周长比方法比以及K因子法都能够达到良好的精度，但是蒙卡模拟是否存在准直器互换效应仍需进一步研究。对于矩形小野总散射因子，由于在模体中剂量分布梯度大，侧向电子难以平衡，因此使用面积周长比公式计算结果误差较大，建议采用B因子算法，模拟计算的结果与该算法计算的结果近似。

Monte Carlo Simulation Study on Total Scatter Factor of 6 MV Siemens Medical Accelerator

Objective： To analyze the total scatter factor of 6 MV Siemens accelerator by the effect of changing radiation fields, and provide reference for clinical dose calculations. Methods： Monte-Carlo program BEAMnrc and DOSXYZnrc are used to simulate this physics process, detailed analysis are presented about the total scatter factor effected by the changing radiation, and different methods to calculate the total scatter factor are discussed. Results： The total scatter factor of square fields de- crease by the decrease of square field dimensions. When the dimension is less than 2 cm, the total scatter factor is very small. When the rectangle field dimension is large than 2 cm, area-perimeter method and K-factor method can get good accuracy. When the rectangle field dimension is less than 2 cm, the B-factor can get good accuracy. Conclusions： Area-perimeter method and K-method can be used to calculate the total scatter factor when the rectangle field dimension is not too small, but it still needed to study whether Monte-Carlo simulation has a problem with collimator-exchange. For small rectangle field dimensions, because field dimensions are too small to ensure later electronic equilibrium, the area-perimeter method can result in high error. So we advise using B-factor method, the results of the simulation algorithm approximates to the results of the calculation.