利用蒙特卡罗模拟评估医用电子直线加速器非均整模式的光子束软化特性

本文利用蒙特卡罗模拟评估不同能量、不同射野下医用电子直线加速器非均整模式(Flattening Filter Free, FFF)光子束的软化特性.基于本院的Varian Edge加速器的组件参数和测量数据，利用通用蒙卡程序EGSnrc/BEAMnrc构建6 MV均整模式(Flattening Filter, FF)、6 MV FFF和10 MV FFF下的机头模型，使用EGSnrc/DOSXYZnrc程序模拟获得5 cm×5 cm和10 cm×10 cm射野下的百分深度剂量(Percentage Dose Depth, PDD)和横向剂量离轴比(Off Axis Ratio, OAR),与测量数据进行比较，确定不同模式的源参数；在此基础上模拟三种光子束模型在上述两种射野射野条件下非均匀模体(水-骨-肺-水)内的PDD,并对其相空间文件进行能谱分析，评估FFF光子束的软化特性.三种光子束模式在非均匀模体内的PDD趋势相似，但在小野时扰动更大，FFF模式相较于FF模式，包含更多的低能光子和更少的机头散射；在低能、小野时，6 MV FFF模式的光子束软化特性带来的剂量影响与6 MV FF模...

Evaluation of photon beam softening characteristics in flattening-filter-free mode of medical electron linear accelerator by Monte Carlo simulation

Monte Carlo simulations were used to evaluate the photon beam softening characteristics of medical electron linear accelerators operated in flattening filter-free mode (FFF) under different energies and field sizes through. Using the parameters of the Varian Edge accelerator and measured data in our hospital, a 6 MV flattening filter (FF), a 6 MV FFF, and a 10 MV FFF model in the accelerator head were constructed using the general Monte Carlo program EGSnrc/BEAMnrc. Using EGSnrc/DOSXYZnrc, three modes were characterized based on percentage depth dose (PDD) and off-axis ratio (OAR), which were compared with simulation data to determine source parameters. By using the above models, we obtained PDDs for three modes of inhomogeneous phantoms (water-bone-lung-water) in two field sizes, and the energy spectrum was calculated from phase space files, then the FFF photon beam softening properties were analyzed. In the inhomogeneous phantom, the PDD trends of the three photon-beam modes are similar, but the fluctuations are greater at small fields. In contrast to FF mode, FFF mode contains more low-energy photons and less head scattering. At low energy and small field, photon beam softening caused by the 6 MV FFF mode does not bring about a noticeable dose effect. however, at high energy and large field, the beam softening effect is gradually apparent. FFF photon beams have a steeper dose drop than FF beams, especially at small fields and low energy, which makes them evidently advantageous in clinical settings; however, at large fields and high energies, FFF photon beams appear to have significant softening characteristics, particularly in low-density areas, requiring careful consideration of the choice of clinical planning modality.