基于蒙特卡罗方法的光子束均整与非均整模式能谱对比分析

目的:对比分析6 MV光子束均整与非均整模式在空气和标准水模中特定深度处的能谱分布。 方法:利用BEAMnrc程序建立美国Varian公司TrueBeam加速器均整和非均整模式的机头模型，分别计算（40×40） cm2照射野下空气和标准水模中SSD=110 cm深度处的相空间文件，并利用BEAMDP程序对射野内不同区域的能谱分布进行对比分析。 结果:空气中（40×40） cm2射野内SSD=110 cm深度处，均整模式能谱分布低能部分随着统计区域增大而增大，与非均整模式分布规律相反；在标准水模体中Depth=10 cm深度处，有无反散的情况下两种模式的能谱分布相差较大，主要在小于0.511 MeV的区域；射野内不同位置的能谱分布均整模式在离轴方向低能部分逐渐减少，而非均整模式分布情况相反；相对于电子和正电子来说，相同射野内光子对能谱分布影响较大。 结论:该研究为医用直线加速器临床剂量学数据的测量和校正提供依据。

Comparison analysis of energy spectra between flattened and flattening-filter-free Monte Carlo models for photon beam

Objective To compare and analyze the energy spectra of 6 MV flattened and flattening-filter-free photon beam in the air medium and standard water phantom at specific depth. Methods BEAMnrc program was used to model Varian TrueBeam accelerator gantry head, including flattened model and flattening-filter-free model. The phase space files of (40×40) cm2 field at SSD of 110 cm in air medium and standard water phantom were calculated. The energy spectrum distribution in different regions of the irradiation field were compared and analyzed with the use of BEAMDP program. Results For the irradiation field of (40×40) cm2 at SSD of 110 cm in air medium, with the enlargement of statistical regions, the low energy part in the energy spectrum of flattened model increased, while that of flattening filter-free model decreased. For the standard water phantom at depth of 10 cm, large differences were found in the energy spectrum of two models with or without scatter, mainly in the area less than 0.511 MeV. The low energy part in the energy spectrum of flattened model in different locations decreased in crossplane direction, while the flattening filter-free model showed opposite trend. Compared with electron and position electron, the photon had great effects on the energy spectrum in the same irradiation field. Conclusion This study provides basis for further clinical dosimetric data measurement and calibration of medical linear accelerator.