Department of Radiology, Faculty of Para-Medicine, Hormozgan University of Medical Sciences, Bandare-Abbas, Iran.
Department of Radiology and Nuclear Medicine, School of Para Medical Science, Kermanshah University of Medical Sciences, Kermanshah, Iran.
J Xray Sci Technol. 2019;27(6):1047-1070. doi: 10.3233/XST-190568.
This study aims to benchmark a Monte Carlo (MC) model of the 18 MV photon beam produced by the Siemens Oncor® linac using the BEAMnrc and DOSXYZnrc codes.
By matching the percentage depth doses and beam profiles calculated by MC simulations with measurements, the initial electron beam parameters including electron energy, full width at half maximum (spatial FWHM), and mean angular spread were derived for the 10×10 cm2 and 20×20 cm2 field sizes. The MC model of the 18 MV photon beam was then validated against the measurements for different field sizes (5×5, 30×30 and 40×40 cm2) by gamma index analysis.
The optimum values for electron energy, spatial FWHM and mean angular spread were 14.2 MeV, 0.08 cm and 0.8 degree, respectively. The MC simulations yielded the comparable measurement results of these optimum parameters. The gamma passing rates (with acceptance criteria of 1% /1 mm) for percentage depth doses were found to be 100% for all field sizes. For cross-line profiles, the gamma passing rates were 100%, 97%, 95%, 96% and 95% for 5×5, 10×10, 20×20, 30×30 and 40×40 cm2 field sizes, respectively.
By validation of the MC model of Siemens Oncor® linac using various field sizes, it was found that both dose profiles of small and large field sizes were very sensitive to the changes in spatial FWHM and mean angular spread of the primary electron beam from the bending magnet. Hence, it is recommended that both small and large field sizes of the 18 MV photon beams should be considered in the Monte Carlo linac modeling.
本研究旨在使用 BEAMnrc 和 DOSXYZnrc 代码对标西门子 Oncor®直线加速器产生的 18MV 光子束的蒙特卡罗（MC）模型。
通过将 MC 模拟计算的百分深度剂量和射束轮廓与测量值相匹配，得出 10×10cm2 和 20×20cm2 射野大小的初始电子束参数，包括电子能量、半高全宽（空间 FWHM）和平均角扩展。然后，通过伽马指数分析，对不同射野大小（5×5、30×30 和 40×40cm2）的 18MV 光子束 MC 模型进行验证。
电子能量、空间 FWHM 和平均角扩展的最佳值分别为 14.2MeV、0.08cm 和 0.8 度。MC 模拟得到了这些最佳参数的可比较的测量结果。所有射野大小的百分深度剂量的 MC 模拟结果与测量结果的伽马通过率（接受标准为 1%/1mm）均为 100%。对于十字线轮廓，5×5、10×10、20×20、30×30 和 40×40cm2 射野大小的伽马通过率分别为 100%、97%、95%、96%和 95%。
通过对西门子 Oncor®直线加速器的 MC 模型进行各种射野大小的验证，发现小射野和大射野的剂量分布对从弯铁出来的初级电子束的空间 FWHM 和平均角扩展的变化非常敏感。因此，建议在 MC 直线加速器建模中应同时考虑 18MV 光子束的小射野和大射野。
