Assessment of the Versatility of Phase Space Files in TrueBeam Accelerator 6 MV Flattening
Filter Free Mode by Monte Carlo Simulation
CHEN Lei1,2, TANG Huanan1, FU Yuchuan1, LI Changhu1, HAN Jifeng2
1. Center for Radiophysical Technology, Cancer Center, West China Hospital Sichuan University, Chengdu Sichuan 610041, China;
2. Key Laboratory of Radiation Physics and Technology, Ministry of Education, Institute of Nuclear Science and Technology,
Sichuan University, Chengdu Sichuan 610064, China
Abstract：Objective To explore the universality of phase space file (PSF) for similar accelerators under Varian 6 MV photon
beam flattening filter free (FFF) mode. Methods Based on the parameters of the Varian Edge accelerator in our hospital, EGSnrc/
BEAMnrc was used to simulate a complete accelerator head model (complete FFF model) and a partial accelerator head model (partial
FFF model) with PSF input source in 6 MV FFF mode provided by Varian. EGSnrc/DOSXYZnrc was used to simulate the dose
distribution in the subsurface model under the two model conditions, and the percentage dose depth (PDD) and the lateral dose off
axis ratio (OAR) were obtained, and compared with the measured data. At the same time, the energy spectrum and energy injection
of the PSF obtained after leaving the accelerator head at the field size of 10 cm×10 cm were analyzed. Results The PSF of the 6 MV
FFF provided by Varian tended to underestimate the incident energy and off axis average energy, resulting in a relatively low PDD
compared to the measured data and a significant deviation in the OAR drop zone. There were differences between the complete FFF
model and the partial FFF model. By comparing the PDD and OAR data of the two models, it is found that the results of partial
FFF models were slightly worse than those of the complete FFF model. In addition, the energy spectrum analysis also showed that
the lower incident electron energy of partial FFF models could affect the proportion of low-energy photons and the average offaxis
energy. Conclusion In this study, Monte Carlo software was used to construct partial FFF model and complete FFF model by
utilizing TrueBeam's 6 MV FFF PSF provided by Varian to simulate the three-dimensional dose distribution of different launching
fields. The results shows that the model constructed with PSF has some problems, such as low PDD, large OAR deviation, and
underestimation of off-axis average energy. It is suggested that this PSF should be compared and verified with the measurement
data of in-house water tanks, and specific measures should be taken to reduce the uncertainty. A whole accelerator head should be
constructed to support diverse study topics if the difference is significant.
陈蕾1,2，唐华南1，傅玉川1，李长虎1，韩纪锋2. 利用蒙特卡洛模拟评估TrueBeam加速器6 MV非均整模式下相空间文件的通用性[J]. 中国医疗设备, 2023, 38(9): 64-68.
CHEN Lei1,2, TANG Huanan1, FU Yuchuan1, LI Changhu1, HAN Jifeng2. Assessment of the Versatility of Phase Space Files in TrueBeam Accelerator 6 MV Flattening
Filter Free Mode by Monte Carlo Simulation. China Medical Devices, 2023, 38(9): 64-68.
