title: Monte Carlo simulation of beam characteristics from small fields based on TrueBeam flattening-filter-free mode.
abstract: OBJECTIVE
  Through the Monte Carlo (MC) simulation of 6 and 10 MV flattening-filter-free (FFF) beams from Varian TrueBeam accelerator, this study aims to find the best incident electron distribution for further studying the small field characteristics of these beams.
  
  
  METHODS
  By incorporating the training materials of Varian on the geometry and material parameters of TrueBeam Linac head, the 6 and 10 MV FFF beams were modelled using the BEAMnrc and DOSXYZnrc codes, where the percentage depth doses (PDDs) and the off-axis ratios (OARs) curves of fields ranging from 4 × 4 to 40 × 40 cm(2) were simulated for both energies by adjusting the incident beam energy, radial intensity distribution and angular spread, respectively. The beam quality and relative output factor (ROF) were calculated. The simulations and measurements were compared using Gamma analysis method provided by Verisoft program (PTW, Freiburg, Germany), based on which the optimal MC model input parameters were selected and were further used to investigate the beam characteristics of small fields.
  
  
  RESULTS
  The Full Width Half Maximum (FWHM), mono-energetic energy and angular spread of the resultant incident Gaussian radial intensity electron distribution were 0.75 mm, 6.1 MeV and 0.9° for the nominal 6 MV FFF beam, and 0.7 mm, 10.8 MeV and 0.3° for the nominal 10 MV FFF beam respectively. The simulation was mostly comparable to the measurement. Gamma criteria of 1 mm/1 % (local dose) can be met by all PDDs of fields larger than 1 × 1 cm(2), and by all OARs of no larger than 20 × 20 cm(2), otherwise criteria of 1 mm/2 % can be fulfilled. Our MC simulated ROFs agreed well with the measured ROFs of various field sizes (the discrepancies were less than 1 %), except for the 1 × 1 cm(2) field.
  
  
  CONCLUSIONS
  The MC simulation agrees well with the measurement and the proposed model parameters can be clinically used for further dosimetric studies of 6 and 10 MV FFF beams.
authors.affiliation: Key laboratory of Radiological Protection and Nuclear Emergency, Chinese Center for Disease Control and Prevention, National Institute for Radiological Protection, Beijing, 100088, China. fengzsxulj@126.com.
authors.forename: Zhongsu
authors.initials: Z
authors.lastname: Feng
authors.affiliation: Key laboratory of Carcinogenesis and Translational Research (Ministry of Education/Beijing), Department of Radiotherapy, Peking University Cancer Hospital & Institute, 52 Fuchen Road, Haidian, Beijing, 100142, China. yuehzh@163.com.
authors.forename: Haizhen
authors.initials: H
authors.lastname: Yue
authors.affiliation: Key laboratory of Carcinogenesis and Translational Research (Ministry of Education/Beijing), Department of Radiotherapy, Peking University Cancer Hospital & Institute, 52 Fuchen Road, Haidian, Beijing, 100142, China. ybzhang77@gmail.com.
authors.forename: Yibao
authors.initials: Y
authors.lastname: Zhang
authors.affiliation: Key laboratory of Carcinogenesis and Translational Research (Ministry of Education/Beijing), Department of Radiotherapy, Peking University Cancer Hospital & Institute, 52 Fuchen Road, Haidian, Beijing, 100142, China. wuhao.bicr@gmail.com.
authors.forename: Hao
authors.initials: H
authors.lastname: Wu
authors.affiliation: Key laboratory of Radiological Protection and Nuclear Emergency, Chinese Center for Disease Control and Prevention, National Institute for Radiological Protection, Beijing, 100088, China. chengjs3393@163.com.
authors.forename: Jinsheng
authors.initials: J
authors.lastname: Cheng
authors.affiliation: Key laboratory of Radiological Protection and Nuclear Emergency, Chinese Center for Disease Control and Prevention, National Institute for Radiological Protection, Beijing, 100088, China. suxu_nirp@126.com.
authors.forename: Xu
authors.initials: X
authors.lastname: Su
mesh_terms: D003198:Computer Simulation; D004583:Electrons; D006801:Humans; D009010:Monte Carlo Method; D010315:Particle Accelerators; D017785:Photons / Q000627:therapeutic use; D011874:Radiometry / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D012984:Software