title: A comprehensive procedure for characterizing arbitrary azimuthally symmetric photon beams.
abstract: OBJECTIVE
  A new Monte Carlo (MC) source model (SM) has been developed for azimuthally symmetric photon beams.
  
  
  METHODS
  The MC simulation tallied phase space file (PSF) is divided into two categories depending on the relationship of the particle track line to the beam central axis: multiple point source (MPS) and spatial mesh based surface source (SMBSS). To validate this SM, MCNPX2.6 was used to generate two PSFs for a 6 MV photon beam from a Varian 2100C/D linear accelerator.
  
  
  RESULTS
  PDDs and profiles were calculated using the SM and original PSF for different field sizes from 5 × 5 to 40 × 40 cm2. Agreement was within 2% of the maximum dose at 100 cm SSD and 2.5% of the maximum dose at 200 cm SSD for beam profiles at depths of 3.5 cm and 15 cm with respect to the original PSF. Differences between the source model and the PSF in the out-of-field regions were less than 0.5% of the profile maximum value at 100 cm SSD. Differences between measured and calculated points were less than 2% of the maximum dose or 2 mm distance to agreement (DTA) at 100 cm SSD.
  
  
  CONCLUSIONS
  This SM is unique in that it accounts for a higher level of energy dependence on the particle's direction and it is independent from accelerator components, unlike other published SMs. The model can be applied to any arbitrary azimuthally symmetric beam and has source biasing capabilities that significantly increase the simulation speed up to 3300 for certain field sizes.
authors.affiliation: Department of Energy Engineering, Sharif University of Technology, Tehran, Iran. Electronic address: ezzati@energy.sharif.ir.
authors.forename: Ahad Ollah
authors.initials: AO
authors.lastname: Ezzati
authors.affiliation: Department of Energy Engineering, Sharif University of Technology, Tehran, Iran.
authors.forename: Mostafa
authors.initials: M
authors.lastname: Sohrabpour
authors.affiliation: Department of Medical Physics, Tehran University of Medical Science, Tehran, Iran.
authors.forename: Seied Rabi
authors.initials: SR
authors.lastname: Mahdavi
authors.affiliation: Department of Radiation Oncology, Dana-Farber/Brigham and Women's Cancer Center, Harvard Medical School, Boston, MA, USA.
authors.forename: Ivan
authors.initials: I
authors.lastname: Buzurovic
authors.affiliation: Department of Radiation Oncology, Thomas Jefferson University, Philadelphia, PA, USA.
authors.forename: Matthew T
authors.initials: MT
authors.lastname: Studenski
mesh_terms: D009010:Monte Carlo Method; D010315:Particle Accelerators; D017785:Photons / Q000627:therapeutic use*; D011336:Probability; D011874:Radiometry; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000295:instrumentation / Q000379:methods*; D015203:Reproducibility of Results
keywords: Monte Carlo; Phase space file; Radiotherapy; Source model