title: Monte Carlo simulation of the effect of miniphantom on in-air output ratio.
abstract: OBJECTIVE
  The aim of the study was to quantify the effect of miniphantoms on in-air output ratio measurements, i.e., to determine correction factors for in-air output ratio.
  
  
  METHODS
  Monte Carlo (MC) simulations were performed to simulate in-air output ratio measurements by using miniphantoms made of various materials (PMMA, graphite, copper, brass, and lead) and with different longitudinal thicknesses or depths (2-30 g/cm2) in photon beams of 6 and 15 MV, respectively, and with collimator settings ranging from 3 x 3 to 40 x 40 cm2. EGSnrc and BEAMnrc (2007) software packages were used. Photon energy spectra corresponding to the collimator settings were obtained from BEAMnrc code simulations on a linear accelerator and were used to quantify the components of in-air output ratio correction factors, i.e., attenuation, mass energy absorption, and phantom scatter correction factors. In-air output ratio correction factors as functions of miniphantom material, miniphantom longitudinal thickness, and collimator setting were calculated and compared to a previous experimental study.
  
  
  RESULTS
  The in-air output ratio correction factors increase with collimator opening and miniphantom longitudinal thickness for all the materials and for both energies. At small longitudinal thicknesses, the in-air output ratio correction factors for PMMA and graphite are close to 1. The maximum magnitudes of the in-air output ratio correction factors occur at the largest collimator setting (40 x 40 cm2) and the largest miniphantom longitudinal thickness (30 g/cm2): 1.008 +/- 0.001 for 6 MV and 1.012 +/- 0.001 for 15 MV, respectively. The MC simulations of the in-air output ratio correction factor confirm the previous experimental study.
  
  
  CONCLUSIONS
  The study has verified that a correction factor for in-air output ratio can be obtained as a product of attenuation correction factor, mass energy absorption correction factor, and phantom scatter correction factor. The correction factors obtained in the present study can be used in studies involving in-air output ratio measurements using miniphantoms.
authors.affiliation: Department of Radiation Oncology, Thomas Jefferson University, Philadelphia, Pennsylvania 19107, USA. jun.li@jefferson.edu
authors.forename: Jun
authors.initials: J
authors.lastname: Li
authors.forename: Timothy C
authors.initials: TC
authors.lastname: Zhu
mesh_terms: D000388:Air; D055592:Biophysical Phenomena; D003198:Computer Simulation; D006801:Humans; D009010:Monte Carlo Method; D010315:Particle Accelerators / Q000706:statistics & numerical data; D019047:Phantoms, Imaging / Q000706:statistics & numerical data*; D017785:Photons / Q000627:therapeutic use; D011880:Radiotherapy Planning, Computer-Assisted / Q000706:statistics & numerical data*; D020266:Radiotherapy, Conformal / Q000706:statistics & numerical data