title: A Monte Carlo calculation model of electronic portal imaging device for transit dosimetry through heterogeneous media.
abstract: OBJECTIVE
  To develop and evaluate a fast Monte Carlo (MC) dose calculation model of electronic portal imaging device (EPID) based on its effective atomic number modeling in the XVMC code.
  
  
  METHODS
  A previously developed EPID model, based on the XVMC code by density scaling of EPID structures, was modified by additionally considering effective atomic number (Zeff) of each structure and adopting a phase space file from the EGSnrc code. The model was tested under various homogeneous and heterogeneous phantoms and field sizes by comparing the calculations in the model with measurements in EPID. In order to better evaluate the model, the performance of the XVMC code was separately tested by comparing calculated dose to water with ion chamber (IC) array measurement in the plane of EPID.
  
  
  RESULTS
  In the EPID plane, calculated dose to water by the code showed agreement with IC measurements within 1.8%. The difference was averaged across the in-field regions of the acquired profiles for all field sizes and phantoms. The maximum point difference was 2.8%, affected by proximity of the maximum points to penumbra and MC noise. The EPID model showed agreement with measured EPID images within 1.3%. The maximum point difference was 1.9%. The difference dropped from the higher value of the code by employing the calibration that is dependent on field sizes and thicknesses for the conversion of calculated images to measured images. Thanks to the Zeff correction, the EPID model showed a linear trend of the calibration factors unlike those of the density-only-scaled model. The phase space file from the EGSnrc code sharpened penumbra profiles significantly, improving agreement of calculated profiles with measured profiles.
  
  
  CONCLUSIONS
  Demonstrating high accuracy, the EPID model with the associated calibration system may be used for in vivo dosimetry of radiation therapy. Through this study, a MC model of EPID has been developed, and their performance has been rigorously investigated for transit dosimetry.
authors.affiliation: Department of Physics, East Carolina University, Greenville, North Carolina 27858.
authors.forename: Jihyung
authors.initials: J
authors.lastname: Yoon
authors.affiliation: Department of Physics, East Carolina University, Greenville, North Carolina 27858.
authors.forename: Jae Won
authors.initials: JW
authors.lastname: Jung
authors.affiliation: Department of Radiation Oncology, University of Pittsburgh Cancer Institute, Pittsburgh, Pennsylvania 15232.
authors.forename: Jong Oh
authors.initials: JO
authors.lastname: Kim
authors.affiliation: Department of Radiation Medicine, Loma Linda University Medical Center, Loma Linda, California 92354.
authors.forename: Inhwan
authors.initials: I
authors.lastname: Yeo
mesh_terms: D001842:Bone and Bones / Q000528:radiation effects; D002138:Calibration; D003198:Computer Simulation; D006801:Humans; D008168:Lung / Q000528:radiation effects; D008953:Models, Anatomic; D009010:Monte Carlo Method; D019047:Phantoms, Imaging; D011874:Radiometry / Q000295:instrumentation* / Q000379:methods*; D014867:Water
chemical_list: D014867:Water