title: A dual two dimensional electronic portal imaging device transit dosimetry model based on an empirical quadratic formalism.
abstract: OBJECTIVE
  This study describes a two dimensional electronic portal imaging device (EPID) transit dosimetry model that can predict either: (1) in-phantom exit dose, or (2) EPID transit dose, for treatment verification.
  
  
  METHODS
  The model was based on a quadratic equation that relates the reduction in intensity to the equivalent path length (EPL) of the attenuator. In this study, two sets of quadratic equation coefficients were derived from calibration dose planes measured with EPID and ionization chamber in water under reference conditions. With two sets of coefficients, EPL can be calculated from either EPID or treatment planning system (TPS) dose planes. Consequently, either the in-phantom exit dose or the EPID transit dose can be predicted from the EPL. The model was tested with two open, five wedge and seven sliding window prostate and head and neck intensity-modulated radiation therapy (IMRT) fields on phantoms. Results were analysed using absolute gamma analysis (3%/3 mm).
  
  
  RESULTS
  The open fields gamma pass rates were >96.8% for all comparisons. For wedge and IMRT fields, comparisons between predicted and TPS-computed in-phantom exit dose resulted in mean gamma pass rate of 97.4% (range, 92.3-100%). As for the comparisons between predicted and measured EPID transit dose, the mean gamma pass rate was 97.5% (range, 92.6-100%).
  
  
  CONCLUSIONS
  An EPID transit dosimetry model that can predict in-phantom exit dose and EPID transit dose was described and proven to be valid.
  
  
  CONCLUSIONS
  The described model is practical, generic and flexible to encourage widespread implementation of EPID dosimetry for the improvement of patients' safety in radiotherapy.
authors.affiliation: 1 College of Medical, Veterinary and Life Sciences, University of Glasgow, Glasgow, UK.
authors.forename: Y I
authors.initials: YI
authors.lastname: Tan
authors.affiliation: 2 Radiotherapy Physics, The Beatson West of Scotland Cancer Centre, Glasgow, UK.
authors.forename: M
authors.initials: M
authors.lastname: Metwaly
authors.affiliation: 2 Radiotherapy Physics, The Beatson West of Scotland Cancer Centre, Glasgow, UK.
authors.forename: M
authors.initials: M
authors.lastname: Glegg
authors.affiliation: 3 Radiation Therapy Centre, National University Cancer Institute, Singapore.
authors.forename: S P
authors.initials: SP
authors.lastname: Baggarley
authors.affiliation: 1 College of Medical, Veterinary and Life Sciences, University of Glasgow, Glasgow, UK.
authors.forename: A
authors.initials: A
authors.lastname: Elliott
mesh_terms: D000465:Algorithms; D006801:Humans; D008297:Male; D019047:Phantoms, Imaging; D011874:Radiometry / Q000379:methods*; D011879:Radiotherapy Dosage; D050397:Radiotherapy, Intensity-Modulated