title: Dose-to-water conversion for the backscatter-shielded EPID: a frame-based method to correct for EPID energy response to MLC transmitted radiation.
abstract: OBJECTIVE
  To develop a frame-by-frame correction for the energy response of amorphous silicon electronic portal imaging devices (a-Si EPIDs) to radiation that has transmitted through the multileaf collimator (MLC) and to integrate this correction into the backscatter shielded EPID (BSS-EPID) dose-to-water conversion model.
  
  
  METHODS
  Individual EPID frames were acquired using a Varian frame grabber and iTools acquisition software then processed using in-house software developed inMATLAB. For each EPID image frame, the region below the MLC leaves was identified and all pixels in this region were multiplied by a factor of 1.3 to correct for the under-response of the imager to MLC transmitted radiation. The corrected frames were then summed to form a corrected integrated EPID image. This correction was implemented as an initial step in the BSS-EPID dose-to-water conversion model which was then used to compute dose planes in a water phantom for 35 IMRT fields. The calculated dose planes, with and without the proposed MLC transmission correction, were compared to measurements in solid water using a two-dimensional diode array.
  
  
  RESULTS
  It was observed that the integration of the MLC transmission correction into the BSS-EPID dose model improved agreement between modeled and measured dose planes. In particular, the MLC correction produced higher pass rates for almost all Head and Neck fields tested, yielding an average pass rate of 99.8% for 2%/2 mm criteria. A two-sample independent t-test and fisher F-test were used to show that the MLC transmission correction resulted in a statistically significant reduction in the mean and the standard deviation of the gamma values, respectively, to give a more accurate and consistent dose-to-water conversion.
  
  
  CONCLUSIONS
  The frame-by-frame MLC transmission response correction was shown to improve the accuracy and reduce the variability of the BSS-EPID dose-to-water conversion model. The correction may be applied as a preprocessing step in any pretreatment portal dosimetry calculation and has been shown to be beneficial for highly modulated IMRT fields.
authors.affiliation: School of Mathematical and Physical Sciences, University of Newcastle, Newcastle, New South Wales 2308, Australia.
authors.forename: Benjamin J
authors.initials: BJ
authors.lastname: Zwan
authors.affiliation: Department of Radiation Oncology, Massey Cancer Center, Virginia Commonwealth University Health System, Richmond, Virginia 23298.
authors.forename: Brian W
authors.initials: BW
authors.lastname: King
authors.affiliation: School of Mathematical and Physical Sciences, University of Newcastle, Newcastle, New South Wales 2308, Australia.
authors.forename: Daryl J
authors.initials: DJ
authors.lastname: O'Connor
authors.affiliation: School of Mathematical and Physical Sciences, University of Newcastle, Newcastle, New South Wales 2308, Australia and Department of Radiation Oncology, Calvary Mater Newcastle Hospital, Newcastle, New South Wales 2310, Australia.
authors.forename: Peter B
authors.initials: PB
authors.lastname: Greer
mesh_terms: D000465:Algorithms; D008962:Models, Theoretical; D019047:Phantoms, Imaging; D011835:Radiation Protection; D011874:Radiometry / Q000295:instrumentation / Q000379:methods*; D011879:Radiotherapy Dosage; D050397:Radiotherapy, Intensity-Modulated / Q000295:instrumentation / Q000379:methods; D012984:Software; D014867:Water
chemical_list: D014867:Water