title: Development and testing of an improved dosimetry system using a backscatter shielded electronic portal imaging device.
abstract: OBJECTIVE
  To investigate the properties of a modified backscatter shielded electronic portal imaging device (BSS-EPID) and to develop a dose model to convert BSS-EPID images to dose in water as part of an improved system for dosimetry using EPIDs.
  
  
  METHODS
  The effectiveness of the shielding of the BSS-EPID was studied by comparing images measured with the BSS-EPID mounted on the support arm to images measured with the BSS-EPID removed from the support arm. A dose model was developed and optimized to reconstruct dose in water at different depths from measured BSS-EPID images. The accuracy of the dose model was studied using BSS-EPID images of 28 IMRT fields to reconstruct dose in water at depths of 2, 5, 10, and 20 cm and comparing to measured dose in water from a two-dimensional diode array at the same depths. The ability of the BSS-EPID system to operate independently of detector position was demonstrated by comparing the dose reconstruction of a 10 × 10 cm(2) field using different detector offsets to that measured by a two-dimensional diode array.
  
  
  RESULTS
  The shielding of the BSS-EPID was found to be effective, with more than 99% of pixels showing less than 0.5% change due to the presence of the support arm and at most a 0.2% effect on the central axis for 2 × 2 cm(2) fields to fully open 30 × 40 cm(2) images. The dose model was shown to accurately reconstruct measurements of dose in water using BSS-EPID images with average γ pass rates (2%, 2 mm criteria) of 92.5%, 98.7%, 97.4%, and 97.2% at depths of 2, 5, 10, and 20 cm, respectively, when compared to two-dimensional diode array measurements. When using 3%, 3 mm γ criteria, the average pass rate was greater than 97% at all depths. Reconstructed dose in water for a 10 × 10 cm(2) field measured with detector offsets as large as 10 cm agreed with each other and two-dimensional diode array measurements within 0.9%.
  
  
  CONCLUSIONS
  The modified BSS-EPID and associated dose model provide an improved system for dosimetry measurements using EPIDs. Several important limitations of the current hardware and software are addressed by this system.
authors.affiliation: School of Mathematical and Physical Sciences, University of Newcastle, Newcastle, New South Wales, Australia. brian.king@newcastle.edu.au
authors.forename: Brian W
authors.initials: BW
authors.lastname: King
authors.forename: Daniel
authors.initials: D
authors.lastname: Morf
authors.forename: Peter B
authors.initials: PB
authors.lastname: Greer
mesh_terms: D055615:Electrical Equipment and Supplies; D011835:Radiation Protection / Q000295:instrumentation*; D011874:Radiometry / Q000295:instrumentation*; D012542:Scattering, Radiation; D014867:Water
chemical_list: D014867:Water