title: Verification of the optimal backscatter for an aSi electronic portal imaging device.
abstract: Electronic portal imaging devices (EPIDs), currently used for determining proper patient placement during irradiation in a radiotherapy treatment, can also be used as dosimeters. However, the Varian aS500 portal imager exhibits dosimetric artefacts caused by non-uniform backscatter from mechanical support structures located behind the imager. Monte Carlo simulations predict that adding 5 mm of Pb behind the imaging cassette will reduce the non-uniform backscatter to <1% for 6 MV and to <1.5% for 18 MV photon beams. This study experimentally tested this hypothesis by comparing images using an unmodified test imager and an imager modified by adding 3 and 5 mm of Pb behind the imaging cassette. Using the modified imager containing 5 mm of Pb, the non-uniform backscatter was reduced to <0.5% for 6 MV and <0.6% for 18 MV beams. Addition of the 5 mm of Pb increased the detector contrast by 3.5% +/- 0.5% at 6 MV and 5.0% +/- 0.7% at 18 MV, and increased the resolution by 0.9% +/- 0.2% at 6 MV and 0.5% +/- 0.12% at 18 MV.
authors.affiliation: Department of Radiation Oncology, Medical College of Virginia Hospitals, Virginia Commonwealth University, Richmond, VA, USA. jamoore@vcu.edu
authors.forename: Joseph A
authors.initials: JA
authors.lastname: Moore
authors.forename: Jeffrey V
authors.initials: JV
authors.lastname: Siebers
mesh_terms: D001822:Body Burden; D004867:Equipment Design; D019544:Equipment Failure Analysis; D011785:Quality Assurance, Health Care / Q000379:methods*; D011829:Radiation Dosage; D011874:Radiometry / Q000295:instrumentation* / Q000379:methods; D011881:Radiotherapy, Computer-Assisted / Q000295:instrumentation* / Q000379:methods; D012062:Relative Biological Effectiveness; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D012825:Silicon / Q000528:radiation effects
chemical_list: D012825:Silicon