Verification of the optimal backscatter for an aSi electronic portal imaging device

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.