Radiotherapy Research Today is a free monthly online journal that collates and summarizes the latest research about Radiotherapy, including details on cancer treatment, side effects.

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

Moore JA, Siebers JV

Department of Radiation Oncology, Medical College of Virginia Hospitals, Virginia Commonwealth University, Richmond, VA, USA. jamoore@vcu.edu

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.

Published 6 May 2005 in Phys Med Biol, 50(10): 2341-50.
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