Poster - Thurs Eve-25: Depth dependence of electron backscatter for electron radiotherapy: A Monte Carlo study

The aim of this study is to investigate the depth dependence of electron backscatter from lead (Pb) for clinical electron beams using Monte Carlo simulations. Phase-space files for the 4 and 9MeV electron beams based on the Varian 21EX Linac were generated using the BEAMnrc code. A 3mm Pb layer was irradiated by the phase-space beams with 0.5 and 1cm depth of water on top. Electron energy spectra at different depths in water with and without the Pb layer were determined using the BEAMnrc code. For energy spectra of the 4 MeV beams with 0.5cm depth of water, electron backscatter was found at both the Pb-water interface and 0.5cm of water over the Pb. However, electron backscatter almost disappeared at 0.5 and 1cm of water over the Pb, when 1cm depth of water was used. This is because the increased attenuations of the (1) 4MeV beam from the thicker water layer, and (2) electron backscatter in water above the Pb. For the 9MeV beams with 0.5cm depth of water, only slight electron backscatter was observed. More electron backscatter was found when 1cm depth of water was used. It is because the electron beam energy was reduced more due to the attenuation of the 1 cm depth of water compared to 0.5 cm. Since the electron energy spectrum varies with depth of water on top of the Pb layer, the electron backscatter depends very much on the thickness of water/water equivalent bolus/human tissue over the Pb shield in electron radiotherapy.