An EGSnrc Monte Carlo investigation of backscattered electrons from internal shielding in clinical electron beams

The ability to accurately predict dose from electron backscatter created by internal lead shielding utilized during various superficial electron beam treatments (EBT), such as lip carcinoma, is required to avoid the possibility of an overdose. Methods for predicting this dose include the use of empirical equations or physically measuring the electron backscatter factor (EBF) and upstream electron backscatter intensity (EBI). The EBF and upstream EBI are defined as the ratio of dose at, or upstream, from the shielding interface with and without the shielding present respectively. The accuracy of these equations for the local treatment machines was recognised as an area that required verification; in addition the ability of XiO's electron Monte Carlo (eMC) treatment planning algorithm to handle lead interfaces was examined. A Monte Carlo simulation using the EGSnrc package of a Siemens Artiste Linac was developed for 6, 9, 12, and 15 MeV electron energies and was verified against physical measurements to within an accuracy of 2 % and 2 mm. Electron backscatter dose distributions were predicated using the MC model, Gafchromic film, and XiO eMC, which when compared showed that XiO's eMC could not accurately calculate dose at the lead interface. Several MC simulations of lead interfaces at different depths, corresponding to energies of 0.2-14 MeV at the interfaces, were used to validate the accuracy of the equations, with the results concluding that the equation could not accurately predict EBF and EBI values, especially at low energies. From this data, an equation was derived to allow estimation of the EBF and upstream EBI, which agreed to within 1.3 % for the EBF values and can predict the upstream EBI to a clinically acceptable level for all energies