Dosimetric characteristics of backscattered electrons in lead

In electron beam therapy, tissue overdose due to electrons backscattered fr om lead has been profusely studied. To quantify this dose enhancement effec t, an electron backscatter factor (EBF) was defined as the ratio of dose at the tissue-inhomogeneity interface with and without the scatterer present. The dependence of the EBF on energy at the scatterer surface is not well k nown for energies lower than 3 MeV which is the most frequent clinical situ ation. In this work, we have done Monte Carlo calculations with the GEANT c ode to study EBF in lead at this energy range. The applicability of this co de and the developed procedure for dose estimation has been experimentally verified. The dependence of the EBF on the beam energy incident on the scat terer has been studied for different nominal beam energies incident at the phantom's surface. The results show a trend of increase of EBF with the bea m energy incident on the scatterer between 0.5 and 1.5 MeV, keeping practic ally constant above this energy up to 3 MeV. Backscattered electron energy spectra and depth dose curves in the 'up-stream' direction have been obtain ed at the various energies of the primary electron beam striking on the lea d scatterer. The results of this work are compared with previously publishe d data.