Room scatter effects in total skin electron therapy: a Monte Carlo study

Total Skin Electron Irradiation (TSEI) is a complex technique which involves the use of large electron fields. Electrons scattered from the treatment room floor and ceiling might contribute to skin dose and distort dose distribution, especially when dual-field approach is used. The purpose of this work is to study effects of scattered electrons on the dosimetry of TSEI by Monte Carlo (MC) simulations. 6 MeV and 9 MeV beams from Elekta Precise linac operated in High-Dose-Rate (HDR) mode are used for TSEI treatments. The EGSnrc code package was used for MC simulation. First, the incident electron beam parameters (energy spectrum, FWHM) were adjusted to match the measured data (PDD and profile) for both energies at SSD=100 cm for 40x40 open field. These parameters were then used to calculate vertical dose profile at 1mm depth at the treatment distance of 400 cm. Floor was modeled within BEAMnrc using JAWS module. LATCH variable was used to track electrons history and calculate dose profile with and without electrons scattered from the floor. Dose profiles were normalized to the maximum dose from one horizontal field (gantry angle 90 degrees) at 1 mm depth. Influence of dual field angle and floor material on the contribution of floor scatter was investigated. Spectrum of the scattered electrons was calculated. Measurements of dose profile were performed in order to verify MC calculations