Poster - Thur Eve - 27: Scattering foil redesign for modulated electron radiotherapy.

Abstract

This work serves to build on existing work that discussed the advantages of removing the scattering foil from the beamline for modulated electron radiotherapy (MERT) much like flattening filter-free beams in photon IMRT. Due to concerns about the ability of the accelerators transmission ionization chamber to function normally with narrow, minimally-scattered beams as well as a limited maximum field size, this study focused on the design of a new custom scattering foil that was optimized for MERT applications. Different foil parameters such as material, thickness and shape were investigated to produce a minimally scattered beam, as opposed to a flat beam in the case of the clinical foil, while preserving the dosimetric benefits of a reduced thickness foil such as dramatically higher dose rate and reduced photon contamination in the bremsstrahlung tail. Using a Monte Carlo model that was verified against measured data, fluence profiles at the level of the transmission ionization chamber were produced and analyzed for all foil configurations, as well as PDDs and profiles in a water phantom. It was found that a simple foil of slab geometry made of low to medium Z material with the thickness determined by the maximum desirable field size and chosen electron energy produced a sufficiently scattered beam that maintained the dosimetric advantages of a scattering-foil-free beamline. These new foils, when applied to MERT, have the advantage of reducing treatment time due to higher dose rates and reducing dose to healthy tissue due to reduced dose in the bremsstrahlung tail.