Boundary Electron Dosimetry: Radiochromic Film Measurement and Monte Carlo Simulation of Electron Absorbed Dose Near Tissue Interfaces

Abstract

Tissue heterogeneity effects present a major challenge to electron beam dosimetry in radiotherapy and radiation protection. The perturbation of the absorbed dose distribution in tissue due to the presence of heterogeneous material boundaries was investigated in this work. Experiments were conducted in a tissue-equivalent phantom in order to quantify electron backscatter from various materials. For these experiments, irradiations were performed using a 6MeV (nominal) electron beam under conditions of one dimensional geometry. Depth-energy degradation of the electron beam provided mean electron energies of 2.3MeV, 1.9MeV, and 1.4MeV at interface locations. Backscatter phenomena were investigated for the following interface geometries: polystyrene/air, polystyrene/cortical-bone-equivalent plastic, polystyrene/copper, and polystyrene/bismuth. Novel radiochromic film dosimetry techniques were developed for these experiments, and the dose and energy response characteristics of GAFChromic Type 37-041 film were investigated. Monte Carlo simulations of the experiments were performed in parallel, using tile ITS TIGER code, and methodologies were developed to determine appropriate input parameters to these simulations. From experimental and Monte Carlo results, the backscatter factor at the interface, its spatial variation with depth, and its dependence on electron energy and scatterer atomic number were investigated.