A model for calculating the effects of small inhomogeneities on electron beam dose distributions

Charged particles penetrating a medium are repeatedly deflected from their line of motion by coulomb interactions with target nuclei. This physical feature should be included in the calculation of dose distributions when the irradiated medium contains small inhomogeneities. We use the mean path taken by particles in homogeneous media to represent their motion through inhomogeneous materials. This procedure adapts the small angle multiple scattering formalism to yield manageable, approximate solutions when a medium contains arbitrarily composed inhomogeneities. To promote the development of this procedure into an electron beam treatment planning program, we discuss the salient features which arise in connection with electron beam inhomogeneity calculations. We use a solid phantom containing an air cavity just below its surface as a particular example of a small inhomogeneity. We discuss qualitatively and quantitatively the dose distributions which result from their irradiation with an 11-MeV electron beam.