Determination of the dose distribution of therapeutic electrons at interfaces

The variation of dose close to tissue/inhomogeneity or tissue/air interfaces was investigated in slab and anthropomorphic phantoms for electron beams between 4 and 20 MeV. Effective source-surface distance (ESSD), beam obliquity, backscatter from internal interfaces, and dose to the sidewalls of air cavities were investigated using an Attix ion chamber, different thicknesses of thermoluminescent dosimeters, and EGS4 and MCNP Monte Carlo simulations. Surface dose was found to vary due to the SSD and beam obliquity. Relative surface dose decreased as the SSD increased due to the effect of low energy scattering from the cone and ESSD determined at the surface differed from the ESSD measured at d max . The difference was found to be most significant for the combination of high energy beams and small field sizes. Surface dose increased with angle for obliquely incident beams from approximately 30° up to a maximum, which was found at approximately 80°, and then decreased rapidly. This dose maximum angle increased with increasing electron beam energy for both surface and near surface depths. Dose variations at internal tissue/inhomogeneity interfaces are due to disequilibrium of electron scatter. The backscatter of electrons (EBF) from inhomogeneities was found to be dependent on the effective atomic number and relative electron density (ρ e ) of the scatterer. Where ρ e =0.03, dose upstream of the scatterer was reduced due to a lack of backscattered electrons. EBF was dependent on the mean energy of the beam incident on the interface, but was independent of the shape of the energy spectrum. The energy of backscattered electrons at the interface was generally low, but increased with increasing atomic number of the inhomogeneity and was related to the upstream extent of backscattered electrons. Dose variation along the side wall of a cavity was dependent on the angular distribution of the beam entering the cavity and dimensions of the cavity.