Contamination of a 15-MV photon beam by electrons and scattered photons

The 15-MV photon beam of a linear accelerator (Siemens Mevatron 20) was studied for electron and scattered photon contamination. The surface dose, attributable almost entirely to contamination electrons, has a Gaussian lateral distribution, a linear dependence on field width for square fields, and an inverse square dependence on distance from the bottom of the fixed head assembly. This geometrical dependence is consistent with the proposal that the field flattening filter is the main source of electron contamination when accessories are absent. A tissue-maximum-ratio curve in the build-up region for the electron and photon contamination was produced utilizing the linearity of dose with respect to field width. The derived contamination curve inside was similar to the measured build-up curve outside the field. The primary photon component, obtained by subtracting the contaminant contribution, showed no dependence on field size, source-to-probe distance, or presence of accessories.