Characteristics of contamination electrons in high energy photon beams

A simple method to estimate the contribution of contaminating electrons to the dose, and to evaluate their dosimetric characteristics is proposed. The method is based on a normalisation of the tissue--maximum ratio curves to a constant primary photon fluence. The contribution of the contaminating electrons to the dose is calculated by subtracting the dose relative to a small field from the dose relative to the field under consideration. The method includes the determination of the mean energy, the linear apparent attenuation coefficient, the 50% range and the maximum range of the contaminating electrons. The extrapolated surface dose normalised to a constant primary photon fluence has been found to be constant for a constant collimator opening whatever may be the source distance.