Abstract
High surface doses and a trend for surface dose to increase with increasing field size were observed with broad electron beams from a Philips SL75-20 linear accelerator. This indicates substantial contamination of the beam by the beam defining system. The EGS4 Monte Carlo code has been used to simulate the transport of the 10 MeV electron beam through the beam defining system. The trend for surface dose to increase with field size has been reproduced and the agreement between simulated and experimental central axis depth dose data is good. Two reasons for the unusual behaviour of surface dose with field size have been found. The adjustable X-ray collimators shield the phantom from secondary electrons. The electron applicators filter the beam as they get narrower, raising the mean energy of the electrons. The variations in photon dose and electron angular distribution with field size were not found to have a significant effect on surface dose. The electron energy spectra are broader than conventional formulae imply, using the half-value-layer depth and practical range. High photon fluence was observed for all field sizes.