Surface dose from 6 MV photon interactions in air

Abstract

Nilsson and Brahme (see ibid., vol.24, p.901, 1979) calculated that the electrons generated in the air volume between a 60Co source and a surface at 100 cm contributed about half of the surface dose at the centre of a 40 cm*40 cm field. They suggested that if air were replaced with helium the surface dose attributable to interactions in the gas volume would be decreased by a factor greater than 5. Prediction of such a large change provided sufficient impetus for the author to carry out an air-replacement experiment, utilising a 6 MV research facility. The purpose of the investigation was to assess the magnitude of the air-generated electron contribution to the surface dose. It is concluded that the experimental results obtained approximately confirm the theoretical predictions of the surface dose due to scattered electrons from 6 MV photon interactions with air in the irradiated volume.