Surface dose from 6 MV photon interactions in air

Nilsson and Brahme (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 x 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 five. The surface dose measurements reported by the present authors do not lead directly to an evaluation of the air generated electron component, because electron scattering in helium is smaller than scattering in air. Thus the reduced generation of electrons in helium, compared with air, is opposed by an increase in the electron contribution from machine components, and consideration of electrons and photons backscattered from the phantom introduced further complications. The authors therefore concluded that they could not quantify the air contribution, but that the data presented and the estimated uncertainty indicate that the surface dose on axis due to air generated electrons is at least 2.5% for a 10 cm x 10 cm field and at least 12.4% for a 35 cm x 35 cm field, the uncertainty in both quantities being approximately +-1.4 percentage points. (U.K.)