Measurements of Superficial Absorbed Dose with 2 MV X Rays Used at Glancing Angles

Abstract

A photographic film method of investigating the variation of absorbed dose near an air-tissue interface is described, which involves the use of a microdensitometer. The corrections necessary to allow for the effect of direct photon absorption in the film are discussed. Application of the method to a situation in which the angle of incidence of the X-ray beam varies from 0 to 180 deg. shows that at 2MV the 80 per cent depth varies from 0·15 to 1·1 mm below the surface. It is suggested that this allows adequate treatment of superficial tissue, while retaining some degree of skin saving even when the angle of incidence to the normal is 90 deg.