Surface dose from megavoltage therapy machines

To determine the cause of increased skin reactions near the edges of the field the authors measured doses at and near the surface of the skin with an extrapolation chamber for the gamma-ray beam of an AECL Eldorado 8 60Co irradiator and the x-ray beams of a Varian Clinac 4 linear accelerator, a Varian Clinac 6 linear accelerator, and an ARCO Mevatron 8 linear accelerator. Buildup curves were measured on each machine to locate the depth of the maximum dose (Dmax). The surface dose of the central axis was measured as a function of field size. Off-axis surface doses were measured on the Clinac 4 linear accelerator and on the Eldorado 8 60Co therapy unit. No measurable increase in surface dose was found leading the authors to believe that the observed skin reaction in the axilla for the Clinac 4 linear accelerator is due to a combination of increased dose at Dmax from the anterior mantle field and an increase in exit dose from the posterior mantle field when compared to similar treatments for 60Co.