Relative Surface Doses From Supervoltage Radiation

Ionization measurements were taken to determine the relative surface dose for several supervoltage modalities. The relationship of relative surface dose and the physical treatment variables was studied for a Varian 4 MeV accelerator. Comparisons were then made between 4 MeV (with 2 different flattening filters), 6 MeV, and cobalt 60 radiation. For a given therapy unit, dependent variables for skin doses are target-skin distance, field size, shadow tray-skin distance, tray construction, beam entrance angle, distance on skin from central axis, and any beam modifying devices such as wedges. Loss of skin sparing for megavoltage therapy results from electron contamination arising from unfavorable combinations of these variables.