INVESTIGATION AND MODELING OF THE SURFACE DOSE FROM LINEAR-ACCELERATOR PRODUCED 6 AND 10 MV PHOTON BEAMS

The radiation reaction from the surface dose to a patient's skin is cl inically important as an indicator of tolerance to treatment and is fr equently the most visible side-effect from external beam radiotherapy. To quantify surface dose, its build-up was investigated in a solid wa ter phantom over a range of field sizes, source to skin distances, wed ge angles and beam shaping conditions. These data were analysed and us ed to produce an empirical model of the effect of these factors which predicted surface dose to within 5% under test conditions. The effects of changing treatment techniques was investigated by comparing fixed to dynamic wedges, and multileaf collimator (MLC) with alloy blocks. I t was found that the MLC reduced the surface dose relative to alloy bl ocks but that dynamic wedges made no change relative to open fields. F ixed wedges reduced the relative surface dose as a consequence of beam hardening. It is hoped that routine clinical use of the model to calc ulate skin dose will increase awareness of situations in which the ski n sparing effect of modern linear accelerators may be compromised.