| Abstract: | Skin-sparing properties of megavoltage photon beams are compromised by electron contamination. Higher energy beams do not necessarily produce lower surface and basal cell layer doses due to this electron contamination. For a 5 ± 5cm field size the surface doses for 6 MVp and 18MVp X-ray beams are 10% and 7% of their respective maxima. However, at a field size of 40x40cm the percentage surface dose is 42% for both 6 MVp and 18 MVp beams. The introduction of beam modifying devices such as block trays can further reduce the skin-sparing advantages of high energy photon beams. Using a 10mm perspex block tray, the surface doses for 6MVp and 18MVp beams with a 5x5cm field size are 10% and 8%, respectively. At 40x40cm, surface doses are 61 % and 63% for 6MVp and 18MVp beams, respectively. This trend is followed at the basal cell layer depth. At a depth of 1 mm, 18 MVp beam doses are always at least 5% smaller than 6 MVp doses for the same depth at all field sizes when normalized to their respective Dmax values. Results have shown that higher energy photon beams produce a negligible reduction of the delivered dose to the basal cell layer (0.1 mm). Only a small increase in skin sparing is seen at the dermal layer (1 mm), which can be negated by the increased exit dose from an opposing field. |
