SUM MARYSkin-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 5xScm field size the surface doses for 6 MVp and 18MVp X-ray beams are 10% and 7 % of their respective maxima. However, a t a field size of 4Ox40cm t h e percentage surface dose is 42% for both 6 M V p and 18 M V p 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, t h e surface doses for 6 M V p and 18MVp beams with a Sx5cm field size are 10% and 8%, respectively. A t 4Ox40cm, surface doses are 61 % and 63% for 6 M V p and 18MVp beams, respectively. This trend is followed a t the basal cell layer depth. A t a depth of 1 mm, 18MVp beam doses are always a t least 5% smaller than 6 M V p doses for t h e same depth a t all field sizes when normalized to their respective Dmax values. Results have shown that higher energy photon beams produce a negligible reduction of t h e delivered dose to the basal cell layer (0.1 mm). Only a small increase in skin sparing is seen a t t h e dermal layer (1 mm), which can be negated by the increased exit dose from an opposing field.
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