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Physical factors affecting absorbed dose to the skin from cobalt-60 gamma rays and 25-MV x rays.

When there is an absence of scattering material adjacent to the skin on the exit side of a megavoltage beam the dose to the skin is less than would be calculated using depth dose tables or isodose distributions measured in "semi-infinite" phantoms. Ionization measurements using a thin-window parallel-plate chamber show that the dose at 4 mg/cm2 from the exit surface is 14% to 16% less for cobalt-60 gamma rays and about 8% less for 25-MV x rays compared to the dose with full backscatter. As the angle of incidence increases the skin dose increases due to radiation scattered toward the surface. A method for the calculation of skin doses from tangential therapy beams is described.