title: Physical factors affecting absorbed dose to the skin from cobalt-60 gamma rays and 25-MV x rays.
abstract: 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.
mesh_terms: D003037:Cobalt Radioisotopes / Q000627:therapeutic use*; D005720:Gamma Rays; D008954:Models, Biological; D011827:Radiation; D011829:Radiation Dosage; D011874:Radiometry / Q000295:instrumentation; D011879:Radiotherapy Dosage; D011882:Radiotherapy, High-Energy; D012542:Scattering, Radiation; D012867:Skin / Q000528:radiation effects*
chemical_list: D003037:Cobalt Radioisotopes