Implications of the quadratic cell survival curve and human skin radiation "tolerance doses" on fractionation and superfractionation dose selection.

An analysis of early published multifraction orthovoltage human acute skin irradiation tolerance isoeffect doses is presented. It indicates that human acute skin radiation reactions may result from the repetition, with each dose fraction, of a cell survival curve of the form: S = e-(alpha D + beta D2) The analysis also shows no need for an independent proliferation related time factor for skin, for daily treatments of six weeks or less in duration. The value obtained for the constant beta/alpha for orthovoltage irradiation from these data is 2.9 x 10(-3) rad-1 for the cell line determining acute skin tolerance. A radiation isoeffect relationship, based on the quadractic cell survival curve, is introduced for human skin. This relationship has some advantages over the nominal standard dose (NSD). First, its use is not restricted to tolerance level reactions. Second, a modification of the relationship, which is also introduced, may be employed in the selection of doses per treatment when irradiation dose fractions are administered at short intervals where repair of sublethal injury is incomplete.