Derivation of Cellular Survival Kinetic Parameters for Experimental Fractionated Irradiation of Rat Skin

Abstract

If different fractionation schedules yield similar normal tissue reactions in a given system, it may be assumed that the degree of cellular depletion in the irradiated tissues wil be about equal in each instance. On this basis, cell population kinetic parameters (mean cellular lethal dose, extrapolation number, slope ratio, regeneration rate, and repopulation limit) can be derived statistically from experimental or clinical data using a cellular lethality model designed to simulate a course of fractionated radiation therapy. Reactions following fractionated irradiation of the feet of rats were analyzed using this model and a computer search program designed for analysis of clinical data. A variety of fractionation schedules and levels of intensity in the radiation reaction of rat skin were analyzed. The parameters derived were internally consistent and were in agreement with known radiobiological parameters for rodent skin and other mammalian normal tissues. Different intensities of the skin reaction were associated with different levels of cellular depletion ranging from a log (surviving fraction) of -3 for moderate erythema to -8 for severe moist desquamation.