Test of equal effect per fraction and estimation of initial clonogen number in microcolony assays of survival after fractionated irradiation

Abstract

Equal effect per dose fraction is assumed in multifraction microcolony assays to infer the low-dose response of in situ renewal systems, e.g., intestinal crypts. The construction of a cell-survival curve requires an initial count of cells capable to repopulating each renewal structure. A method of designing fractionation protocols which provides a regression estimate of the initial number of clonogens per renewal structure and a test of the equal effect per fraction is described. The essential factor in the design is using common dose fractions (the same dose per fraction in series with different numbers of fractions). The method is applied to data for which the assumption of equal effect per fraction holds (4-h fractionation interval in mouse testis) and does not hold (1-h fractionation interval in jejunal crypt).