Repair kinetics in pig epidermis: An analysis based on two separate rates of repair

Pig skin was irradiated using Sr-90/Y-90 plaques and the dose-related incidence of induced moist desquamation was determined. The repair of radiation-induced sublethal damage (SLD) was studied by fitting these response data to the generalized LQ equation for incomplete repair using quasilikelihood methods with binomial statistics, and either a Poisson or logistic link to relate the probability of response to the covariates. A Poisson response analysis based on the assumption that SLD was governed by two repair processes gave estimated repair half-times of 0.20 [(95% confidence limits) 0.12, 0.34] and 6.6 [4.3, 10.0] h. The estimates of the short and long repair half-times were significantly different, although there was no significant difference between the results using the Poisson and logistic modes of analyses. The partition coefficient for the longer repair process was 0.5 [0.34, 0.71] indicating that about 33% of SLD-derived lethal damage is associated with the longer repair process in the case of 'complete repair' protocols. However, this proportionation is, in general, protocol dependent for incomplete repair protocols. A chi(2) test on the residual deviance showed that the assumption of two repair processes for SLD gave a superior fit to the data than a single repair process at a significance level > 99%. The radiation dose to the assumed target cell population depends upon their depth from the skin surface, due to the relatively short range of the electron emission from the Sr-90/Y-90 plaques. However, further modelling analyses have shown that the estimated repair half-times were independent of the assumed target cell distribution in the skin. This is in contrast with the alpha/beta ratio, where different (clinically significant) estimates can be obtained depending upon the assumed target cell distribution. If the target cells were at 16 mu m depth from the surface of the skin, the estimated value for the alpha/beta ratio using the biphasic repair model would be 4.6 [3.6, 5.6] Gy (Poisson analysis). However, the estimates decrease with the assumed depth (distribution) of the target cells.