Recovery from radiation damage in mouse lung: interpretation in terms of two rates of repair.

A reanalysis was performed of the extensive data set obtained with fractionated irradiations of mouse lung reported by Travis et al. (Int. J. Radiat. Biol. 52, 903-919, 1987). The possibility was investigated that the poor fit of these data to the linear-quadratic model might have been the result of the presence of two rates of repair of sublethal damage instead of one. Therefore, the incomplete-repair linear-quadratic model was adapted to incorporate two independent rates of repair and the data were analyzed using this two-component incomplete-repair model. The results which are subjected to certain qualifications with respect to the assessment of the validity of the confidence limits indicated the presence of two significantly different repair rates, corresponding to a fast-repair half-time (t1/2) of 0.40 h (0.28, 0.53) and a slow t1/2 of 4.01 h (1.55, 6.57). A weight factor determined simultaneously indicated that the fast component has approximately four times more weight than the slow component. The alpha/beta value calculated for the entire data set using the same model was 3.8 Gy (3.0, 4.6), which is not significantly different from the alpha/beta of 3.6 Gy (2.8, 4.5) calculated for the 8- and 12-h data only, using the complete-repair linear-quadratic model. An experiment specifically designed to test the significance of the fast-repair component was performed in which mouse lungs were irradiated with two equal dose fractions, separated by intervals ranging from 10 min to 6 h. Data obtained from this experiment allowed only one repair rate to be determined, corresponding to a t1/2 of only 0.4 h. This finding confirms the presence of a very fast repair rate in mouse lung.