Repair rate in mouse lung after clinically relevant radiation doses per fraction

Data published previously have shown that repair of sublethal damage in mouse lung proceeds with two significantly different repair half-times of 0.4 h and 4.0 h and that the fast component has approximately four times more weight than the slow component. None of these data, however, were obtained after small dose fractions similar to those used in clinical radiotherapy. The purpose of the experiments presented here was to determine the half-time of the fast component only of repair in mouse lung after doses per fraction of 2.0 Gy. We irradiated the whole thoraces of mice with six equal doses of 2.09 Gy given at intervals ranging from 0 to 45 min. The dose was topped up 24 h later by a range of single doses designed to bring the response, i.e. breathing rate and death from pneumonitis, into the observable range. Data on breathing rate were converted into quantal response data. All data were analyzed by the linear-quadratic model that contains two rates of repair (H. D. Thames et al., Radiother. Oncol. 15, 49-53, 1989). The data showed that the repair rate is very rapid, giving a t( 1/2 ) ranging from 0.25 to 0.75 h for breathing rate and mortality, in agreement with our data published previously for higher dose fractions. There were no differences between the t( 1/2 )'s obtained from the two assays of damage. These data indicate that the half- time of the fast component of repair in mouse lung is approximately 0.4 h after clinically relevant dose fractions.