Renal damage in the mouse: repair kinetics at 2 and 7 Gy per fraction

The kinetics of repair of radiation damage during the intervals between fixed X-ray dose fractions of 2 or 7 Gy has been measured in the kidneys of mice. Two-gray fractions were given as one pair per day for a total of 9 days, with intervals of 0 to 12 h between the two doses in each pair. Seven-gray fractions were given as a single pair, also with different interfraction intervals. Variable top-up doses of d(4)-Be neutrons were given as two fractions separated by 1 week, starting 1 day after the last X-ray dose in each of these schedules, to increase underlying renal damage to the level at which late functional injury could be measured by reduction in renal clearance and decrease in hematocrit to 45 weeks after irradiation. Two separate experiments were carried out, and when the data from the two studies were pooled, there was no difference at all in the repair rate between the schedules using 2- or 7-Gy fractions; values of repair half-time were within 1% of one another, demonstrating no dependence on dose per fraction. In both the individual experiments and the pooled data set, repair occurred single-exponentially at both 2 and 7 Gy per fraction and there was no evidence of a "slow" component. Analyzing the whole data set together gave a repair half-time of 1.29 +/- 0.16 h (95% confidence limits) and an alpha/beta ratio of 3.22 +/- 0.16 Gy.