Radiation induced renal damage in mice: influence of overall treatment time

The influence of overall treatment time on the radiation response of the mouse kidney was studied by varying the time over which 2 or 5 fractions of X-rays were administered. Two functional assays (urine output and 51Cr-EDTA excretion), and renal weight at sacrifice were used to obtain dose-response curves and estimate isoeffective doses. Split dose experiments showed Elkind recovery of about 5 Gy in 24 h. With a 7-day interval between fractions a transitory increase in isoeffect dose was observed in the first experiment. In the second, more extensive, experiment the recovered dose did not increase significantly even if the interval between two fractions was prolonged up to 25 days. Therefore, if slow repair occurred it was not worth more than 1 Gy because this was the limit of resolution of the assays used. As overall time was prolonged to 60 days an additional 1-2 Gy were recovered: it is difficult to explain this delayed sparing effect on the basis of a compensatory proliferative response, because the labelling indices of the likely target cells in the kidney are so low. Whatever the mechanisms involved, an increase in overall time had only a slight effect on isoeffect dose in these experiments and values for the "T" exponent were low (0.0-0.09). Recovery from sublethal injury between fractions has a much larger effect.