Time course of loss of residual radiation damage in murine skin assessed by retreatment.

The amount of radiation damage remaining in mouse foot skin has been assessed by retreatment from 10 days to 6 months after a range of first doses. The acute skin reaction was used as the endpoint. Mice hind feet were first irradiated with a range of single doses (15-37.5 Gy) covering zero to near full effect. Feet were retreated with a full range of single doses together with groups of non-previously treated age-matched control mice. No age-related changes in radiation sensitivity were observed. Dose-response curves were constructed for all retreatment times for each priming dose, and isoeffect doses were calculated for both peak and average skin reactions. If 2-6 months were allowed to elapse before retreatment, the skin could be reirradiated as if it were previously untreated. However, if only 1 month was allowed to pass before retreatment, damage was 'remembered' after all first doses. The amount of damage 'remembered' in terms of dose was 11 Gy after a first dose of 37.5 Gy, and was less after the lower first doses.