Progressive development of radiation damage in mouse kidneys and the consequences for reirradiation tolerance

The influence of protracted overall treatment times on the development and repair of renal irradiation injury was studied in mice. There was no difference between time of onset or rate of development of damage after two equal fractions in 1 day or 1 month, but there was a time lag of 7-15 weeks (depending on dose) before development of damage after 2F given in 6 months, when the rate of progression of damage was the same for 2F/6 months as for 2F in the shorter intervals. Tolerance of mouse kidneys to reirradiation at 6 months after single doses of 6-12 Gy was also assessed. All previously irradiated animals developed more severe renal impairment after reirradiation than age-matched controls. Severest damage occurred in mice receiving highest initial radiation doses, but doses of 6 Gy were sufficient to markedly reduce tolerance to reirradiation. It is concluded that no additional dose-sparing took place in the kidneys during a 6-month interval, even when initial radiation dose alone was insufficient to cause measurable renal dysfunction. (U.K.)