The kinetics of repair of sublethal damage in the rat cervical spinal cord during fractionated irradiations.

The kinetics of repair of sublethal damage were investigated in the cervical spinal cord of rats. Two and 4 fractions have been given with intervals ranging from 20 min to 24 h. The occurrence of paralysis within 7 months after irradiation (due to white matter necrosis) was used as the endpoint. From dose-response curves, ED50 values (dose at which 50% of the animals develop paralysis) were determined, from which the proportion of the dose repaired (FR) during the different intervals can be calculated. It was found that the rate of cellular repair of sublethal damage was exponential, with a half-time of approximately 110 min after a fraction size of 11-15 Gy and 85 min after 7-11 Gy. The corresponding time to complete cellular repair was calculated to be 8 and 6 h, respectively. This suggests that the rate of cellular repair is faster after smaller fractions.