Re-irradiation tolerance in the rat spinal cord: influence of level of initial damage.

The influence of the level of initial radiation damage on the long term recovery and re-irradiation tolerance in the rat spinal cord was investigated. Rats were irradiated with 0, 10, 20, 30 and 36 daily fractions of 2.15 Gy initially representing 0, 25, 50, 75 and 90% of cord tolerance. After an interval of 20 weeks, retreatments were given using graded single doses of X-ray. The end-point was paralysis of the forelimbs due to white matter necrosis. Latent times to paralysis were inversely proportional to the level of initial injury and retreatment doses. The retreatment ED50S were 19.0, 17.0, 15.7, 14.0 and 11.8 Gy for the control animals and animals irradiated initially with 10, 20, 30 and 36 fractions of 2.15 Gy respectively. Using the extrapolated response dose (ERD) concept, alpha/beta of 3.0 Gy, the retreatment ED50S in % ERD were 81, 70, 58 and 42% after initial doses of 25, 50, 75 and 90% ERD respectively. The level of initial injury appeared to influence the proportion of residual injury. For an initial injury of 25 and 90% of ERD, the respective residual injury was 74 and 65% of the initial damage; for an initial injury of 50 and 75% ERD, the residual injury decreased to 59 and 57% respectively. It is concluded that there was significant long-term recovery in the rat spinal cord, and that the level of initial radiation damage influenced both the retreatment tolerance and the time to expression of injury.