Repair kinetics as a determining factor for late tolerance of central nervous system to low dose rate irradiation

The effect of contineaous irradiation, delivered at four different dose rates (107.6, 14.7, 3.9 and 2 GyZh-1 has been investigated using the rat cervical spinal cord biological system. The endpoint was the induction of foreleg paralysis at 9 months which corresponds, as has been described before, to white matter necrosis. Paralysis occurring in 50% of the animals was taken as the isoeffect, and the ED50 (radiation dose leading to paralysis in 50% of the animals) was calculated by probit analysis. There was a constant increase in the ED50 with the decrease in the dose rate, resulting from the repair of sublethal damage (SLD) occurring during irradiation. A comparison was made with the previously published results of high dose rate (100-120 GhZh-1) fractionated irradiations (2, 4 and 10 fractions). α/β (1.6 Gy for the pooled fractionation and dose rate data) and the half-time of SLD repair (82 min) were derived. (author). 22 refs.; 4 figs.; 2 tabs