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Radiation damage to the rat spinal cord: the effect of single and fractionated doses of X rays.

The tolerance of the rat lumbar spinal cord to single and multiple doses of X irradiation has been studied using paralysis of the hind legs as the endpoint. The paralysis, which has an acute onset progressing from the first symptoms to full paralysis within two to three days, occurs within one year post-irradiation. Dose/effect relationships were estimated at one year. Experiments which were designed to study the effect of varying the overall treatment time between two equal doses of X rays showed that a large amount of recovery from irradiation damage occurred during the first 24 hours after irradiation (D2-D1 at 24 hours was 950 rad). No further recovery took place for another 15 days but at 32 days another wave of recovery seemed to be in progress (D2-D1 at 32 days was 1250 rad). The effect of varying the number of fractions independently of the overall treatment time was investigated. The tolerance of the cord increased rapidly as the number of fractions given in a six-week period was increased from four to 30. The slope of a Strandqvist-type plot of this data was about 0.4. The data for the spinal cord have been compared with similar data for late damage in the mouse lung. The latent period between irradiation and the onset of paralysis was also investigated. At high doses, when the probability of paralysis occurring was great, the latent period was independent of the dose. At lower doses which gave rise to a lower incidence of paralysis, the latent period was inversely related to the dose. A second type of paralysis occurred later than one year after irradiation. This had a very protracted development and occurred in unirradiated as well as irradiated animals.