RECOVERY OF SHEEP AFTER WHOLE-BODY IRRADIATION: A COMPARISON OF CHANGES IN RADIOSENSITIVITY AFTER EITHER ACUTE OR PROTRACTED EXPOSURE.
NAVAL RADIOLOGICAL DEFENSE LAB SAN FRANCISCO CALIF
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The kinetics of recovery of sheep in terms of time-dependent changes in radiosensitivity LD50 have been evaluated as a function of dose rate and size of exposure to 60Co gamma or 1 Mvp X-rays. Although there were certain quantitative differences in recovery pattern, as related to both dose rate and size of the conditioning exposure, the basic pattern can be qualitatively described as consisting of three distinct phases. The first phase, lasting 5 - 10 days, consists of a shoulder or very slow recovery. This is followed in the 15 - 20 day post-conditioning period by rapid recovery which proceeds to an overrecovery or radioresistance of LD50s considerably greater than normal controls. The resistance was only a transient phenomenon and during the 24 - 30 day period the animals had returned to a more sensitive condition with LD50s slightly less than normal. This third phase appeared to remain at least as long as 75 days and may represent either a very slow recovery or the irreparable fraction. Changes in the survival time of decedents in the challenge LD50 studies appeared related to the recovery period. The mean survival times were shorter in the 20 day recovery studies and longer in those in the 20 day experiments. Speculation is offered as to physiological changes that might account for such a relationship. Author