Radiobiology of Large Animals.
STANFORD RESEARCH INST MENLO PARK CALIF
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Significant accumulation of radiation injury in sheep continuously irradiated Cobalt 60 at dose rates of less than 1 Rhr has been demonstrated. The estimated LD5060 at 0.91 Rhr is 1277 R, while the amount of residual injury after an accumulated exposure of 727 R at 0.46 Rhr is indicative of an LD5060 of 1957 R. Increasing the dose rate to 3.4 Rhr, with simultaneous reduction in exposure time so that incremental weekly doses 140 R remain the same as at 0.91 Rhr, results in a significant lowering of the estimated LD5060 to 1054 R. Combined analysis of previous results from a kinetic standpoint indicates that counts of circulating leucocytes in the sheep are not primarily determined by the size of the parent stemcell population. In addition, it has been found that hematologic measures in the sheep tend to show marked departures from the normal distribution. Finally, radioactive tracer studies have shown that the total bone-marrow cellularity per kilogram of body weight is similar in mice and in sheep. Author