Radiobiology of Large Animals.
STANFORD RESEARCH INST MENLO PARK CALIF
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The relationships between dose rate and radiation response were studied in sheep and in mice. For sheep, the LD 5060 varies inversely with dose rate, even when the dose rate is several hundreds of Rhr. An initial exposure to as little as 10 R midline-air at a high dose rate appears to affect the pattern of injury accumulation during subsequent low dose-rate exposure. The LD 5060 for sheep exposed at 0.84 Rhr appears to be well in excess of 800 R. Continuous exposure to death at 3.8 Rhr results in a compression in survival time, so that all animals die within a period of seven days. Studies in mice indicate that there is no simple direct dependency of LD 5030 on fractional survival of hematopoietic stem cells as the LD 5030 increases with decreasing dose rate. A technique for evaluating erythroid activity in the bone marrow has been used to demonstrate that increased erythroid activity can occur in mice at a time post-irradiation when marrow cellularity is depressed. This technique has been found to be feasible for future irradiation studies using sheep. Author