The Relationship of Exposure Rate and Exposure Time to Radiation Injury in Sheep

Radiation lethality in sheep has been studied with⁶⁰ Co gamma radiation at five exposure rates ranging from 660 to 2.0 R/hr. The data show that a decrease in the exposure rate (or protracting the exposure over longer time periods) results in an increase in the LD₅₀. An analysis of these data shows linearity through the range 660 to 30 R/hr. However, between 30 and 3.6 R/hr, a sharp departure from this relationship occurs, with the LD₅₀' s at 3.6 and 2.0 R/hr being considerably higher than would be predicted from extrapolation. In another series of experiments, the kinetics of injury accumulation at 3.6 R/hr has been studied. The nature of the curve would indicate that the rate of injury accumulation is decreased as the total exposure and time of exposure are increased. The departure from linearity which occurs in both studies might suggest that at least two "recovery processes" are operative. Speculation is offered relating these results, and "recovery" mechanisms as described for cell-culture systems. The results are compared with expressions of dose-rate effectiveness derived for rodents and man.