The relationship of exposure rate and exposure time to radiation injury in sheep.

Abstract : Radiation lethality has been studied with Co60 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, e.g., the LD50 at 660 R/hr is 237 R while at 2.0 R/hr it is 637 R. An analysis of these data show linearity through the range 660 - 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. Although the injury accumulation data do not preclude the possibility of a strictly linear relationship, the best fit to the data may be curvilinear. 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. Therefore, the departure from linearity which occurs both in studies of injury accumulation and LD50 would suggest that at least two 'recovery processes' are operative. Speculation is offered relating these results and 'recovery' mechanisms described for cell-culture systems. The results are compared with dose-rate effectiveness expressions derived for rodents and man. They do not support the assumption used in several military and civil defense documents that for man and livestock, the biological effect of an exposure received in 96 hours (termed 'brief') is the same as that received within a few minutes. Protracting an exposure from a few minutes to 96 hours results in a doubling of the LD50.