The Effect of Exposure Rate on Animal Lethality and Spleen Colony Cell Survival

In this study we examined the response of mice to x-rays delivered at various exposure rates from 2.86 to 105 R/min. Mouse lethality was observed over a 30-day period postirradiation. The LD50/30 increased with decreasing exposure rate throughout the range studied with the greatest change occurring at lower exposure rates. Survival times and death distributions of the mice were analyzed and the results indicated that the time at which the mice died was independent of the exposure rate employed. The greatest mortality occurred between 9 and 30 days postexposure. From these results, bone marrow injury was interpreted as the primary cause of death of the mice at all exposure rates. The effect of exposure rate variation upon hematopoietic stem cell survival as estimated by the ability to form colonies (CFU) was studied. Cells obtained from the femoral marrows of mice were transplanted into irradiated recipient mice and subsequently irradiated within the recipients. The survival...