Tumor and normal tissue response to irradiation in vivo: variation with decreasing dose rates

Continuous irradiation in vivo at dose rates ranging form 0.54 to 274 rads per minute was performed in mice using the EMT6 tumor, the gut clone system, and the bone-marrow CFU system. A progressive increase in D0 and decrease in ñ were seen with decreasing dose rates in the EMT6 tumor and the gut clone system. The D0 showed little change in the bone-marrow CFU system. These findings are related to the D2-D1 values obtained from split-dose experiments at conventional dose rates. The results do not fit the various mathematical models proposed for correlating the effects of continuous low-dose-rate irradiation and acute exposure.