Female noninbred Sprague-Dawley rats were exposed to single doses of 0.28, 0.56, and 0.85 gray (Gy = 1 J/kg or 100 rads) of X-rays or 0.001, 0.004, 0.016, and 0.064 Gy of 430-keV neutrons at 62 +/- 1 days of age and were then observed over the rest of their lives for the appearance of mammary neoplasia. As mammary neoplasms were detected, they were removed and given a classification of adenocarcinoma(s) (AC) or fibroadenoma(s) (FA) after microscopic study. All irradiated groups exhibited an increased incidence of mammary neoplasia. The tumor rate increased steeply with age of the animals, and the effect of the irradiation could be adequately described as a forward shift in time of the spontaneous incidence. The cumulative prevalence was derived from first neoplasms only, and a formalism was presented that makes it possible to derive the integral tumor rate from all neoplasms in all animals. Mortality-corrected cumulative prevalences and integral tumor rates as a function of age were given for the different doses and separately for FA and AC. The mammary FA response and the total mammary neoplastic response (including both FA and AC) were approximately proportional to the absorbed dose of X-rays or the square root of the neutron dose. The relative biological effectiveness (RBE) of the neutrons increased with decreasing dose and reached values exceeding 100 at a neutron dose of 1 mGy; the single dose of 1 mGy of neutrons produced a significant increase of the tumor rate that corresponded to a foward shift or roughly 35 days of the spontaneous incidence. The AC, taken separately, were subject to considerable statistical uncertainties due to their small numbers. However, their RBE-dose dependence was consistent with that for the FA and, even at the highest dose studied, the RBE value exceeded 10. The nonrandom development of multiple FA within individual animals appeared to be the result of differences in susceptibility to radiation. However, mammary FA and AC within individual animals were not statistically correlated.
Adenocarcinoma Adenofibroma