Neutron-induced Mammary Neoplasms in the Rat1

R, and the frequency of induced mammary tumors. This apparent linear dose-effect relation does not hold after exposure to fission neutrons, at least from 10 to 250 rads, in single whole-body exposures. The female Sprague-Dawley rat has a relatively high spontaneous incidence of mammary tumors. However, these neoplasms appear late in life in unirradiated controls. Only 2% of 89 control rats showed any mammary tumors during their first year, and more than one-half of these untreated control rats showed no spontaneous tumors during their entire lives. Female Sprague-Dawley/ANL rats were irradiated with single whole-body exposures of 5 rads of fission neutrons. More than 77% (21/27) of the exposed animals developed at least one mammary neoplasm during their lifetimes. We have compared the frequency of mammary neoplasms in this neutron-irradiated group with that of similar rats exposed to either 100 or 300 R of 250-kVp X-rays, given also as single whole-body doses. The frequency of mammary tumors in the 5-rad neutron group was higher than that in the group of rats exposed to 100 R of X-rays over their entire lifetime. The relative biological effectiveness (X/n) for this system appears to lie between 20 and 60, an extremely high value for any mammal. Approximately 70% of the diagnosed tumors were benign, with fibroadenomas being the most common type of neoplasm. A comparison was made between partialand whole-body exposures of female rats to a dose of 35 rads of fast neutrons. In the former case, one mammary gland was irradiated with neutrons of a mean energy of 540 ±50 keV. In the latter case, rats were exposed to Argonne's CP-5 research reactor to a