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Neutron-induced Mammary Neoplasms in the Rat¹

Section of Radiation Biology, Department of Radiology, University of Tennessee College of Medicine, Memphis, Tennessee 38103

Fission neutrons are effective in producing mammary neoplasms in the female Sprague-Dawley rat. Other workers have established a linear relationship between a single whole-body dose of X- or -rays, at least between 25 and 400 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 partial- and 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 whole-body exposure of 35 rads of fission neutrons (mean energy, about 1 MeV). Seventy-five and 80% of these 2 groups of irradiated rats showed palpable mammary neoplasms during their lifetimes. No significant differences were observed in the histopathology of their neoplasms.

Histopathological data were summarized for 126 mammary neoplasms in rats exposed to fission neutrons (single whole-body doses varying from 5 to 250 rads).

Comparisons were made of mammary neoplasms induced by X- and -rays in Sprague-Dawley female rats in studies from 2 different laboratories. The results of the X- and -ray exposures were similar.

¹ This research was initiated (in 1961) under the auspices of the United States Atomic Energy Commission at Argonne National Laboratory, Division of Biological and Medical Research, Argonne, Ill. The work has been continued since 1967 in the department of Radiology at the University of Tennessee College of Medicine, supported in part by USPHS Grant CA 10635. A paper on this work was read at the Fourth International Congress of Radiation Research held in July 1970 in Evian, France.

Received 3/ 5/71. Accepted 2/ 3/72.