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Results of Fractionation and Protraction of Total-Body Radiation on Rat Mammary Neoplasia

Brookhaven National Laboratory, Upton, New York³

Female Sprague-Dawley rats showed a high incidence of mammary neoplasia and a large number of mammary neoplasms following exposure to a single dose of 500 r of ⁶⁰Co gamma, total-body radiation. When studied over the entire life-span, these responses were not different when the 500 r were given at either 40 or 160 days of age. Fractionation and protraction of this dose into 4 x 125 r spread over 2 weeks or 8 x 62.5 r spread over 4 weeks or 16 x 31.25 r spread over 8 weeks or 32 x 15.625 r spread over 16 weeks did not change the incidence of rats with mammary neoplasia or the total number of mammary neoplasms compared to a single dose of 500 r. However, a decreased yield of adenofibroma-fibroadenomas and an increased yield of adenocarcinomas were observed. Mammary adenocarcinomas occur very late in the life of nonexposed females long after a significant incidence of adenofibroma-fibroadenomas has been established and then only in a ratio of approximately 1:5. All radiation exposures tended to shift the appearance of the adenocarcinomas forward in time so that they appeared before the adenofibroma-fibroadenomas with the resulting ratio of approximately 1–4 adenocarcinomas to 1 adenofibroma-fibroadenoma.

¹ Present address: Zoology Department and Medical School, University of Michigan, Ann Arbor, Mich. Supported in part by USPHS Grant NIH CA-05496.

² Present address: Pathology Department, The Jefferson Medical School, Philadelphia, Pa.

³ Brookhaven National Laboratory supported by the USAEC.