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The Effect of Infection, Hormonal Environment, and Genetic Constitution on Mammary Tumor Induction in Rats by 7,12-Dimethylbenz (a) anthracene¹

Cancer Research Centre, University of British Columbia, Vancouver, Canada

Mammary carcinomas arose in 82% of virgin Sprague-Dawley rats kept in constant light and given a single dose p.o. of 20 mg of 7,12-dimethylbenz(a)anthracene at 55 days of age. Fibroadenomas occurred in 44%. Other rats of the same age and strain given the same dose of the carcinogen but kept in total darkness developed a similar incidence of breast cancer (87%) but only 1 fibroadenoma among the group of 39. The incidence of mammary cancer was reduced to 30% in Sprague-Dawley rats exposed to chronic lung infection, and there was also a reduction in the number of fibroadenomas. The injection of Metopirone, deoxycorticosterone, or progesterone for 0.5 day prior to and 4.5 days after carcinogen dosage reduced the breast cancer incidence to 20, 25, and 42%, respectively. Spontaneous regression of some of the mammary tumors was observed in all groups. Only 5 of 20 rats of a hooded strain developed mammary carcinomas after dosage with the carcinogen, and the latent period of these was much longer than in the Sprague-Dawley strain.

¹ Supported by the National Cancer Institute of Canada.

Received 3/28/66. Accepted 6/14/66.