Interaction of Dimethylbenzanthracene and Diethylstilbestrol on Mammary Adenocarcinoma Formation in Female ACI Rats1

It has been reported that X-irradià tion and diethylstilbestrol (DES) act synergistically on mammary adenocarcinoma for mation in female ACI rats. The physical carcinogen, X-irradia tion, was replaced by a chemical carcinogen, dimethylbenzan thracene (DMBA), and their interaction was studied in this system. Thirty-three female ACI rats were given 13.3 mg of DMBA per 100 grams of body weight. A total of 10 mammary adenocarcinomas were found, 8 in rats with a single mammary adenocarcinoma and 2 in a single rat, over a 266-day study period. Twenty-nine rats were implanted with a cholesterol , pellet containing 5 mg of DES, and a total of 47 mammary adenocarcinomas were found, 5 in rats with a single mammary adenocarcinoma and 42 in 5 rats with 2 or more mammary adenocarcinomas. Twenty-four rats were given a combined treatment of both compounds, DES 2 days before DMBA, and a total of 126 mammary adenocarcinomas were found, 2 in rats with a single mammary adenocarcinoma and 124 in 18 rats with 2 or more mammary adenocarcinomas. The interac tion between DMBA and DES was interpreted to be synergistic in regard to the proportion of rats with one or more mammary adenocarcinomas, the proportion of rats with two or more mammary adenocarcinomas, and the median times of appear ance of both first and second mammary adenocarcinomas. These interactions between DMBA and DES resemble the previously reported synergistic interactions between radiation and DES on mammary adenocarcinoma formation in female ACI rats.