Site of origin of mammary tumors induced by 7,12-dimethylbenz(a)anthracene in the rat.

Serial sections of mammary glands from female Sprague-Dawley rats were examined at different intervals after rats were treated with 7,12-dimethylbenz[a]anthracene (CMBA). DMBA was given either by intravenous (iv) injection or by local application. Lesions with histologic characteristics typical of mammary adenocarcinomas were observed in the mammary glands as early as 20 days after DMBA treatment, when no other lesions were detectable. Whereas hyperplastic alveolar nodules in the mammary glands began to appear 35 days after iv injection of DMBA, they were never observed after local application of the carcinogen. All mammary tumors were adenocarcinomas of ductal origin.