A series of 82 basaloid adenomas (BA) occurred in the mammary glands of 29/171 female beagle dogs administered oral contraceptive steroids for 5–7 years. These BA, comprising approximately 10% of the mammary nodules that developed during the study, were observed in 29 of 85 dogs (34%) that developed mammary nodules. The BA generally were less than 1 cm in diameter and usually remained static after an Initial short period of growth. Evidence of metastasis or invasion was not seen. The tumors were well delineated, having expansive, noninfiltrating borders, and were composed of uniform cords and clusters of monomorphic epithelial cells. By light and electron microscopy, the peripheral cells were palisaded and oriented against a thin basal lamina. The centrally located cells were randomly arranged and occasionally showed squamous and glandular differentiation. These tumors resembled the basal cell adenoma described in the human salivary gland as well as the cutaneous basal cell tumor and certain adnexal tumors of man and dog. The BA were seen only in bitches treated with investigational progestins, given either alone or in combination with estrogens. BA were not observed in 18 control beagles or in 30 dogs administered mestranol alone. The Incidence of BA was highest in female dogs administered WY-4355 plus mestranol or anagestone acetate plus mestranol and lower in dogs administered ethynerone plus mestranol or ethynerone alone. To our knowledge, this canine mammary tumor has not been reported previously. The mammary lesions represented a distinct morphologic entity in this series of female beagle dogs and were associated with prolonged administration of certain progestin contraceptives.
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Kwapien et al. (1977) studied this question.