Canine mammary tumors.

A retrospective study was conducted of 1,366 cases of canine mammary neoplasms derived from 3 different populations of dogs. The mean ages for occurrence of mammary tumors varied between populations studied from the ninth to eleventh years with malignant neoplasms tending to occur later than benign. Irradiation had no effect on incidence. Poodles had the largest numbers of papillary adenomas and benign mixed tumors. Dachshunds, chihuahuas, and cocker spaniels had a large proportion of all other types. Papillary adenomas arose from the ductile or alveolar epithelium and accounted for only 5.1% of the tumors. Benign mixed rumors comprised 45.1% and consisted of an adenomatous epithelial component and proliferative fibrous connective tissue or myoepithelium which frequently had undergone metaplasia to cartilage or bone. Malignant mixed tumors accounted for 8.5%, their malignant component being most frequently epithelial. Carcinomas represented 39.7% of the tumors. More carcinomas were seen among neutered than intact females except in the closed beagle colony where carcinoma was the most frequent neoplasm in the exclusively intact female population. Among the carcinomas, an infiltrating type, usually comprised of ductile cells infiltrating an abundant fibrous stroma, was the most common. A solid type, comprised of masses of epithelial cells with scant or inapparent stroma, was next in frequency. Papillary, lobular, and squamous types of carcinomas occurred in descending order. Malignant myoepitheliomas comprised 1.1% of the tumors; fibrosarcomas, osteosarcomas, and chondrosarcomas were rare, accounting for less than 1%. Approximately 2% of the neoplasms were from males which had no predilection for a particular type of rumor.