Although mammary gland tumors are among the most commonly diagnosed neoplasms in female dogs, they seem to be rare in male dogs. Medical databases from Louisiana State University, Texas A&M University, and University of Wisconsin-Madison Veterinary Medical Teaching Hospitals from 1994 to 2004 were searched for male dogs diagnosed with primary mammary gland tumors. A single pathologist reviewed biopsy samples from all dogs included to confirm the diagnosis of primary mammary gland tumor and to determine surgical margins, degree of malignancy, and subtype. Eight male dogs, all purebreds, diagnosed with primary mammary gland tumors were identified. All dogs were treated with surgical excision alone and had favorable prognostic factors, including relatively small tumor sizes, benign or well differentiated malignant epithelial tumors, and no metastatic disease at diagnosis. All but 1 of the tumors were an incidental finding, which emphasizes the importance of a thorough examination of the mammary chains in all dogs, regardless of sex. Based on population data from UW and TAMU over a 6.5-year period, annual incidence rate of mammary gland tumors in females was recorded as 207/100,000, whereas annual incidence rate for males was 4/100,000.

COMMENTARY: The lack of malignant behavior in these dogs is noteworthy. In humans, the clinical course of male breast cancer is similar to that in women although the disease is much less frequent. If this were true in dogs we would have expected 50% of the male dogs should have a diagnosis of malignant neoplasia instead of the 13% reported here. I doubt we know the real prevalence of this disease as many neoplasms may be overlooked because of the benign nature, the small size, and most of these cases being detected when the dogs are presented for another problem.

Mammary gland tumors in male dogs. Saba CF, Rogers KS, Newman SJ, et al. J VET INTERN MED 21:1056-1059, 2007.