Biologic behavior and clinical characteristics of mammary gland tumors in male dogs

BACKGROUND: Reports of mammary-gland tumors in male dogs are lacking. OBJECTIVE: To describe the clinical characteristics of mammary-gland tumors in male dogs. ANIMALS: Eight male dogs diagnosed with mammary-gland tumors. METHODS: Retrospective study. Medical databases from 3 institutions were searched. Medical records were abstracted, and owners and referring veterinarians contacted for follow-up information. Tissues were reviewed for histologic type, and immunohistochemical staining for estrogen and progesterone receptors (ER, PR) was performed. RESULTS: Eight dogs with histologically confirmed mammary-gland tumors were included in this retrospective study. Median age at diagnosis was 11.5 years. Four dogs were sexually intact; 4 were neutered. All were purebred. Mammary-gland tumors were incidental findings in 7 of 8 dogs. All dogs were treated with only surgical excision. All but 1 dog had benign epithelial tumors. The dog with the malignant tumor was the only dog to develop possible local recurrence but de novo tumor development cannot be excluded. No dog had evidence of metastatic disease at diagnosis. Based on institutional population data, it was determined that female dogs are 62 times more likely to develop mammary-gland tumors than male dogs (P < .001). Estrogen-receptor expression was strong in the majority of tumors; progesterone-receptor expression, although present in all tumors, was less intense. CONCLUSIONS/CLINICAL IMPORTANCE: This study suggests that mammary-gland tumors in male dogs are rare, usually benign, and surgery alone can provide long-term control in most dogs.