How common is cancer in dog mammary gland lumps?

Abstract

OBJECTIVE: To determine the prevalence of malignancy in masses from the mammary gland region of dogs with single or multiple masses. ANIMALS: 95 female dogs from which mammary gland masses had been excised. PROCEDURES: Medical records of all female dogs from which mammary gland tissue was submitted to the Angell Animal Medical Center Pathology Department from 2009 through 2014 were reviewed. For each dog, data were obtained on breed, body weight, age, reproductive status, and number, location, and histologic classification of masses. The prevalence of malignancy was compared between dogs with single versus multiple masses and among the 5 pairs of mammary glands. Dogs with single versus multiple masses were also compared with respect to age and reproductive status. RESULTS: Among 161 evaluated masses, 137 (85%) were classified as benign or nonneoplastic and 24 (15%) as malignant. Five of 95 (5%) dogs had masses that were not of mammary gland origin. Age, reproductive status, and quantity of masses (single vs multiple) were not significantly associated with the prevalence of malignancy. The prevalence of malignancy in masses from the fourth (caudal abdominal) mammary gland was significantly lower than that in the other 4 mammary glands combined. CONCLUSIONS AND CLINICAL RELEVANCE: Dogs with multiple masses in the mammary gland region were not significantly more likely than dogs with single masses to have a malignancy, suggesting that these 2 groups could be managed similarly. Further studies are needed to evaluate the clinical relevance of the lower prevalence of malignancy in masses from the fourth mammary gland.