A mammary tumor is a tumor originating in the mammary gland. It is a common finding in unspayed female dogs. Mammary gland tumors are the most frequently diagnosed neoplasms in female dogs. It is a common finding in unspayed older female dogs (the incidence rate is one in 4 in unspayed female dogs over the age of 4). While this condition is more likely to occur in the female population, it does also affect male dogs, albeit rarely. When a male dog is affected by a tumor of the breast, the prognosis is much more guarded and grave.
The mammary glands in female dogs vary in number and can be determined by counting the nipples. The typical bitch has 10 mammary glands, five on each side of the midline, beginning on the chest and extending to the groin. The largest glands are located near the groin. The risk of this tumor can be greatly reduced by spaying a female dog, specially before it goes into heat.
There are two forms of mammary gland tumors; benign and malignant. Approximately 50% of all mammary tumors in dogs are benign, which may be classified as complex adenomas, simple adenomas, fibroadenomas, and duct papillomas while the other 50% are malignant which are classified as osteosarcomas, fibrosarcomas, solid carcinomas, and papillary cystic adenocarcinomas, amongst others.
There are some breeds that are predisposed to this condition including toy and miniature poodles, English springer spaniels, Brittanys, cocker spaniels, English setters, pointers, German shepherd dogs, Maltese, and Yorkshire terriers. The average age is about 10 years and the neoplasm is less likely to occur in dogs younger than five years.
The principal sign is a painless lump or mass. Most lumps occur in the larger glands closest to the groin. A mass may be large or small, with boundaries that are distinct or indefinite. Some lumps are freely moveable, while others adhere to the overlying skin or underlying muscle. Occasionally, the mass ulcerates the skin and bleeds.
Other signs and symptoms of this tumor are
Usually slow-growing single or multiple masses in the mammary glands – about half of patients have multiple tumors
May have superficial loss of tissue on the surface of the skin over the mammary tissue, frequently with inflammation
Mass may be freely movable, which implies benign behavior
May be fixed to skin or body wall, which implies malignant behavior or cancer
