Mammary Gland (Breast) Tumors in Dogs

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.

Mammary gland tumors are the most common tumors in dogs. In fact, among unspayed females the risk of a mammary tumor is 26 percent. This is three times the risk of breast tumors in women. Most mammary gland tumors occur in bitches over 6 years of age (the average age is 10). Forty-five percent are cancerous and 55 percent are benign. An increased incidence occurs in sporting breeds, Poodles, Boston Terriers, and Dachshunds. Multiple tumors are common. If a bitch has one tumor, she is three times more likely to have or develop a second tumor.

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.

Inflammatory cancer is a rapidly progressive neoplasm that spreads throughout the chain of mammary glands and into surrounding skin and fat. Death usually comes in a matter of weeks. Inflammatory cancer may be difficult to distinguish from acute septicmastitis.

Malignant tumors spread widely, primarily to the pelvic lymph nodes and lungs. Before embarking on treatment, a chest X-ray should be taken to rule out lung metastases, present in 30 percent of these cancers. Ultrasonography is useful in determining whether the pelvic lymph nodes are involved. Biopsy of the tumor may not be necessary if surgical removal is contemplated. Inflammatory cancer, however, must be biopsied, because there is little to be gained in attempting aggressive treatment in these tumors.

Treatment: Removing the lump with adequate margins of normal tissue is the treatment of choice for all mammary tumors, whether benign or malignant. How much tissue will be removed depends on the size and location of the tumor. Removing a small tumor with a rim of normal tissue is called a lumpectomy. A simple mastectomy is the removal of the entire mammary gland. A complete unilateral mastectomy is the removal of all five mammary glands on one side of the body. The inguinal lymph nodes are often included in a unilateral mastectomy. A specimen is then submitted to a pathologist for a tissue diagnosis to determine the prognosis.

The success rate of surgery depends on the biological potential and the size of the tumor. Benign tumors are cured. Bitches with small malignant tumors less than 1 inch (25cm) across have favorable cure rates. Those with large, aggressive tumors are more likely to have metastatic disease and a poor prognosis.

The addition of chemotherapy, immunotherapy, and complete ovariohysterectomy does not improve cure rates, although chemotherapy may offer some relief in bitches with advanced cancers that cannot be surgically excised.

Prevention:Spaying a female before the first heat cycle reduces her risk of breast cancer to less than 1 percent. If she is spayed after one heat period, her risk is still only 8 percent. After two heat cycles, however, there is no reduction in risk.

It is important to examine the mammary glands of unspayed bitches every month, starting at 6 years of age or younger. If you feel a suspicious lump or swelling, take the dog to your veterinarian at once. Experience shows that many owners procrastinate for several months hoping that a lump will go away. Thus, the opportunity to cure many mammary cancers is lost.

This article is excerpted from “Dog Owner’s Home Veterinary Handbook” with permission from Wiley Publishing, Inc.