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208 Breeds, 422 Health Conditions  |  Find a Vet

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Mammary Gland (Breast) Tumors




Condition Overview

The mammary glands in female dogs vary in number and can be determined by counting the nipples. The typical female has 10 mammary glands, 5 on each side of the mid-line, beginning on the chest and extending to the groin. The largest glands are located near the groin.


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 movable, while others adhere to the overlying skin or underlying muscle. Occasionally, the mass ulcerates the skin and bleeds.

Inflammatory cancer may be difficult to distinguish from acute septic mastitis.


Inflammatory cancer is a rapidly progressive neoplasm that spreads throughout the chain of mammary glands and into the surrounding skin and fat. Death usually comes in a matter of weeks.


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 metastates, present in 30% 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. There is little to be gained in attempting aggressive treatment in these tumors.


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 5 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. Females 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 (spreading) and a poor prognosis.

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


Spaying a female before the first heat cycle reduces her risk of breast cancer to less that 1%. If she is spayed after one heat period, her risk is 8%. After 2 heat cycles, however, there is no reduction to risk.

It is important to examine the mammary glands of un-spayed females every month, starting at age 6 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.


Please contact your veterinarian with questions regarding this condition.

Show Sources & Contributors +


Dog Owners Home Veterinary Handbook

Publisher: Wiley Publishing, 2007

Website: http://www.wiley.com/WileyCDA/

Authors: Debra M. Eldredge, Liisa D. Carlson, Delbert G. Carlson, James M. Giffen MD

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