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.
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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