Issue Description A mammary tumor is a tumor originating in the
mammary gland. It is a common finding in older female dogs and cats
that are not spayed, but they are found in other animals as well. The
mammary glands in dogs and cats are associated with their nipples and
extend from the underside of the chest to the groin on both sides of
the midline. There are many differences between mammary tumors in
animals and breast cancer in humans, including tumor type, malignancy,
and treatment options. The prevalence in dogs is about three times
that of women. In dogs, mammary tumors are the second most common
tumor (after skin tumors) over all and the most common tumor in female
dogs. The lifetime risk of intact (not spayed) female dogs to develop
mammary tumors has been estimated to be about 25 percent, with a much
lower risk (about 1 percent) in male dogs and a risk in cats about
half that of dogs.
Causes Female dogs who are not spayed or who are
spayed later than the first heat cycle are more likely to develop
mammary tumors. Specifically, dogs spayed before their first heat have
0.5 percent the risk of intact female dogs, and dogs spayed after just
one heat cycle have 8 percent the risk. The tumors are often multiple.
The average age of dogs with mammary tumors is ten to eleven years
old. Obesity at one year of age and eating red meat have also been
associated with an increased risk for these tumors, as has the feeding
of high fat homemade diets.
Breeds At Increased Risk
Symptoms There can be a single or several tumors, and
they can occur in one or more glands. The last two sets of glands (the
4th and 5th glands) are most commonly affected. The tumors can be firm
or soft, well-defined lumps or diffuse swellings. Tumors can be
attached to underlying tissues or moveable, skin-covered or ulcerated.
They can be different sizes, and they may grow slowly or quite fast.
Most dogs are seen by the veterinarian for signs associated with the
primary tumor and are otherwise feeling well. A few dogs are diagnosed
with advanced metastasis (tumors that have spread to elsewhere in the
body, such as the lungs and lymph nodes) and might be feeling ill from
their tumors when they come for treatment.
Types Of Tumors Fibroadenoma A benign glandular tumor for which no treatment
"Mixed" Mammary Tumor What is mixed is the type of cell that makes
up the tumor: the epithelial cells that line the glandular tissue and
the mesenchymal cells that make up the non-glandular portion. ("Mixed"
does not refer to a mix of benign and malignant cells.) The mixed
tumor can be either benign or malignant and the biopsy will indicate
Adenocarcinoma Adenocarcinomas can be "tubular" or "papillary"
depending on that gland cells the tumor arises from. Adenocarcinomas
behave malignantly but how aggressively malignant they are depends not
on whether they are tubular or papillary but on other cellular
characteristics described by the pathologist (such as how quickly the
cells appear to be dividing and how closely they resemble normal gland
cells). When the oncologist reads the description he or she will be
able to determine how aggressively to combat the tumor.
Inflammatory Carcinoma A highly malignant tumor that generates
tremendous inflammation locally with ulceration, pus, and discomfort.
This type of tumor tends to spread early in its course and is
difficult to treat. Fortunately, this especially tragic tumor type
accounts for less than 5% of mammary tumors.
Diagnosis Appearance and location of the tumor is enough
to identify it as a mammary tumor. Biopsy will give type and
invasiveness of the tumor.
Treatment Surgical removal is the treatment of choice,
but chest x-rays should be taken first to rule out metastasis. Removal
should be with wide margins to prevent recurrence, taking the whole
mammary gland if necessary. Because 40 to 50 percent of dog mammary
tumors have estrogen receptors, spaying is recommended by many
veterinarians. A recent study showed a better prognosis in dogs that
are spayed at the time of surgery or that had been recently spayed.
Chemotherapy is rarely used.
Prognosis Size of the tumor is thought to be a good
prognostic indicator. Dogs with smaller (less than 5 cm in diameter)
malignant tumors have a better prognosis for long-term survival.
Metastasis is observed in about half of malignant tumor cases.
Histologic evaluation is critical not only to determine the origin of
the neoplastic cells, but also to find microscopic evidence of
possible metastases. Early surgical intervention and the method of
surgery play important roles in prolonging survival.
Ovariohysterectomy at the time of tumor excision has been recognized
to increase survival time.