Mammary gland tumors remain one of the most common cancers in our canine patients. Despite the high incidence, there have
been few significant advances in the treatment of these tumors. In this article, I review the current standard of care and
present new insights into the biology of these tumors.
In dogs, the biologic behavior of mammary gland tumors is highly variable. The rule of thumb has been that 50 percent of mammary
gland tumors are benign, and of the 50 percent that are malignant, half can be cured by surgery alone. This means that 25
percent of patients are at high risk of metastasis and require therapy beyond surgery alone.
It is well-documented that early ovariohysterectomy (OHE) significantly decreases the risk of development of mammary gland
tumors in the future. Body mass index as a juvenile may also influence the risk for future development of these tumors. Dogs
assessed as being lean at 1 year of age have a lower risk of developing mammary gland cancer. New areas of interest include
looking into dietary manipulation as another factor to consider in preventing these tumors.
For those dogs that do develop mammary tumors, it has been shown that up to 60 percent will develop multiple tumors, and dogs
that have had a malignant tumor are at higher risk of developing a second malignant tumor compared with dogs with benign tumors.
However, these separate primary tumors likely share a common etiology. It has been suggested that mammary tumors in dogs develop
as a result of a phenomenon known as field carcinogenesis, which involves the idea that multiple tumors are the result of all of the mammary tissue being exposed to the same hormonal
Well-known prognostic factors include tumor size, tumor type, grade of tumor, degree of differentiation, stage and the presence
of vascular or lymphatic invasion. Equally important to note, factors that have not been shown to be prognostic include tumor
location, number of tumors or the type of surgery performed.
Most studies have shown that tumors 3 cm or less have a better prognosis, although a recent study showed the same to be true
of tumors up to 5 cm.1 In this study, tumors > 5 cm were more likely to be associated with nodal metastasis, as were tumors that were present less
than six months before surgery. Results also indicated that an OHE performed at the time of tumor removal significantly improved
the two-year survival rate. These findings indicate that while tumor size is an important prognostic factor, it is necessary
to look at other variables when estimating survival times.
Tumor types that have a negative prognosis include mammary gland sarcomas and inflammatory mammary gland tumors. Mammary gland
sarcomas (fibrosarcoma or osteosarcoma) are uncommon but have been reported. The average survival time for mammary gland sarcomas
of any kind is short, usually between 9 and 12 months. Inflammatory mammary gland tumors are highly aggressive and typically
present as a rapidly growing tumor with ulceration, edema and erythema. On histopathologic examination, these tumors are poorly
differentiated, with invasion of dermal lymphatics. Treatment typically consists of palliative care only, and the survival
time for these patients is one to two months. Surgery is not recommended for patients with this tumor type.
Tumor grade has also been used as a prognostic indicator. Tumors that are classified as high grade have a high mitotic index,
poor nuclear differentiation and significant necrosis. However, pathologists have not adopted a standard grading scheme for
mammary tumors, so often the grade must be inferred from the microscopic description.
In addition, other molecular factors such as estrogen and progesterone receptor status, proliferative indices and the presence
of certain gene mutations can predict biologic behavior. These factors have advanced our understanding of the biologic behavior
and progression of mammary gland tumors but are not widely available for use in routine evaluation of clinical patients. In
a recent study, 113 dogs with both malignant and benign mammary tumors were evaluated for the presence of estrogen and progesterone
receptors.2 Almost all of the benign tumors had both estrogen and progesterone receptors, but in the malignant tumors, only 50 percent
had estrogen receptors and 70 percent had progesterone receptors. Tumors that were positive for both types of receptors were
more likely to be < 5 cm and were less likely to have metastasized at the time of diagnosis. Particularly, the dogs that had
the highest one-year survival rate were the dogs that were positive for progesterone receptors. The results of this study
suggest that determining the presence of these hormone receptors may help predict tumor behavior.