Mammary Tumors in Pets

In honor of breast cancer awareness month, I thought it would be great to provide a little information on mammary tumors in dogs and cats! First, what is a mammary tumor? Dogs and cats have glandular tissue surrounding each teat that extends from the underside of the thorax to the abdomen. The cells in this mammary tissue can undergo mutations and become abnormal as a pet gets older. Most commonly this is due to stimulation by the hormone progesterone in an intact female. Spaying a dog or cat at an early age (between 4-6 months) reduces the risk that she will develop mammary cancer later in life.

It is important to note that mammary tumors in dogs and cats behave very differently.  In dogs, if a mammary tumor is diagnosed, around 50% will be benign and around 50% will be malignant. Of those that are malignant, roughly half will spread to other parts of the body. On the other hand, out of the cats who develop mammary tumors, around 80% will be malignant.

These masses often go unnoticed by the pet parent due to their location and size, which is why most mammary masses are diagnosed at a veterinary visit during a physical exam. A sample of the mass can be taken using a needle and examined underneath the microscope. This is an inexpensive and convenient way to start ruling in or out potential causes of the mass.

If abnormal cells are seen, further testing can be done to confirm a diagnosis. If cancer is suspected, it is important to determine whether or not it has spread using additional diagnostics. These can include full bloodwork, thoracic radiographs, and sampling nearby lymph nodes. In animals where the cancer is localized to the mammary tissue, surgical removal is the preferred method of treatment with potential follow up with radiation or chemotherapy. If the cancer has spread, chemotherapy and radiation therapy are generally considered. If surgery is performed, a sample of the tissue, known as a biopsy, can be sent for complete analysis and identification. Pathology reports will provide more specific information about the tumor type, behavior, and the best treatment recommendations.

Overall, prognosis tends to be better for pets with masses less than 3 centimeters in size. If the cancer has already spread by the time of diagnosis, the survival time is further limited.