Breast Cancer in Dogs and Cats

Breast tumors are pretty common in pets, but they usually go by the name “mammary masses.” These lumps are most often noticed during a grooming appointment or belly rub, and they frequently resemble a BB lodged under the skin. I’ve often been asked whether I thought that someone could have shot the pet with an air rifle (“the neighbors” are often advanced as potential suspects), but that hard little mass is usually composed of abnormal mammary tissue. What lies ahead when we discover a tumor (or a handful of them) on the belly of a furry friend?

As with any other unidentified lump, our primary concern is finding out whether it intends to grow quietly (benign) or spread aggressively into other parts of the body (malignant). With most other kinds of masses, your veterinarian can obtain that answer within a day or two using just a needle, a microscope slide, and the help of a friendly pathologist. Mammary tumors are special, however, because they generally refuse to yield reliable answers when sampled with a needle. In this particular case, you will not know if your pet needs surgery to remove the mass until that surgery has already been completed. In other words, they all have to be removed.

Because these tumors can’t be reliably identified before performing surgery, we have to make our treatment plan based on their anticipated behavior. Canine mammary masses can arise several at a time, but a good portion of those will be benign. All of them should be removed in a single surgery, if possible, but each one will have to be examined independently by the pathologist since each one may be of a different tumor variety. While feline mammary masses usually form just one at a time, they tend to be much more aggressive. These need to be addressed immediately if we hope to prevent invasion of distant organs, such as the lung. A careful inspection of the lungs can tell us if the disease has already spread there, so we usually take a series of x-rays prior to surgery. Once a cancerous tumor has moved into the lung, we know that surgery will no longer be adequate to cure the pet. That knowledge motivates some pet owners to pursue chemotherapy after surgery, but others may decide to simply cancel surgery and make the most of the time they have remaining.

There is a bright side to this condition: it is largely preventable. By getting our female cats and dogs spayed at the traditionally recommended age of six months, we can shift the balance dramatically away from aggressive tumors and toward benign ones. These will still have to be removed by surgery, but they will form less frequently and pose less of a risk to our beloved pets’ survival. If you have a dog or cat at home, roll her over for a belly rub and check today for any unwanted lumps or bumps. The earlier we can detect this common tumor, the better chance we’ll have of success.

Dr. M.S. Regan