Cancer in general afflicts an estimated 30 percent to 40 percent of all cats, and one-third or so of these malignancies involve the mammary glands. Tumors originating in these glands account for the third most common type of feline cancer (after lymphoma and skin cancer). Occurring more than 95 percent of the time in females, it is the most frequently diagnosed type of feline cancer in cats older than 10 years of age.

According to Margaret McEntee, DVM, a professor of oncology at Cornell University’s College of Veterinary Medicine, approximately 85 percent of feline mammary cancers are lethally malignant adenocarcinomas. These growths originate in the epithelial tissue of a gland beneath a nipple and eventually spread (metastasize) to the lymph nodes, lungs, pleura, liver, adrenal gland, kidney, or other parts of the body.

A cat has two “chains” of four mammary glands and nipples running parallel on each side of its belly. “A tumor can start as a small, firm nodule, about the size of a BB, just beneath or next to a nipple,” explains Dr. McEntee. “Although the tumor can become fixed at its point of origin, it will usually spread to the lymph nodes and then to other parts of the body. In its initial stage, when the growth is small, it’s not going to be painful, you won’t be able to feel it, and there will be no obvious clinical signs. It may take weeks or months before it is noticeable.”

The underlying causes of feline mammary gland cancer are unknown. Genetic influence has not yet been found to play a role. Likewise, while links between external cancer-causing agents—such as environmental carcinogens, exposure to sunlight, viruses, and vaccine injections—have been established in relation to various other forms of feline cancer, these factors do not appear relevant to the onset of mammary cancer in cats.

On the other hand, says Dr. McEntee, the hormone status of a female cat is significant—specifically the roles played by the two female reproductive hormones, estrogen and progesterone.  “If these hormones are given to unspayed cats as contraceptives or for behavior modification either orally or by injection,” she notes, “their risk of mammary cancer can triple.”

A cat’s breed may also play a role. For example, says Dr. McEntee, “Siamese cats, for unknown reasons, have twice the risk of other breeds, and they also tend to get the cancer at an earlier age.” (Although the average age of cats with mammary cancer is between 10 and 12 years, the disorder can affect animals anywhere from less than one year to 20 years of age or older.)

Definitive diagnosis of mammary cancer is usually achieved by removing a portion of affected tissue—or all of it, if possible—and submitting the sample for biopsy, a histopathologic examination that will determine whether the cancer is benign or malignant. Chest x-rays and abdominal ultrasound imaging may also be used to determine whether the mammary mass has spread to the lymph nodes and subsequently to the internal organs.

If the cancer has spread throughout a cat’s body, treatment would center on chemotherapy. But if the tissue involvement is confined to the mammary glands, mastectomy would be performed to remove one or both chains of the patient’s nipples, mammary glands, and underlying tissue. “If caught early,” says Dr. McEntee, “the survival chances are very good—three years or more if a tumor is smaller than two centimeters in diameter.”

To dramatically reduce the risk of feline mammary cancer, Dr. McEntee urges owners to have their female cats spayed before they enter their first heat cycle, as young as three to four months of age.  Indeed, one study indicated that cats spayed prior to six months of age had a 91 percent reduction in risk and those spayed prior to one year of age had an 86 percent reduction.

In addition, an owner can play a valuable role in recognizing the presence of a mammary tumor at an early stage of its development by routinely palpating a cat’s underside in the areas of the mammary glands and nipples. This should be done on a weekly basis, the objective of which is to detect the formation of obvious masses or lumps of tissue when they are most treatable. Of course, any such alarming discovery should be reported to a veterinarian without delay.