Mammary Gland Cancer in Dogs and Cats

Mammary Gland Cancer is a common form of cancer in female dogs and cats who have undergone two or more heat cycles before being spayed. It appears as one or many nodules on the underside of your dog or cat near the mammary gland underneath the nipple. In the following article, I will discuss mammary gland-associated cancers. The differences between this cancer in dogs and cats, how to prevent your pet from getting this disease, and the treatment options available.

Mammary Gland Adenocarcinoma is the term given for cancer associated with the mammary gland. It is divided into sub-types with either benign or malignant tumors. Benign tumors are unlikely to spread to other tissue, whereas malignant tumors can potentially spread to other areas and tissues. While mammary tumors’ exact appearance can vary, many nodules will feel like hard, irregular growths associated with the mammary gland. They may be red, swollen, and painful, grow, become infected, and rupture, leaking a pus-like, bloody discharge.

Several factors affect the risk of mammary cancer in dogs and cats. Spaying and obesity appear to be two of the most important factors. Dogs spayed before their first heat cycle have less than a 0.5% chance of acquiring mammary cancer, compared to about 8% in intact female dogs. The risks increase with the number of heat cycles. Dogs and cats who have gone into heat twice may have a cancer rate as high as 25%—meaning 1 in every 4! Additionally, breed and genetics may play a role in disease incidence, alongside early-in-life obesity.

It is important to note that only about 50% of mammary tumors in dogs are cancerous. Meaning half are benign and not dangerous and half are malignant/cancerous. In cats, unfortunately, the overwhelming majority of mammary tumors are cancerous. To correctly identify cancerous vs. benign tumors, a biopsy is required, which can be done as a stand-alone test or part of removing the mass or masses. Aspirates or needles placed into the mass to acquire cells are not diagnostic and cannot differentiate malignant vs. benign growths.

Surgery is the main treatment option for pets with mammary cancer without evidence that they have spread to other organs. Single tumors may be removed or the entire mammary chain, depending on a case-by-case evaluation. Routine bloodwork and radiographs are recommended before surgery to identify signs of cancer elsewhere in the body and to assess the surgical risk to the patient. Other treatment options include radiation therapy, chemotherapy, and anti-inflammatory medications, which may help limit the inflammation. These may also improve outcomes and limit associated pain. Studies also suggest that the tumor size can predict outcomes, with smaller tumors having better long-term outcomes than larger tumors.

An ounce of prevention is worth a pound of cure. For this and many other reasons, it is recommended to Spay your female dogs and cats before their first heat cycle. While there is a long-standing debate about the correct time to spay your pet, spaying at a young age before the first heat cycle almost eliminates the risk of mammary cancer in your dog or cat. Spaying at about six months of age is the most common veterinary recommendation at this time.

Veterinary Hospital for Mammary Gland Cancer

Mammary tumors are painful and often dangerous, and it’s best to avoid them by spaying early. To find out more about spaying your dog or cat, or if you think your pet may have a mammary tumor, I recommend calling one of our offices to schedule an appointment with a member of our Veterinary Hospital. With a little information and a lot of love, we’ll help keep your dog’s tail wagging and your cat purring for years to come.


Jeffrey Stupine  V.M.D.

Medical Director

World of Animals Veterinary Hospitals