Skip to main content


News & Events

Animal Health: Mammary Tumors in Dogs and Cats









Just as breast cancer is very common in women, dogs and cats can develop tumors in their mammary glands as well. Luckily, there is a way to decrease the risk of dogs and cats developing mammary cancer to almost zero - spaying at an early age.

When a puppy is spayed before her first heat cycle, she has an almost 0% chance of developing mammary cancer. If she is spayed after her first heat cycle, the occurrence is still quite low, at 7%. After 2 heat cycles, incidence increases to 25%, or 1 in 4 dogs. Rates are similar for cats. The reason spaying is such an important protection against mammary cancer is related to the female hormones that course through the body throughout her heat (estrous) cycle. These hormones can continue to contribute to the growth of tumors, so spaying is also an important part of treatment in animals who already have mammary tumors. 

Mammary tumors can usually be easily detected on a routine physical exam by your veterinarian. Dogs and cats considered to be high risk for developing mammary cancer are those who are not spayed, who were known to have had litters of puppies or kittens, or if they were spayed in adulthood. Dogs and cats typically have 8-12 mammary glands; not every animal has the same number, and they may be asymmetrical (the average dog has 9 glands). If you notice a lump, swellings, or discharge from one or more glands, you should take your pet to a veterinarian for evaluation. All high-risk animals should have an exam performed at least twice a year to catch signs of mammary cancer early.

Treatment for mammary cancer is based on the size and location of the tumor, as well as the type of mammary cancer. Typically staging is performed to determine if the tumor has already spread prior to surgery. This is most routinely done by palpating lymph nodes and taking chest x-rays. A fine needle aspirate, a procedure where a small number of cells are taken from the lump and looked at under the microscope, may also be performed. Biopsy is the gold standard for diagnosis – the tumor and possibly some surrounding tissue can be surgically removed and sent to a pathologist for interpretation. These pathology results dictate the type of treatment required after surgery.

Approximately 50% of mammary tumors in dogs are benign, while the other 50% are malignant (cancerous). In cats, only 20% of mammary tumors are benign, with 80% being malignant. Prognosis depends on the size of the tumor, the type of tumor, whether or not the tumor has spread, and the grade of tumor as determined by the pathologist. Further treatment may include radiation, chemotherapy, as well as further surgery. 

Because mammary cancer is easily prevented by having dogs and cats spayed early in life, it is currently recommended to spay prior to their first heat cycle. Please visit our Spay/Neuter Clinic page for more information or to schedule a spay/neuter appointment.