Mammary Masses in Cats by Shann Ikezawa, DVM

The next time you’re giving your cat a belly rub, you may want to take advantage of the opportunity and check for any unusual lumps or bumps. Just like people, cats can get mammary gland tumors (breast cancer) and early diagnosis and treatment can affect long term prognosis. According to The Cornell Feline Health Center, estimates show cancer affects 30-40% of all cats and about ⅓ of these cases involves the mammary glands.

Not all cat mammary gland masses are cancerous, but unfortunately, the majority of them are. It is estimated that 90% of mammary gland masses in cats are malignant (cancerous). This is very different than their canine companions, where malignancy is estimated at 50%. Other than administration of progestin medications, there are no known causes of mammary cancer in cats. Intact and spayed female cats are most commonly affected, but male cats can develop these tumors as well. Siamese and tricolor (calico and tortoiseshell) cats appear to be predisposed to this disease.

What do you do if you feel a lump along your cat’s chest or belly? Your veterinarian will likely want to test the mass to determine what it may be. If there is evidence that it is a mammary gland tumor, surgery to remove the mass, with wide margins, is likely your best option. Testing, such as chest x-rays, a blood panel, and abdominal ultrasound, may also be recommended beforehand to ensure there is no evidence of cancer anywhere else in the body. Once the mass is removed, the biopsy report will provide further information on how aggressive the mass is, if the mass was completely removed, and whether it may have invaded the local lymphatic system.

Prognosis with mammary tumors is largely dependent on the size of the tumor. One study showed cats with tumors less than 3 cm in size have an average median survival time of around 21 months (range 3-54 months). Cats with masses larger than 3 cm had a much poorer prognosis with median survival times of 4-12 months. Radical surgery to remove all the mammary glands has been shown to increase the time before the disease recurs in patients, but not necessarily increase the median survival time. In one study, follow up chemotherapy has been shown to increase the amount of time before disease recurrence in affected cats, but not necessarily to increase survival time. In general, there is still a great deal that is unknown as far as how to best manage and prevent recurrence of mammary cancer in cats. Each individual patient will need a medical and surgical plan tailored to their unique situation and needs.

What can you do to try to prevent mammary cancer in your cat? Spaying your female cat is the most important thing you can do. A study has shown that spaying before 6 months of age decreases risk of mammary cancer by 91%, before 1 year of age, 86%. Spaying before 2 years of age decreases risk by 11%. Spaying after 2 years of age did not reveal any preventative benefits. Having kittens also did not show any protective benefits. 

Dr. Ikezawa is a 1997 graduate of the Veterinary School at the University of California, Davis, where she focused on both small animal and exotic animal medicine. She completed an internship in Small Animal Medicine and Surgery in 1998, and practiced for 2 years with an avian specialist. She was on staff at Crow Canyon Veterinary Clinic prior to its merger to become Bishop Ranch Veterinary Center. Her special interests are soft tissue surgery, internal medicine, and avian/exotic medicine (including rabbits, rodents, and reptiles). She also works closely with pug rescue and has a special interest in pugs and their unique medicine and needs