The frequency of mammary tumors in different species varies tremendously. The dog is by far the most frequently affected domestic species, with a rate that is about 3 times that found in women. About half of all tumors in female dogs are mammary tumors. Approximately 40% of canine mammary tumors are malignant (cancerous).

There are 10 sets of mammary glands in dogs, any of which may be the site of a tumor. Most tumors occur in the glands closest to the hind legs.

The cause of mammary tumors is unknown, but hormones play an important role in their development. Mammary tumors in dogs occur most often in non-spayed female dogs or females spayed late in life; they are extremely rare in male dogs. Female dogs that are spayed before their first heat cycle are no more likely to develop mammary tumors than male dogs. Breast tumors are usually diagnosed by physical examination, but confirmation and identification of the type of tumor requires a biopsy.

There are several treatment choices, including surgery to remove the tumor or the entire breast and anticancer drug treatment. The outlook for recovery depends on multiple factors. Most canine mammary tumors that are going to cause death do so within 1 year. The risk of this disease can be greatly reduced by spaying the dog before it first comes into heat.

Last full review/revision July 2011 by Cheri A. Johnson, DVM, MS, DACVIM (Small Animal); Brad E. Seguin, DVM, MS, PhD DACT; Autumn P. Davidson, DVM, MS, DACVIM; Fabio Del Piero, DVM, DACVP, PhD; James A. Flanders, DVM, DACVS; Mushtaq A. Memon, BVSc, MS, PhD, DACT; Paul Nicoletti, DVM, MS; Robert C. Rosenthal, DVM, PhD, DACVIM (Small Animal, Oncology), DACVR (Radiation Oncology)