The frequency of mammary tumors in different species varies tremendously. They are relatively common in cats. Approximately 90% of mammary tumors in cats are malignant (cancerous). The cause of mammary tumors is unknown, however hormones play an important role in their development. Mammary tumors in cats are most often seen in older (average age 11 years) nonspayed females. Spaying at a young age reduces the risk, but the degree of protection is less precisely documented than that for dogs. Breast tumors are diagnosed by physical examination, x‑rays, and tissue samples (biopsy). They often spread to the lungs, so chest x-rays are also taken to check for that possibility. Treatment includes removal of the tumor or the entire breast and anticancer drug treatment. The outlook is worse in cats with larger tumors and those with a high grade of malignancy, as determined by the biopsy.

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