Merck Manual

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Mammary (Breast) Tumors in Cats


Michelle Kutzler

, DVM, MBA, PhD, DACT, Oregon State University

Last full review/revision Aug 2018 | Content last modified Aug 2018

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). They are prone to spread (metastasize) to the lungs and lymph nodes. 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 (less than 6 to 12 months of age) reduces the risk. The mammary glands on or close to the chest are most likely to be affected. Mammary tumors should not be confused with mammary hypertrophy, a benign overgrowth of breast tissue.

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, the entire breast, the breast and surrounding breasts, or the entire mammary chain and associated lymph nodes. Anticancer drug treatment may also be appropriate. The outlook is worse in cats with larger tumors and those with a high grade of malignancy, as determined by the biopsy.

Also see professional content regarding mammary tumors.

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