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Pet Owner Version

Mammary (Breast) Tumors in Dogs

By

Michelle Kutzler

, DVM, PhD, DACT, Department of Animal and Rangeland Sciences, College of Agricultural Sciences, Oregon State University

Last full review/revision Jun 2018 | Content last modified Sep 2022
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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 45% of canine mammary tumors are malignant (cancerous).

Mammary glands and tumor, dog

Mammary glands and tumor, dog

The cause of mammary tumors is unknown, but hormones play an important role in their development. Obesity is also associated with mammary tumors in dogs. 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. Additional tests (such as x-rays) are needed to evaluate whether the tumor has spread to other parts of the body.

The treatment for mammary tumors is surgery. Surgery can involve removal of just the mass, only the affected teat, the affected teat and the surrounding teats, or all 5 of the teats on the affected side of the body. Lymph nodes may also be removed. Anticancer drugs, radiation treatment, and hormonal treatments do not appear to be effective. However, medication may be helpful for dogs with inflammatory mammary carcinoma, a type of mammary tumor that is difficult to treat with surgery. The outlook for recovery depends on multiple factors, including tumor type, size, and spread to other parts of the body. 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.

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