Mammary tumors are the third most common tumor type seen in the cat, and they account for approximately 20% of cancers in the female cat. These tumors arise from the mammary (breast) tissue and are typically malignant (invasive with a high chance of spreading). Most mammary tumors in cats are classified as adenocarcinomas. About 85% of mammary tumors in cats are malignant; meaning they are very invasive to the surrounding tissues and have a high rate of spreading to other areas of the body. Mammary cancer is often a disease of middle aged to older cats, with Siamese cats having a higher risk.

What causes this type of cancer in cats?

Hormonal influences are involved in the development of mammary cancer in cats. Several studies have found that cats that were spayed had a 40-60% reduced risk of developing mammary cancer.  Read more about Mammary Cancer in Dogs.

What are the common signs of this cancer?

The first sign of this type of cancer may be a fluid filled or firm lump associated with the mammary gland, or discharge originating from the nipple. These masses do not tend to be painful but can be associated with increased grooming behavior if discharge is present.

How is it diagnosed?

A biopsy of the affected tissue is needed for making a definitive diagnosis. The majority of mammary tumors in cats are malignant in nature and very likely to metastasize (spread to other organs) early in the course of disease. The most common locations for this tumor to metastasize to are the local lymph nodes and lungs. Prior to any biopsy or treatment, palpation (manually feeling) of those local lymph nodes and chest radiographs are recommended to look for evidence of metastasis.

How is this cancer treated?

Surgery can be very effective at removing the masses, but the success of surgery is hindered by the invasive nature of the disease. Due to this, an aggressive surgery is typically recommended, which involves removing all four mammary glands on the affected side (or both sides if needed), called unilateral or bilateral mastectomy. This procedure significantly reduces the chance for local re-growth of the tumor. It is important to evaluate the local draining lymph nodes during surgery due to the high risk of cancer spreading to those sites.

Since mammary cancer in cats typically invade the lymphatic system and spread to other locations, chemotherapy is recommended following surgery. Most effective protocols involve the use of a chemotherapeutic agent called doxorubicin alone or in combination with other drugs. Most cats tolerate chemotherapy exceptionally.

What is the prognosis for cats with this cancer?

The average survival time for cats with mammary cancer is variable and depends on several factors including size of the tumor, extent of surgery, the grade (how aggressive it is shown to be on the biopsy report), and whether additional therapy (chemotherapy is needed/used).
The average survival times range from 4 months to over 3 years depending on these factors.

What is on the horizon for this cancer?

As in women, the use of targeted therapies like Herceptin—a treatment that targets certain molecular and genetic defects found in cancer cells-- holds great promise for improving the outcome of therapy for the treatment of mammary cancer. In veterinary medicine there are currently two targeted therapies - Palladia and Masivet. There is currently a great deal of research looking for targeted therapies.


Contributed by: Veterinary Cancer Center