Pet Factsheets

Breast cancer in cats

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Breast cancer occurs most frequently in older, female pets that have not been spayed. While the cause of breast cancer is unknown, hormones are thought to play a role. Any suspicious lump in the mammary area should be examined by a vet as soon as possible.

What is breast cancer?

Breast cancer is the uncontrolled growth of abnormal mammary gland (breast) cells. If left untreated, certain types of breast cancer can metastasise (spread) to other mammary glands, lymph nodes, the lungs, and other organs throughout the body.

While any pet can develop mammary tumours, these masses occur most often in older, female cats that have not been spayed. Most (80%-90%) mammary tumours in cats are malignant (cancerous). Siamese cats have a higher risk for breast cancer than other feline breeds.

What causes breast cancer?

The exact cause of mammary gland cancer is unknown. However, cats that are spayed before their first heat cycle are less likely to have breast cancer, so hormones may play a role.

Treatment with hormones for other conditions may increase the risk for this type of cancer. In the past, hormones were used to treat some behaviour and skin problems in cats, but this has generally fallen out of favour.

What are the signs of breast cancer?

Signs of breast cancer include firm nodules in the tissue around the nipples, ulcerated skin, and swollen, inflamed nipples with or without discharge.

There's no way to determine if a lump is cancerous simply by feeling it. Any lump in the mammary area has the potential to be cancerous, so it's a good idea to check your pet regularly.

Mammary tumours tend to be firm, nodular masses that feel like BB pellets under the skin. Tumours may be located in a single mammary gland (the area around one nipple), or they may be in several mammary glands at once. The skin covering the tumour may be ulcerated or infected. Nipples may be swollen or red, and there may be discharge from the nipple itself.

How will my vet diagnose breast cancer?

Breast cancer is best diagnosed with a surgical biopsy. Blood work and radiographs (x-rays) are usually recommended to help determine if the cancerous cells have spread to other parts of the body. Biopsies generally require some form of anaesthesia or sedation, so your vet may recommend a preanaesthetic evaluation and/or blood work.

How is breast cancer treated?

Early detection and surgical removal of the masses is the best treatment option. Before performing surgery, your vet will most likely recommend blood work and radiographs (x-rays). Chest radiographs are important to check for metastases to the lungs, and abdominal radiographs may show signs of enlarged lymph nodes. If the radiographs show no evidence of metastasis, the pet has a better prognosis.

Because of the high rate of malignancy in cats and the fact that cancer often invades several mammary glands along the same side of the body, a radical mastectomy with removal of all mammary glands on the same side is often recommended. For cats with masses on both sides, two separate surgeries several weeks apart may need to be performed.

If your pet still has her ovaries and uterus, your vet may recommend spaying your pet at the time of mammary surgery. Spaying female pets before their first heat cycle is the best way to prevent breast cancer.

Following surgery, your vet may recommend radiation therapy or chemotherapy. Radiation therapy is designed to kill any potentially cancerous cells in a focused area. Chemotherapy involves systemic drugs that treat cancerous cells that may have travelled to other parts of the body.

Can breast cancer be prevented?

The best way to prevent breast cancer is to have your pet spayed before her first heat cycle. Even spaying your pet by 1 year of age can help reduce breast cancer risk. Pets that are spayed later in life will be at higher risk for breast cancer.