Breast cancer in dogs and cats

Updated: Aug 24, 2021


Breast cancer in a bitch

Breast cancer occurs most commonly in older, intact female pets. While the cause of breast cancer is unknown, hormones are thought to play a role. Usually, these are suspected during a physical examination by the veterinarian when a mass is detected in the rear abdominal mammary glands (dogs) and front thoracic mammary glands (cats) respectively.

Breast cancer in a queen cat

The dog is by far the most frequently affected pet, with an occurrence that is three times that in women. About 50% of all cancers in the bitch are mammary tumours. Approximately 45% of breast cancers in dogs are malignant, ie, they spread to other parts of the body, whereas in cats about 90% are malignant.

What is breast cancer?


Breast cancer refers to the uncontrolled growth of abnormal mammary gland (breast) tissue. When left untreated, the breast cancer cells can spread to other mammary glands, lymph nodes, the lungs, and other organs throughout the body. The development of mammary tumours in dogs and cats is most often common in older, female dogs and cats that have not been spayed. 50% of mammary masses in dogs are malignant.

What is the cause of breast cancer in dogs and cats?


The exact cause of mammary gland cancer is unknown in many instances. However, the following are the probable causes:-


Hormones play an important role in the development of mammary tissue cancer, but the exact mechanism is unknown. Dogs and cats that are spayed, however, before their first heat cycle are less likely to have breast cancer, confirming that hormones may play a role. Treatment with hormones for other conditions may increase the risk for this type of cancer.


Animal genetics

Genetics also play a role in canine and feline breast cancer with recent findings showing that certain genes are overexpressed in dogs and cats with this condition.


Pet obesity

It has also been demonstrated that the consumption of red meat, obesity at one year of age, and obesity a year before diagnosis are associated with an increased risk of mammary gland tumours in intact or spayed dogs.

Red meat

What are the signs of breast cancer in dogs and cats?


Signs of breast cancer include firm nodules in the tissue around the nipples, ulcerated skin, and swollen, inflamed nipples with or without discharge. Feeling alone cannot determine whether a lump is cancerous or not. 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 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 discharged from the nipple itself.

How is breast cancer diagnosed in dogs and cats?


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.

In dogs with large masses, it may be possible to obtain a fine needle aspirate of the tumour, which involves placing a needle into the mass and extracting cells for examination under the microscope. This procedure may be more difficult with smaller masses or in cats. Since a biopsy usually provides a larger tissue sample (likely to yield a more definitive diagnosis), this is the best option. Biopsies generally require some form of anaesthesia or sedation, so your vet may recommend a pre-anaesthetic evaluation and/or blood work.

How is breast cancer treated in dogs and cats?

Breast cancer on a bitch


Early detection and surgically removing the masses is the best treatment option. Before performing surgery, it is recommended that blood work and radiographs (x-rays) are done. Chest radiographs are important to check for any cancer that could have spread to the lungs, and abdominal x-rays may show signs of enlarged lymph nodes. If the radiographs show no evidence of spreading, the pet has a better prognosis. Submission of the tissue for microscopic examination will determine if the tumours have been completely removed. Surgical removal of the tumour with the regional lymph node will increase disease-free time but, in cases of malignancy, may not increase survival time.

Radiotherapy & Chemotherapy


Radiation therapy or chemotherapy are usually considered and carried out after surgery as adjuvants. Radiation therapy is administered to kill any remaining potentially cancerous cells in a focused area.

Chemotherapy involves the administration of systemic drugs that treat cancerous cells that may have spread to other parts of the body. The drugs involved in chemotherapy include deracoxib, doxorubicin, cyclophosphamide, vincristine or a combination of deracoxib and doxorubicin or doxorubicin and cyclophosphamide.