Breast Cancer in Dogs

Mass under a dog teat.
Breast cancer often shows up as a firm lump under the nipple.

Getty Images/Elen11

Breast cancer is nothing anyone wants to think about, but, unfortunately, our pets are sometimes at risk of developing this serious disease. Knowing the potential causes of this disease, how it can be prevented, and what can be done if your dog is diagnosed with breast cancer can be helpful.

What Is Breast Cancer in Dogs?

Cancer is a disease that occurs when cells divide abnormally and can affect any part of the body. Breast cancer develops specifically in the mammary or breast tissue of dogs.

Mammary tissue is in and around the nipples but also extends between each nipple under the skin. The tissue is often found from the armpit to between the hind legs on each side of the abdomen of a dog. Since dogs usually have eight to ten nipples on their belly, this means that there is also a lot of mammary tissue that can be affected by breast tumors.

Breast tumors can be classified as benign or malignant. Benign tumors are non-cancerous and malignant tumors are cancerous. About half of all breast tumors in dogs are cancerous but a dog can have both cancerous and non-cancerous breast tumors at the same time.

Symptoms of Breast Cancer in Dogs


  • Lump on the mammary chain
  • Nipple discharge
  • Enlarged nipple
  • Painful breast
  • Lethargy
  • Decreased appetite
  • Coughing

The most common sign of breast cancer in dogs is a lump or multiple lumps somewhere on the mammary chain. These lumps are usually firm and located just under the skin. They can be large or small, located under a nipple, or even between nipples.

If the lumps aren't close to a nipple, owners may not think they could be breast cancer, but since the mammary chain runs up and down the belly of a dog, these lumps could still be involved with the breast tissue. These lumps may also become ulcerated, bleed, and even cause the breast to be painful if they are gently squeezed. Dogs with breast cancer therefore may not want their belly rubbed as they normally would.

Lumps that are located near or under the nipples may result in the nipple being enlarged. Comparing the size of each nipple is a good way to tell if one is grossly enlarged but be aware that minor variations in size can be normal. Discharge from a nipple, however, is not normal (unless a dog is lactating) so fluid coming from the nipple can also indicate breast cancer.

If the breast cancer has spread to other parts of your dog's body, a lack of appetite, a decrease in energy, coughing, and other symptoms may also be seen.

Causes of Breast Cancer in Dogs

Female dogs are at risk of developing breast cancer, especially if they are not spayed or were spayed after their first heat cycle. Male dogs can also develop breast cancer but it is very rare.

The hormone called estrogen that intact female dogs secrete from their ovaries contributes to the development of breast cancer. The longer estrogen is secreted in a dog, the more likely it is to develop breast cancer. This helps explain why older, intact, female dogs are more likely to develop breast cancer than younger, intact, female dogs.

Some dog breeds also show a predisposition to developing breast cancer more than others. Cocker spaniels, Maltese, Shih Tzus, toy and mini Poodles, Yorkshire Terriers and some other breeds are considered high-risk.

Diagnosing Breast Cancer in Dogs

If you suspect your dog has breast cancer, your veterinarian should examine it as soon as possible. A full medical history including whether or not your dog has been spayed and when may be important to know and a physical examination will also be performed. If a lump is palpable a biopsy or fine needle aspirate may be obtained to determine whether or not it is breast cancer.

Treatment of Breast Cancer in Dogs

If your dog is diagnosed with breast cancer, X-rays are often taken to look for signs of cancer elsewhere in the body. This spread of cancer is called metastasis and the X-rays may be referred to as a "met-check" because of this.

Bloodwork and some other tests may also be run to ensure your dog is healthy enough to have surgery. Surgery will be recommended to remove the lump or lumps if your dog is deemed to be able to undergo anesthesia after running these tests.

If multiple lumps are present or both mammary chains are affected, full or bilateral mastectomies may be performed to remove all of the mammary tissue.

If your dog is still intact, spaying will also be recommended. Chemotherapy or radiation may be needed if the breast cancer has spread to other parts of the body and other medications are routinely prescribed to manage other symptoms of the disease. If the cancer has spread, depending on the severity, palliative care may be the only option.

How to Prevent Breast Cancer in Dogs

The best way to prevent breast cancer from developing in your dog is to get them spayed when your veterinarian recommends doing so. Spaying helps to substantially decrease the likelihood of your dog developing breast cancer.

Article Sources
The Spruce Pets uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Kim HW, Lim HY, Shin JI, Seung BJ, Ju JH, Sur JH. Breed- and age-related differences in canine mammary tumors. Can J Vet Res. 2016;80(2):146-155.