Breast Cancer in Dogs

Causes, Treatment, and Prevention

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

Getty Images/Elen11

Breast cancer is a common concern among humans, but dogs can also develop this disease. While not all mammary tumors in dogs are malignant (cancerous), the early symptoms of cancer, benign (noncancerous) growths, and other mammary conditions look very similar. Lumps in the mammary tract, redness, or nipple discharge should be evaluated by a veterinarian to determine the cause and treatment options.

What Is Breast Cancer?

Cancer is a disease that occurs when cells divide abnormally and grow rapidly. It can affect any part of the body. Breast cancer develops specifically in mammary tissue in and around the nipples. In dogs, this tissue covers the length of the abdomen and chest (dogs usually have eight to ten teats).

Symptoms of Breast Cancer in Dogs

Breast tumors can be classified as benign or malignant. 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 are often the same and require medical diagnostics to differentiate between malignant and benign conditions.


  • Lump within the mammary tract
  • Nipple discharge
  • Enlarged nipple
  • Redness anywhere in the mammary area
  • Ulcers or bleeding sores
  • Reluctance to have belly touched or rubbed
  • 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 the full length of a dog's underside, these lumps could involve breast tissue. These lumps may also become ulcerated and bleed. Because such lumps are painful, dogs with breast cancer may not want their bellies touched or rubbed.

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 (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.

What Causes 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. Hormonal influences contribute to cancer, including:

  • 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. Older, intact, female dogs are more likely to develop breast cancer than younger female dogs.

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

How Do Vets Diagnose Breast Cancer in Dogs?

If you suspect your dog has breast cancer, your veterinarian should examine it as soon as possible. After a physical examination, a palpable lump may require a biopsy or fine-needle aspiration to determine whether or not it contains cancerous cells.

If your dog is diagnosed with breast cancer, X-rays are often taken to look for signs of cancer elsewhere in the body.

Bloodwork and some other tests may also be run to ensure your dog is healthy enough to have surgery.

How to Treat Breast Cancer

Surgery will be recommended to remove the lump or lumps if your dog can undergo anesthesia and the procedure.

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, while other medications are prescribed to manage pain and hormonal imbalance.

If the cancer has spread, depending on the severity, palliative care may be the only option.

Prognosis for Dogs with Breast Cancer

Early detection and treatment of breast cancer are critical because small, isolated tumors may be successfully removed, offering the best chance of survival. Larger tumors or those that have spread are more difficult to treat and therefore more likely to result in earlier fatality.

How to Prevent Breast Cancer

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

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  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.

  2. Malignant Mammary Tumors in Dogs. VCA Animal Hospitals.