Breast Cancer in Dogs: Early Detection is Key

One in four dogs will develop mammary tumors.
By Bobbi Leder, February 2012, Updated July 2021
breast cancer in dogs

We have all heard of breast cancer in women but chances are dog owners have no idea that their dogs can actually get breast cancer. The bad news is that breast (mammary) cancer in dogs is common—in fact even more prevalent than humans; the good news is that the disease can be treated successfully if caught early.

One in four unspayed dog is affected by breast cancer1 
and approximately 50% are malignant.2

According to veterinarian Dr. Race Foster, the most common type of tumor in female dogs is the mammary tumor—especially in (unspayed) dogs between the ages of 5 to 10 years-old. There are male dogs that do develop breast cancer and, sadly, their prognosis is not good because this type of breast cancer is very aggressive.

Signs of Breast Cancer in Dogs

Similar to human breast cancer, mammary tumors in dogs can range in size. Breast tumors in dogs often grow quickly with an irregular shape. These malignant tumors can also cause bleeding and ulceration. However, if your dog’s tumor does not exhibit these signs, that does not mean your dog is free from breast cancer; small tumors that have been present for a while can suddenly grow aggressively as well. As with most other types of cancer, once malignant tumors in dogs start to grow, the cancerous cells can spread to other parts of the body.


  • Growing lumps or sores that don’t heal
  • Drastic changes in a pet’s appetite or weight
  • Offensive odors
  • Bleeding or discharge from any opening in the body
  • Difficulty chewing or swallowing
  • Unwillingness to exercise
  • Persistent lameness or stiffness
  • Difficulty breathing, urinating or defecating

If you find a lump on your dog, do not wait to go to the veterinarian. It is always best to play it safe and have your dog examined by a licensed veterinarian who will perform a biopsy. Half of all mammary tumors in dogs are benign, but do you really want to play guessing games when it comes to your dog’s health?


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Treatment of Canine Breast Cancer

Treatment of a malignant tumor usually involves surgery. Similar to breast cancer in humans, dogs will either have just the tumor removed or the entire mammary tissue along with lymph nodes. Dogs’ mammary glands are different than humans in that they are outside of the muscle, so the surgery is not as radical. Dr. Race Foster suggests that unlike humans, chemotherapy and radiation in dogs are not successful.

What Determines Prognosis

A biopsy is typically performed to determine the type of breast cancer tumor and the best treatment options. In addition, to the type and location of the tumor.

  • The size of the tumor
  • Whether the tumor has spread
  • History of rapid growth

Preventing Breast Cancer in Dogs

The best way to prevent breast cancer in female dogs is to spay them before they go into heat for the first time—just another benefit of spaying. By doing this, dog owners can practically eliminate the chances of their dog developing mammary cancer.

According to Wendy Brooks, DVM, DABVP, “approximately 50 percent of malignant mammary tumors in dogs have receptors for either estrogen or progesterone. Having these female hormones promotes the growth of these tumors...This means that spaying is important even if a tumor has already developed; in one study, female dogs spayed at the time of their tumor removal (or in the two years prior to the tumor removal) lived 45 percent longer than those who remained unspayed.”

Diet is also a factor, dogs fed a fatty diet or who are overweight have an increased risk at developing breast cancer.

Photo by Victor Grabarczyk / Unsplash