Breast Cancer - In Your Pet?

|| October 4, 2016

The month of October is known as Breast Cancer Awareness Month, and all across the nation, pink is prominently displayed to raise awareness in the fight against the disease. According to estimates from the American Cancer Society, about 231,840 new cases of invasive breast cancer will be diagnosed in women. Additionally, about 40,290 women will die from breast cancer, which is the second deadliest cancer in women, exceeded only by lung cancer. 

These figures demonstrate the need to raise awareness of the disease and continue the fight against breast cancer in humans. What might not be so commonly known, however, is that pets are also at risk for breast cancer. It's more commonly referred to as mammary cancer in dogs and cats, though. 

The Risk


Mammary gland tumors are the most common tumors in dogs and the third most common in cats. Unspayed females have the greatest risk of developing mammary cancer, and this risk can increase with each heat cycle. Mammary gland tumors can be benign or malignant. Approximately 50 percent of canine mammary tumors are malignant, and at least 85 percent of feline mammary tumors are malignant. 

The mammary glands are located along the entire belly region of dogs and cats, and the most common sign is a painless lump or mass. A mass can vary in size and can feel just like a small "bb." Some masses are freely moveable, while others are adherent to the overlying skin or underlying muscles. Occasionally, the mass ulcerates the skin and bleeds. 

Malignant tumors can metastasize (spread) to lymph nodes, lungs, and other organs. Staging tests, including chest x-rays and abdominal ultrasound, are recommended prior to definitive treatment in order to determine if there is any evidence of cancer spread. 


Treatment involves surgical removal of the lump with adequate margins of normal tissue. How much tissue will be removed depends on the species, size, and location of the tumor. Removing a small tumor with a rim of normal tissue is called a lumpectomy. A simple mastectomy is the removal of the entire mammary gland. A complete unilateral mastectomy is the removal of all mammary glands on one side of the body. Sometimes a bilateral mastectomy is the recommended treatment, especially in cats, since they tend to have the most aggressive form of disease. The tissue is then submitted to a pathologist for a definitive diagnosis, assessment of surgical margins, and to determine a prognosis. 

The prognosis for mammary cancer depends on many factors including individual tumor characteristics, as well as the results of staging tests. Some patients with aggressive tumors or evidence of spread may require chemotherapy treatment after surgery, and some patients survive long-term.

Being Proactive

The most reliable way to reduce mammary cancer risk in dogs and cats is to have them spayed/neutered prior to adulthood. Regular examinations by your veterinarian and early detection of masses are also cornerstones of successful treatment and improved prognosis. The earlier we can detect a problem, the sooner we can address it in both humans and our pets!

- Kimberly Wirth, DVM, DACVIM (Oncology)

WVRC - Board Certified Oncologist