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Mammary masses on dog

Q: I adopted a 12-year-old female shelter dog with mammary masses. I have two questions about them.

First, my veterinarian said the tumors could be benign cancer or malignant cancer, that any growth is cancer. I thought cancer was always malignant. Please clarify cancer terminology for me.

Also, my vet recommends removing both mammary chains in separate surgeries. If I decide not to put my elderly dog through these surgeries, what will happen?

A: Veterinary oncologists describe a lump or mass as a tumor. The tumor may be benign, which means it is unlikely to invade local tissues or spread elsewhere in the body.

Alternatively, the tumor can be malignant, meaning it can invade adjacent sites and move to distant parts of the body through the blood or lymphatic vessels, a process called metastasis (“me TAS ta sis”). Most oncologists reserve the term cancer for malignant tumors.

Mammary masses, the most common type of tumor in unspayed female dogs, develop between the ages of seven and 13. Spaying prior to the first heat protects females from the disease and also prevents uterine infection. Certain breeds are predisposed to mammary cancer, and obesity when young also increases risk.

About 70 percent of dogs have multiple mammary tumors at the time of diagnosis. Half are benign, and the rest are malignant. Without surgery, these tumors usually grow and eventually ulcerate, oozing blood and pus and causing discomfort.

The tumors may remain localized in the mammary glands, or the cancer may spread to nearby lymph nodes, lungs, liver, kidneys and bones. Survival is longest when the masses are less than 3 centimeters (just over an inch) in diameter and the cancer hasn’t spread through the body. Surgery and chemotherapy prolong life.

For more details about your dog’s prognosis with and without surgery, make an appointment with a veterinary oncologist.

Lee Pickett, V.M.D. practices companion animal medicine. Contact her at

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