Mammary Tumors Common in Unsterilized Female Dogs

Lee Pickett
Thu, 22 Jan 2026 21:00:00 -0800
Lifestyle

Q: I recently adopted Missy, my mother's middle-aged, mixed-breed dog. She was never spayed, and she has two small masses in her mammary glands. Are these masses likely to be cancer? If not, should I have her spayed?

A: Mammary tumors develop in one in four unsterilized female dogs, making them the most common tumor in this group. Dogs that had puppies, females spayed after their heat cycles began and dogs that were never spayed are at highest risk.

Mammary tumors develop between the ages of 7 and 13. About 70% of dogs have multiple mammary tumors at the time of diagnosis. The mammary glands closest to the hind legs are most often affected.
Updated: Thu Jan 29, 2026

Q: I recently adopted Missy, my mother's middle-aged, mixed-breed dog. She was never spayed, and she has two small masses in her mammary glands. Are these masses likely to be cancer? If not, should I have her spayed?

A: Mammary tumors develop in one in four unsterilized female dogs, making them the most common tumor in this group. Dogs that had puppies, females spayed after their heat cycles began and dogs that were never spayed are at highest risk.

Mammary tumors develop between the ages of 7 and 13. About 70% of dogs have multiple mammary tumors at the time of diagnosis. The mammary glands closest to the hind legs are most often affected.

Half of mammary tumors are benign, and half are malignant. Of the malignant tumors, half spread to other parts of the body.

It makes sense to have Missy spayed when her mammary tumors are removed for a couple of reasons. If the tumors are benign, spaying will prevent uterine infection, a common, life-threatening emergency in unspayed females. If the mammary tumors are malignant, spaying decreases the risk that additional tumors will form and prolongs survival.

Mammary tumors have multiple causes, the most important of which is hormonal. Exposure to estrogen and progesterone, whether from the dog's ovaries during each heat cycle or through medications, increases risk. Spaying prior to the first heat protects females from mammary tumors.

Genetics probably also play a role, since specific breeds and small dogs in general are predisposed.

In addition, dogs that are overweight before they are a year old are more likely than dogs that are slim at that age to develop mammary tumors later in life.

Treatment begins with surgery, which cures 75% of mammary tumors. Without surgery, mammary tumors can grow and eventually ulcerate, oozing blood and pus and causing discomfort.

Malignant tumors may spread to nearby lymph nodes as well as the lungs, liver, kidneys and bones. Your veterinarian may recommend a chest X-ray before making a decision on surgery and additional treatment to be sure Missy's lungs are clear.

Survival is longest when malignant mammary masses are less than 3 centimeters in diameter (just over an inch) and the cancer hasn't spread. While little research has been done on radiation treatment in dogs with mammary cancer, surgery and chemotherapy have been shown to prolong life.

Q: We live in the country, and our cats play outdoors in the garden during the day. Our newest cat, Willow, occasionally chews on the shrubs around the house. We have a lot of yews and also some azaleas and rhododendrons. Are these plants safe for cats?

A: Any ingested plant material can cause gastrointestinal problems, including loss of appetite, drooling, vomiting and diarrhea.

But azaleas, laurels, rhododendrons and yews are particularly toxic to pets. If enough of the plant is ingested, death can occur.

These plants contain toxins that cause cardiac and neurologic problems manifested by weakness, breathing trouble, trembling, loss of coordination, seizures and collapse.

Yews are especially dangerous. Veterinary medical journals contain reports of dogs that chewed yew branches and died of heart failure before other clinical signs were evident. Humans have even used yew toxins to commit suicide and as chemical weapons during warfare.

The dried plants retain their toxicity for months, so burn or bury plant material after you prune or remove the shrubs.

Encourage Willow to stay indoors by placing cat perches adjacent to windows that look out on bird baths, feeders and nesting boxes.