Mammary Tumors in a Small Chihuahua


By: Dr. Josie Arkush from Animal Care Center

Mickey is an eight-year old female spayed chihuahua whose owner brought her to the clinic. Her owner had noticed a small mass on Mickey's chest, associated with her mammary gland. Mickey had a litter of puppies a few years ago, and was spayed about one year prior to the owner noticing a mass. Other than the mass, her owner indicated Mickey was acting completely normal. Her appetite was normal and her bathroom habits had not changed. There was no change in her energy level, either.

On physical exam, Mickey was bright and alert. All of her vitals were normal. There was a small mass - about 1 cm in diameter - that could be felt under the skin. It seemed to be associated with her mammary tissue.

A fine needle aspiration (needle biopsy) was offered to the owner, but she decided to make an appointment to have the mass removed ASAP.

Mickey's blood work was completely normal and she was cleared for surgery.

The mass was removed under general anesthesia. Mickey did very well during the procedure and recovered with no problems. The mass was submitted for biopsy. A few days later, it was determined that Mickey's mass was a mammary carcinoma, and the entire tumor had been removed (clean margins) with surgery.


Mammary carcinomas are very aggressive tumors that can develop in dogs and cats. They are more common in animals who are not spayed or who have been spayed later in life. When discovered, it is important that they be removed with very large margins, because they are very aggressive.


Since 50 percent of mammary tumors have already spread by the time of surgery (Ref:, it is recommended that these patients undergo chemotherapy for the best possible outcome. Mammary carcinomas often will spread to the lungs, so it is important to check those as well.

X-rays of Mickey's lungs were performed, and they appeared to be free of obvious tumors. It is very difficult to assess lymph nodes on a chest x-ray.


Chemotherapy is intensive, and is usually performed by a veterinary oncologist. It can cost thousands of dollars, and requires frequent trips to the hospital.

Mickey's owner secured a grant for Mickey to undergo chemotherapy.

She did well with her chemo treatments and, months later, is currently living a happy life.


It is common for these masses to return - even with chemotherapy. Since there is no blood test for cancer, the only way to monitor for recurrence is with routine checks of the patient's mammaries and lung x-rays.

Mickey's owner performs them every two weeks. If she ever noticed another mass, the owner is encouraged to bring Mickey in ASAP to have the mass removed.


Owners can reduce the chance of their female dogs/cats developing mammary carcinomas by spaying them early. With every heat cycle, the risk of developing mammary cancer increases. After one heat, the chances go up by seven percent. After two heat cycles, the chances go up to 25 percent (Ref: The ideal time to spay your puppy/kitten is between 4-7 months old.