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Mammary Tumors in Dogs

Posted July 23, 2015 in Dog Diseases & Conditions A-Z

AJ Debiasse, a technician in Stroudsburg, PA, contributed to this article.

Unspayed female dogs are at risk for mammary or breast tumors. In dogs, there is a 50-50 chance that a mammary mass is cancerous vs. benign. This is the reason why veterinarians often insist on the importance of spaying your dog.

Dog laying on the carpetWhat causes mammary tumors in dogs?
Mammary tumors develop because of spikes in female hormone (estrogens) that take place during a dog’s heat cycle. By spaying a dog at 6 months of age or before the first heat cycle, it virtually eliminates the risk of getting mammary tumors, which starts at only about 0.5%. Once a dog goes through one single heat cycle, the risk increases to 8%. After a second heat cycle, the risk shoots up to 26% (says the American College of Veterinary Surgeons).

If a dog is spayed after 2 years of age, then there is no more protection. Over 25% of non-spayed female dogs will develop mammary tumors. This is a huge percentage! Being obese or receiving hormones (estrogens, progesterone) can further increase that risk.

What if your dog already has mammary tumors?
Since spaying prevents mammary tumors, should a dog older than 6 months old with mammary tumors be spayed? Yes, without any doubt. Again, spaying at that age will not prevent mammary tumors. However, it would eliminate the risk of pyometra, a life-threatening infection of the uterus. Therefore, if you are in this situation, you should have an open discussion with your veterinarian to understand the pros and cons, and decide on what makes the most sense.

How will your veterinarian check for mammary tumors?
Most mammary masses are found on physical exam by a veterinarian. Occasionally, a mass is found by the guardian or by a groomer, especially in a dog who enjoys belly rubs. Most masses are not painful. When they outgrow the skin or rub on the floor, they can become ulcerated, which means that the skin has opened up and the mass can bleed.

How can your veterinarian tell if a mammary mass is benign or cancerous?
Looking at a mass, or feeling it, doesn’t help vets determine the diagnosis. The only way to diagnose the type of mammary mass is by taking a biopsy and sending it to the lab for analysis. In most cases, the mass is surgically removed first, and then sent to a pathologist.

How will your veterinarian decide whether to biopsy the mass first or not?
As a general rule, I recommend taking a biopsy first if there is a chance it will change the treatment plan. This is the case when the tumor is an inflammatory carcinoma. This type of cancer, although rare, has a very poor prognosis and surgical removal is not recommended. This is one situation where a good exam should make us very suspicious.

What’s inflammatory mammary carcinoma?
Inflammatory mammary carcinoma is red, firm and very painful. This is unusual for a mammary tumor. Unfortunately, there is no good treatment for inflammatory carcinoma. Radiation therapy, pain medications and non-steroidal anti-inflammatory drugs can be used to control pain.

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Dr. Phil Zeltzman is a board-certified veterinary surgeon and author. His traveling practice takes him all over Eastern Pennsylvania and Western New Jersey. You can visit his website at, and follow him at

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