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Mammary tumors in dogs

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Breast cancer is relatively common in female dogs, mainly in middle-aged to older animals. It is rare in male dogs. Benign mammary tumors are more common than malignant ones. The single most important risk factor for mammary tumors in dogs is the influence of sex hormones. For this reason, the risk of breast cancer is significantly decreased if dogs are spayed before their first estrous cycle.

Breast cancer in dogs appears as a lump or lumps in the mammary glands. Some lumps produce clear, milky, or blood-stained fluids that may be expressed from the nipple. Malignant mammary tumors are often firmly attached to surrounding tissues, and they may ulcerate and bleed. Inflammation and secondary infection of the tumors are common. Malignant mammary tumors frequently spread to the lungs and cause difficult breathing. Weight loss, vomiting, and other nonspecific signs of disease can be seen.

Mammary tumors can be diagnosed on physical examination, but a biopsy is needed to determine whether the tumor is benign or malignant. The most common treatment is surgical removal. This usually cures benign tumors, but malignant tumors frequently recur and spread to other parts of the body. In these cases, in addition to removing the lump(s), a single or chain mastectomy may be needed, along with removal of all nearby lymph nodes. Many glands and lymph nodes are removed if there are multiple tumors. Additional more specialized treatments (eg, chemotherapy) may be helpful.



What pets are most at risk for mammary tumors?

Mammary tumors are most common in middle-aged to older female pets that were never spayed or were spayed later in life. 

What do mammary tumors look like?

Mammary tumors appear as lumps in the mammary glands, often associated with swelling, redness, or ulceration. 

How are mammary tumors treated?

The most common treatment is surgery to remove one or more of the affected glands.  This usually cures benign tumors, but malignant tumors frequently recur and may spread to other parts of the body. 

How can mammary tumors be prevented?

Mammary tumors are caused largely by sex hormones, so the best way to prevent them is to have your pet spayed before her first heat period. 


Credit: Written and reviewed by John A. Bukowski, DVM, MPH, PhDand Susan E. Aiello, DVM, ELS
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