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Canine Mammary Tumors | College of Veterinary Medicine

From the above link:

Canine Mammary Tumors

Mammary tumors are extremely common in dogs; approximately 50% of them are malignant. Mammary tumors are more common in intact than in spayed females; in fact spaying before the first or second heat cycle significantly reduces the risk of developing mammary tumors. Median age on presentation is 10 to 11 years. Dogs fed a high-fat diet or overweight at 1 year age are at increase risk of developing mammary tumors. Appropriate early treatment, even if the tumor is malignant, is often curative.