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Resource Library: Mammary tumors, canine

Tumors of the mammary gland are the most common tumor in the female dog. These tumors occur in older female dogs who are either unspayed or who were spayed at an older age. Mammary tumors in dogs tend to be less aggressive than those seen in people but the incidence is about three times greater. No specific breed has been found to be predisposed to mammary tumors but poodles, cocker spaniels and German Shepherd can be more frequently affected

Half of all canine mammary gland tumors are malignant. Many dogs with mammary gland tumors have multiple glands involved. The same factors that predispose a dog to a single tumor can contribute to development of tumors in the other glands. Risk factors associated with tumor development are either not being spayed or spayed after 2 years of age. If a dog is spayed before their first heat the risk of mammary tumors is less than 0.05%, if spayed after the first heat the risk increases to 8%, and if spayed after their second heat the risk increases to 26%.

Surgery is the first line of treatment for all dogs with mammary gland tumors with removal of the mass and affected tissue with wide and deep margins appears to be effective. Recent studies have also shown that performing a spay at the same time decreases the incidence of future tumors.

When treated with surgery alone, dogs with tumors less than 3 cm in diameter have an average survival of 3 years or greater. In dogs with tumors greater than 3 cm in diameter, 80% have tumor regrowth and spread within a year, particulary if the margins were not clean after surgery. It is very important that all tissues removed at surgery be submitted for histopathology. The biopsy can also help determine important prognostic factors such whether or not the tumor is malignant, completely removed, invading blood or lymph vessels, or appears to be growing rapidly. All of these factors are taken into consideration when determining the best treatment for each patient.

A canine mammary tumor that is less than 3 cm in size and that has been completely removed at surgery with no evidence of tumor spread usually requires no follow up treatment. Dogs whose tumors are greater than 3 cm in size, that are of an aggressive type, or have presence of lymphatic or vascular invasion on the biopsy report (signs that the tumor is trying to spread), should receive chemotherapy following surgery.

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