Canine Mammary Tumors (Breast Cancer)

Mammary tumors are common in middle age to older females. They are rare in the male dog with risk factors less than one percent. The risk factors for females are influenced by if she has had an ovarianhystectory (been spay) and at what age the surgery was performed.

• Risk factor if spayed before the first estrus (heat cycle) 0.5%
• Risk factor if spayed before the second estrus (heat cycle) 8%
• Risk factor if spayed after second estrus (heat cycle) 26%

Clearly it is important to spay female dogs early in order to reduce the risk for mammary tumors.

There are ten mammary glands with five on each side. Tumors many be a single tumor or multiple tumors in multiple glands. Mammary tumors are tumors of mammary tissue. About fifty percent are malignant and fifty percent are benign. Of the malignant tumors, fifty percent will metastasize to the regional lymph nodes or lungs.

It is important to collect cells and verify in the office that the tumors are mammary tumors. It is possible for other types of tumors to develop in the mammary gland. Cells are collect by fine needle aspirate (collecting a small sample of cells with a needle) to verify it is a mammary tumor.

The tumors are staged by size and location.

Stage 1 – less than 3 cm
Stage 2 – 3 to 5 cm
Stage 3 – greater than 5 cm
Stage 4 – any size with lymph node involvement
Stage 5 – any size with a tumor in the lung

In order to identify if the tumor has spread to lymph nodes, thoracic radiographs (x-rays) of the chest and ultrasound of regional lymph nodes is necessary. The lower the stage, the better the prognosis. Generally, Stage 1 and 2 are treated with a high degree of success with surgery.

The type of surgery will vary depending on which glands are involved and how many tumors are present. Stage 3 or 4 may have prolonged survival times when combined with other therapies. Stage 5, while surgery may be performed to make the patient more comfortable, there is no treatment effective for the tumors in the lungs. Therefore survival time is relatively unaffected.

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