Mammory Tumors

By Dr. Karen Burgess

What is mammary cancer?
Mammary cancer or better known as “breast cancer” in humans is an abnormal growth of tissue in the mammary gland. 25% of dogs spayed after their second heat will develop mammary cancer at some point in time. Of these dogs, half will have malignant tumors and half will have benign tumors. Benign masses do not cause risk to the animal as a whole while malignant masses tend to spread to other parts of the body (metastisize) or cause additional local damage. The only way to differentiate between these types and determine the specific behavior is to perform a biopsy of the tumor’s tissue.

What causes mammary cancer in pets?
Cancer is caused by a variety of factors, many that are still unknown thus making it that much more difficult to prevent. In general genetic damage or mutations to cells initiates excessive duplication of cells and eventually what is diagnosed as a tumor. The greatest correlation to occurrence has been made with lack of early spaying. Dogs spayed before their first heat have close to a 0% chance of developing mammary cancer. This incidence increases to 7% and 25% after one and two heats respectively. Obesity at 1 year of age and high fat diets have also been linked to mammary cancer in dogs. For cats early spaying has similar benefits.

What are signs of mammary cancer?
Most commonly a palpable bump is felt along the mammary chain on the belly. There are often more than one of these at the time of diagnosis. While redness, pain, and discharge are also possible, they are not as common.

How is mammary cancer diagnosed?
A sample of the abnormal tissue is collected and submitted for histopath (microscopic) analysis. This typically takes anywhere from three to five days after submission. This will not only differentiate between benign and malignant but will also determine cell of origin and grade which directly affect prognosis and future tumor behavior.

How are mammary tumors treated?
If diagnosed early, prior to spread, initial treatment of choice is surgical removal of the mass. Sometimes the associated lymph node will also be removed. Prior to any surgery xrays of the lungs and general blood work are recommended to look for any additional concerns including obvious spread to other areas of the body. Spaying may also be indicated in pets that are still intact. Based on the tumor’s histopath results, additional drug or radiation therapy may be indicated.

What is my pet’s prognosis?
Prognosis or survival is almost solely dependent on pathology results, tumor type, and evidence of spread at time of diagnosis. While 50% of tumors are malignant, 70% of these are cured with surgery.