Mammary cancer is the third most common cancer among cats. The average age at which a cat will become infected is ten to eleven years. About 90 percent of these cancers are malignant, although even benign tumors should be surgically removed immediately. They've been known to transform into the malignant variety. Surgery consists of a radical mastectomy (mast-ECK-tuh-me), in which all four mammary glands on the affected side are removed, as well as the lymph nodes in the armpit and groin areas. Following surgery, the cat's activities should be severely limited while the incision heals. In the case of malignant tumors, surgery should be followed up with radiation or chemotherapy. Your veterinarian can explain the various procedures available.