Yes, mammary (breast) cancer is the most common form of cancer in dogs. Dogs develop both malignant (cancerous) and benign (noncancerous) tumors of the mammary glands, but about half of all canine mam-mary tumors are malignant.
When compared to unspayed female dogs, females spayed before their first heat cycle have a 0.5 percent risk of developing mammary cancer, those spayed after their first but before their second heat cycle have an 8 percent risk, and those spayed after their second heat cycle have a 26 percent risk. Therefore, spaying protects a dog from developing mammary cancer, and spaying at an early age. (before the first heat cycle) essentially eliminates the risk, contrary to traditional belief that dogs should have a heat cycle (and even a litter of pups!) before they are spayed.
As in humans, males can develop mammary cancer, too, but it is extremely rare. The first sign of canine mammary cancer is a lump in the mammary gland; half of all cases have multiple lumps. The most reliable method for diagnosis is removal of the lump, which is examined by a histopathologist (cell specialist) who determines whether it is cancerous.
Never “wait and see” if you discover your dog has a mammary lump. Have it removed and examined as soon as possible after discovery. The treatment for canine mammary cancer is removal of the lump and all surrounding tissue. Most experts recommend radical mastectomy, which consists of removal of all eight mammary glands.
Chemotherapy and radiation therapy are sometimes used to control and eliminate cancerous cells that cannot be removed with surgery. If malignant mammary cancer is left untreated, or if all of the cancer cells are not successfully removed, the tumors will reappear, and cancer may also occur in the lungs, the lymph nodes throughout the body, and in the nervous system. The tumors in the mammary glands often become large, heavy, and ulcerated, and are quite uncomfortable. Sadly, most dogs with malignant mammary cancer live only a few months to a few years, even with treatment.