Mammary gland tumors are common in the dog, and surgery to remove cancerous mammary glands, called a mastectomy, is a routinely done procedure in veterinary medicine. Male dogs almost never get mammary tumors; as well, female dogs that have been spayed before their first heat cycle rarely acquire breast cancer. Statistics demonstrate that if a female dog is spayed after her first heat cycle but before her second cycle, her potential to develop mammary tumors is slightly greater than the dog that was spayed prior to a heat cycle. If two cycles occur, then the spay procedure, an even higher incidence of breast cancer is demonstrated. And spaying after three heat cycles has no effect on diminishing the potential to develop mammary tumors. In short, the sooner a dog is spayed the less the chances for mammary tumors to develop in the future; but after three or four heat cycles, spaying has almost no effect on protection against tumor development. Keep in mind that spaying any dog at any time (as long as the patient is healthy) may be advisable to prevent a very serious uterine infection called PYOMETRA .