The word "mammary" refers to something related to breasts. It is commonly used to describe the glands and tissues responsible for producing milk in female mammals, including humans. Full definition
• For females, the risk of mammary gland tumors, ovarian and / or uterine cancer is reduced or eliminated, especially if done before the first heat cycle which is fatal in about 50 percent of dogs and 90 percent of cats. (bjbangs.net)
One can expect virtually zero risk of the development of mammary tumors in a female dog that is spayed prior to experiencing the first heat cycle. (web-dvm.net)
Quite often she will develop mammary cancer in her mid to later life. (puppys-place.com)