Breast cancer in situ is a common type of early breast cancer. Breast cancer in situ is noninvasive, meaning it does not spread beyond the milk ducts or lobules. Breast cancer in situ is not considered life-threatening. However, women with breast cancer in situ are at greater risk for developing more aggressive forms of breast cancer in the future. Follow the links below to find WebMD's comprehensive coverage about how breast cancer in situ develops, what it looks like, what raises your risk, how to treat it, and much more.