Doctors call two different kinds of growth in breast tissue "Stage 0" breast cancer. Lobular carcinoma in situ (LCIS) is a benign tumor consisting of abnormal cells lining the small "lobules" where milk is made in the breast. LCIS does not behave like cancer, and there's no evidence LCIS will spread. Having LCIS does mean, however, that a woman has an increased risk of developing breast cancer. Doctors usually say no treatment is necessary for LCIS, and surgery is not usually recommended.
Ductal carcinoma in situ (DCIS) is also called "Stage 0" breast cancer. This is a non-invasive, malignant tumor in cells lining one of the small ducts that carry milk to the nipple. Although DCIS has not spread beyond the duct, it can progress and become invasive. Doctors usually say treatment is necessary for DCIS. This can include surgery, radiation, and hormonal therapy.