Ductal carcinoma in situ (DCIS) is the presence of abnormal cells inside a milk duct in the breast. DCIS is considered the earliest form of breast cancer. DCIS is noninvasive, meaning it hasn't spread out of the milk duct to invade other parts of the breast. DCIS is usually found during a mammogram done as part of breast cancer screening or when there is another concern with a woman's breast. Because of increased screening with mammograms, the rate at which DCIS is diagnosed has increased dramatically in recent years.