Ductal carcinoma in situ of the breast, light micrograph of a section from an affected breast. Ductal carcinoma in situ (DCIS) is the most common type of non-invasive breast cancer. Ductal means that the cancer starts inside the milk ducts. While DCIS isnt life-threatening, it does require treatment to prevent the condition from becoming invasive. Most women with DCIS are effectively treated with breast-conserving surgery and radiation. Magnification: x 100 when printed at 10 centimetres wide. Human tissue.