NCI: The most common type of breast cancer. It begins in the cells that line the milk ducts in the breast.,NCI: A breast carcinoma arising from the ducts. While ductal carcinomas can arise at other sites, this term is universally used to refer to carcinomas of the breast. Ductal carcinomas account for about two thirds of all breast cancers. Two types of ductal carcinomas have been described: Ductal carcinoma in situ (DCIS) and invasive ductal carcinoma, not otherwise specified. The latter often spreads to the axillary lymph nodes and other anatomic sites. The two forms of ductal carcinoma often coexist.