Breast cancer arises in the milk-producing glands of the breast tissue. Groups of glands in normal breast tissue are called lobules.
The products of these glands are secreted into a ductal system that leads to the nipple. Depending on where in the glandular or ductal unit of the breast the cancer arises, it will develop certain characteristics that are used to sub-classify breast cancer into types.
The pathologist will denote the subtype at the time of evaluation with the microscope. Ductal carcinoma begins in the ducts, and lobular carcinoma has a pattern involving the lobules or glands. The more important classification is related to the evaluated tumor's capability to invade, as this characteristic defines the disease as a true cancer.
The stage before invasive cancer is called in situ, meaning that the early malignancy has not yet become capable of invasion. Thus, ductal carcinoma in situ is considered a minimal breast cancer.