Normal cells
Ductal hyperplasia or "overgrowth" means that too many cells are present.
Atypical ductal hyperplasia means that there are too many cells (hyperplasia) and they are starting to take on an abnormal appearance (atypical or "not typical").
Ductal carcinoma in situ means that there are too many cells and they have the features of cancer, but they are still confined to the inside of the duct.
DCIS-MI (DCIS with microinvasion) means that a few of the cancer cells have started to break through the wall of the duct. This is considered to be a slightly more serious form of DCIS.
Invasive ductal cancer means that the cancer cells have broken beyond the breast duct. The breast cancer is no longer a DCIS but an invasive ductal carcinoma, the most common type of breast cancer.
There are three grades of DCIS: low or grade I; moderate or grade II; and high or grade III. The lower the grade, the more closely the cancer cells resemble normal breast cells and the more slowly they grow. Sometimes it's difficult to figure out where the cells are on in the range from normal to abnormal. If the cells are in between grades, they may be called "borderline.