Carcinoma in situ is cancer that remains where it first began. It has not spread into nearby tissue.
In the context of breast cancer, ductal carcinoma in situ (DCIS) is Stage 0 breast cancer. The cancer cells remain within the breast duct and do not invade surrounding tissues within the breast or out of the breast. Presently, the recommendation for DCIS is complete excision to negative margins, ideally a rim of 2 mm of normal breast cells around the periphery of the tumor to decrease the chance of local recurrence.
On the other hand, lobular carcinoma in situ (LCIS) does not require excision since it has been reclassified as NOT CANCER. However, patients diagnosed with LCIS are possibly recommended to take antihormone medication such as tamoxifen or aromatase inhibitors to decrease their risk of breast cancer.