Ductal carcinoma in situ (DCIS) and lobular carcinoma in situ (LCIS) represent a proliferation of malignant epithelial cells within the ducts of the breast without invasion through the basement membrane. It is assumed that all invasive carcinomas of the breast are preceded by DCIS or LCIS. However, it is not known what proportion of DCIS or LCIS--if left untreated--will progress to invasive carcinoma. Standard treatment for DCIS is breast-conserving surgery with free margins and radiation therapy. LCIS is usually treated by observation alone.