The hormone-sensitive epithelial cells within the lobules are the major source of ductal carcinoma in situ (DCIS) of the breast. The neoplastic cells grow, and fill and increase the volume of the spaces bound by the basement membrane until they disrupt them. Even extensive cases of DCIS are unifocal in three dimensions and are usually confined to a single segment of the mammary duct system. The neoplastic cells can proliferate within the spaces that have been altered by benign proliferative

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... ases such as adenosis and multiple papilloma. The concept of unfolding is the key to understanding the morphology of DCIS as well as benign breast cysts, both of which have larger and fewer structures although they originated in the small blindending structures within the lobules. Atypical ductal hyperplasia (ADH) can be understood as minimal low-grade DCIS that incompletely fills the spaces bound by the basement membrane. Although ADH, atypical lobular hyperplasia, and lobular carcinoma in situ (LCIS) carry a general risk for later development of invasive mammary carcinoma, DCIS carries a localized risk. The management of the DCIS should be determined based on pathological evidence including grade, size, and surgical margin status.