Intraepithelial neoplasia of the breast

Hyperplastic lesions of the breast may be categorized histopathologically as either ductal or lobular. The nomenclature of lobular intraepithelial neoplasia (LIN) and ductal intraepithelial neoplasia (DIN) have recently been preferred. LIN includes atypical lobular hyperplasia (ALH) and lobular carcinoma in situ (LCIS) deriving from the terminal duct-lobular unit (TDLU) and showing a discohesive small-cell hyperplasia with or without Pagetoid extension into terminal ducts. Three grades as LIN1, LIN2 and LIN3 based on the extent and degree of cell proliferation and/or cytological features have been recommended. Ductal intraepithelial neoplasia (DIN) has diverse cytological and architectural features distinct from LIN, not only morphologically, but also in clinical significance and treatment. By definition, they originate from the TDLU and are confined to the mammary duct system, most often involving small-to-medium sized ducts, less often large or lactiferous ducts. Ductal intraepithelial neoplasia (DIN) include the lesions of usual epithelial hyperplasia (UDH), atypical ductal hyperplasia (ADH), ductal carcinoma in situ (DCIS) and flat epithelial atypia (FEA).