Epithelial proliferations of ductal type.

The diagnosis of ductal proliferations requires analysis of both architectural and cytological features. Architectural characteristics of conventional ductal hyperplasia include persistence of the duct lumen as peripheral, crescent shaped spaces; streaming of cells; formation of irregular, slit-like fenestrations; and "maturation." Hyperplastic ductal cells exhibit indistinct cell borders, irregular placement of nuclei, irregular nuclear shapes, granular chromatin, and uniform small nucleoli. The cells do not show polarization or dishesion. Low-grade ductal carcinoma in-situ exhibits both architectural and cytological atypicality. The architectural atypicality takes two forms: the formation of cribriform spaces or their variants and the regular arrangement of cells. The former reflects the polarization of the carcinoma cells and the latter their dishesion. Cytological atypicality includes distinct cell borders; smoothly contoured, oval or round nuclei; homogeneous chromatin; and inconspicuous nucleoli. Atypical ductal hyperplasia shows low-grade cytological atypicality but lacks the architectural atypicality of ductal carcinoma in situ. Proliferations lacking cytological atypicality do not merit the diagnosis of atypical ductal hyperplasia whatever their architectural characteristics. Although not usually necessary, immunohistochemical staining for high molecular weight keratin can help resolve difficult cases. Current evidence does not support the belief that conventional ductal hyperplasia represents an obligate precursor to ductal carcinoma in situ.