Morphological observations regarding the origins of atypical cystic lobules (low-grade clinging carcinoma of flat type)

This study seeks to define the morphological pathogenesis of a form of cyst ic transformation of lobules referred to as atypical cystic lobule or low-g rade clinging carcinoma of flat type. We collected 25 examples that seem to represent the early stages in the formation of atypical cystic lobules and made a careful study of their morphology. Our observations indicate that t his lesion arises from pre-existing, structurally normal terminal duct-lobu lar units and that it seems to develop by the direct transformation of indi genous luminal cells. The transformed cells first become evident in the sma ll duct or terminal ductule, where they appear as slightly enlarged columna r cells containing atypical nuclei. In more advanced examples, these altera tions affect all luminal cells of the terminal duct-lobular unit, but the c ells of the terminal ductule continue to show more pronounced changes than the cells lining the acini. The atypical cells within the lobule do not see m to displace the normal acinar cells. Instead, the cells form a continuum, making it impossible to define the point at which atypical cells and norma l cells meet. Within the small duct, however, the atypical epithelium comes to an abrupt halt as the duct courses towards the nipple. Considering thes e observations, we ascribe the formation of atypical cystic lobules to the accumulation of atypical cells in pre-existing terminal duct-lobular units.