Uncommon morphologic patterns of lobular neoplasia

Discussions of the morphology of lobular neoplasia (atypical lobular hyperplasia and lobular carcinoma in-situ) usually emphasize the commonplace characteristics. Occasional cases deviate from the expected appearance and thereby cause diagnostic confusion. Unusual cytologic alterations include the formation of signet-ring cells and the presence of nuclear pleomorphism. The growth of lobular neoplasia in breast tissue altered by atrophy, sclerosing adenosis, collagenous spherulosis, and benign tumors gives rise to unexpected patterns that one might mistake for normal tissue, invasive carcinoma, or ductal carcinoma in situ. Solid involvement of ducts by lobular neoplasia and uncommon forms of "pagetoid" growth both resemble the appearance of atypical ductal hyperplasia or ductal carcinoma in situ. The authors illustrate these unusual cytologic and architectural findings and discuss features that permit proper diagnosis and interpretation.