Differential diagnosis of solid carcinoma in situ

Although most cases of carcinoma in-situ are easily classified as either ductal or lobular on the basis of their pattern of involvement and the nature of their cell-to-cell relation, a few pose diagnostic problems. Attention to the presence of associated lesions and the results of immunohistochemical staining for E-cadherin and, to a lesser extent, cytokeratin can help to categorize problematic cases. This article reviews the histological criteria employed to separate ductal carcinoma in situ from lobular carcinoma in situ, the role of immunohistochemistry in the diagnosis of equivocal lesions, and the evidence suggesting the existence of combined and truly hybrid forms.