Differentiation of hidroacanthoma simplex from clonal seborrheic keratosis--an immunohistochemical study.

Hidroacanthoma simplex (HS) is an uncommon poroid neoplasm confined within the epidermis. The clinical features of HS are not distinctive and histopathologically HS may be confused with clonal seborrheic keratosis (CSK) if cystic or ductal structure is not present. The purpose of our study was to differentiate HS from CSK by the immunohistochemical expressions of various cytokeratins, CEA, CD1a, and S-100 protein, as well as by the degrees of deposition of melanins and glycogen. Four cases of HS and seven cases of CSK were included in the research. In contrast with CSK, HS showed a very low density of Langerhans cells (19.9 +/- 7.7 versus 3.1 +/- 1.0 CD1a (+) cells/mm, P = 0.027) and sparse melanin deposition in the nests. However, HS could not be set apart from CSK by the expressions of cytokeratins. The nests of both HS and CSK showed very similar patterns of cytokeratin expression and seemed to be mainly composed of basaloid cells with focal differentiation toward epidermal suprabasal cells.