Trichoadenoma of Nikolowski is a distinct neoplasm within the spectrum of follicular tumors

Background: Trichoadenoma is a rare benign follicular tumor first described by Nikolowski 50 years ago. Both trichoadenoma and desmoplastic trichoepithelioma are composed of cords of epithelial cells and cornifying cysts embedded in sclerotic stroma. In trichoadenoma the cystic component predominates, while desmoplastic trichoepithelioma is a mostly solid neoplasm. Therefore trichoadenoma was suggested to represent a cystic variant of desmoplastic trichoepithelioma. Objective: The aim of this study was to investigate whether the morphologic overlap between trichoadenoma and desmoplastic trichoepithelioma translates into a similar immunohistochemical profile. Methods: We studied 19 trichoadenomas and 21 desmoplastic trichoepitheliomas for cytokeratin 20, Ber-EP4, and androgen receptor expression. Results: Eighteen of 19 trichoadenomas and all desmoplastic trichoepitheliomas demonstrated the presence of Merkel cells as detected by a monoclonal antibody against cytokeratin 20. In contrast, while all desmoplastic trichepitheliomas were positive for Ber-EP4, only 4 of 19 trichoadenomas showed any kind of reactivity for this marker. None of the trichoadenomas or desmoplastic trichoepitheliomas expressed androgen receptor. Limitations: This study is limited by the moderate number of these rare tumors available for immunohistochemical analysis. Conclusion: Our data demonstrate that trichoadenoma typically retains cytokeratin 20-positive Merkel cells but lacks Ber-EP4 and androgen receptor expression. Trichoadenoma is a distinct follicular tumor related but not identical to desmoplastic trichoepithelioma.