Proliferating tricholemmal cyst: a possible pitfall in cytological diagnosis

We report a case of proliferating tricholemmal cyst, which made difficulties in the cytological analysis of the fine-needle biopsy aspirates. This rare dermatopathologic entity is a possible diagnostic pitfall due to its close cytological resemblance to squamous cell carcinoma-the abundance of material, with predominance of squamoid cells, which form concentric structures (so-called squamous eddies), that simulate keratin pearls of well-differentiated squamous cell carcinoma. However, careful and strict application of the cytological criteria prevents overdiagnosis; the small squamoid cells that predominate in the smears occur mainly in clusters and sheets, they do not show prominent atypia, and the squamous eddies may not undergo complete keratinization.