MELANOCYTES IN EPITHELIAL NEST: A DIAGNOSTIC CLUE FOR TRICHOEPITHELIOMAS / TRICHOBLASTOMAS

Introduction: Epithelial pilar neoplasms are relatively rare and heterogeneous, revealing pigment exceptionally. The presence of melanocytes has not been studied and there is no systematic evaluation of histologic features, resulting in a low diagnostic reproducibility.<br>Methods: We selected trichoepithelioma/trichoblastoma (46) with appropriate archival material. We systematically evaluated growth pattern (multifocal or not), type of squamous differentiation, cytological evidence of catagen, presence of dual cell population, epithelial and stromal pigment, epithelial and stromal calcification, stromal reaction (myxoid, lamellar fibrosis, hyalinization), nuclear features, and mitotic figurell0 HPF. The presence of melanocyte was investigated by immunohistochemistry (S100, HMB-45). Results: All cases revealed a dual population, epithelial cells revealing bland nuclei and distinct nucleolus and interstitial cells in the nests showing hyperchromatic spindle nuclei, The latter were revealed S-100- and HMB-45-positive. Epithelial pigment also correlated with catagen cytological features (71%). Tumors revealing multifocal growth pattern showed mitotic figures (80%), while these were less frequently present in non-multifocal neoplasms (49%). The other histological variables revealed no correlation.<br>Conclusions: Melanocyte colonization is a very common finding in trichoepitheliomas-trichoblastomas and can help identifying these neoplasms. Multifocal growth pattern should be investigated especially in mitotically active neoplasms, for which a complete excision is more difficult.