| Summary: | Epithelioid fibrous histiocytoma (EFH), also known as epithelioid cell histiocytoma (ECH), is an uncommon benign cutaneous mesenchymal neoplasm that typically occurs in the limbs and consistently harbors ALK gene rearrangements. In this study, we report a unique case of EFH that arose in the labia majora, an unusual site which has been rarely described. The tumor occurred in a 41-year-old woman who presented with a slowly growing painless nodule in the vulva. Histological examination revealed a circumscribed lesion located within the dermis, which was composed of sheets or nests of large polygonal epithelioid cells with abundant eosinophilic cytoplasm. Nuclear atypia was minimal and mitotic figures were rare. Immunohistochemically, the neoplastic cells showed diffuse cytoplasmic staining of ALK protein. Subsequent fluorescence in situ hybridization (FISH) assessment demonstrated a balanced rearrangement of ALK gene. Further next-generation sequencing (NGS) analysis identified SQSTM1-ALK gene fusion. This case demonstrated that EFH could develop in the female external genitalia albeit exceedingly rare. EFH should be included in the differential diagnostic list of vulvar neoplasms with epithelioid cytomorphology. NGS played an important role in the genomic profiling.
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