Epithelioid Fibrous Histiocytoma with LRRFIP1::ALK Fusion and Smooth Muscle Hyperplasia.

Epithelioid fibrous histiocytoma (EFH) is a rare benign mesenchymal skin neoplasm with uncertain differentiation. In contrast to classical histiocytomas, EFH cells have a distinctive epithelioid appearance. Molecular analyses have demonstrated that EFH is biologically distinct from other histiocytomas and is associated with rearrangements in the anaplastic lymphoma kinase (ALK) gene, with various identified fusion partners. A 47-year-old man presented with a nodular cutaneous tumor on his left arm that was excised. Histopathological examination revealed a well-delimited intradermal nodule composed of non-atypical epithelioid cells associated with hyperplastic bundles of smooth muscle. Immunohistochemistry revealed diffuse ALK immunostaining in the tumor cells. Therefore, EFH was diagnosed. RNA sequencing revealed a fusion transcript consistent with LRRFIP1::ALK gene rearrangement. This is the first report of an LRRFIP1::ALK fusion in EFH. The influence of this fusion partner on the histological features of EFH remains undetermined.