Epithelioid fibrous histiocytoma (EFH) is a benign cutaneous neoplasm that is now recognized to be largely driven by ALK rearrangements. Rare cases of EFH and EFH-spectrum tumors with other receptor tyrosine kinase (RTK) fusions including RET and NTRK3 have been described, demonstrating the molecular heterogeneity of this entity. Herein, we report a 9-year-old female with a 1.5 cm right lateral chest wall lesion showing classic EFH morphology. Histology demonstrated a nodular dermal tumor extending to dermo-epidermal junction above and subcutaneous tissue below. The lesion was characterized by sheets of bland epithelioid/histiocytoid cells with a vaguely whorled architecture, prominent vessels, focal myxoid stroma, and no significant atypia, mitoses, or necrosis. On immunohistochemistry, the lesional cells were highlighted by CD68 and CD163, with patchy CD99 positivity. The lesional cells were negative for AE1/AE3, CD34, SMA, Desmin, S100, SOX10, CD10, ERG, CD1a, Langerin, CD30, and CD31. INI-1 was retained. Targeted solid tumor fusion analysis showed an ETV6::NTRK3 fusion. This case expands the molecular spectrum of EFH beyond ALK rearrangements and previously described RET and NTRK3 fusions.
Challa et al. (Tue,) studied this question.