Abstract

Epithelioid fibrous histiocytoma (EFH), a rare variant of fibrous histiocytoma (dermatofibroma) characterized by a well-circumscribed proliferation of plump epithelioid cells in the dermis with an epidermal collarette, typically presents as a nodule on the extremities. Most tumors show expression of ALK by immunohistochemistry, reflecting an underlying ALK fusion. Here, we describe a case of an epithelioid fibrous histiocytoma from the right posterior thigh of a 50-year-old female with a striking and unusual low-power configuration. Histologic examination revealed a cellular lesion arranged as a solitary, thick, circumscribed column extending from the dermis to the subcutis. The lesion was composed of plump spindled to epithelioid cells with amphophilic cytoplasm, irregular nuclear contours, vesicular chromatin, and small nucleoli. By immunohistochemistry, the lesion was diffusely positive for ALK, focally positive for Factor XIIIa and CD68, and negative for CD34, AE1/AE3, smooth muscle actin, desmin, SOX10, and CD30. This case illustrates an unusual appearance of epithelioid fibrous histiocytoma, expanding the histologic spectrum of this neoplasm.

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