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Abstract

Ossifying fibromyxoid tumor (OFMT) is a rare tumor of intermediate malignancy characterized by epithelioid to spindled tumor cells in fibrous to myxoid stroma surrounded by a peripheral shell of bone in up to 80% of cases. Approximately 85% have gene fusions, most commonly involving PHF1. The great majority arise in superficial soft tissue, but only rarely involve the dermis. We present a case of a nonossifying variant of OFMT that presented as a 2 cm nodule on the upper arm of a 50-year-old female. A punch biopsy demonstrated an unencapsulated, multinodular proliferation of epithelioid and plasmacytoid cells arranged in nests and cords embedded in myxoid stroma. The individual tumor cells had uniform, small, oval nuclei and eosinophilic cytoplasm. No significant cytologic atypia, mitotic activity, or necrosis was present, and the lesion did not have peripheral ossification. By immunohistochemistry, the tumor cells were positive for desmin, S100 protein, MyoD1, and negative for AE1/AE3, SOX10, SMA, EMA, PRAME, myogenin, and MUC4. Next generation sequencing identified an EP400::PHF1 fusion, confirming the diagnosis of nonossifying OFMT. This case highlights the fact that OFMT does not always have the namesake feature of ossification and that OFMT can present as a dermal tumor. The combination of S100 and desmin positivity, along with SOX10 negativity, can be a clue to the diagnosis of OFMT. MyoD1 expression occurs in greater than 50% of OFMT, but it is an underrecognized characteristic that can lead to diagnostic confusion. Molecular studies can help confirm the diagnosis in challenging cases such as this one.