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Case Reports

Abstract

Cellular neurothekeomas are uncommon dermal neoplasms of uncertain histogenesis, typically presenting as lobulated fibrohistiocytic lesions lacking S100 expression. We report a diagnostically challenging case of a 65-year-old patient with a hand mass that, on biopsy, raised concern for an infiltrative sarcomatous process. The biopsy was composed predominantly of dense fibrous tissue with scattered large, atypical cells and focal weak S100 positivity. Broad immunohistochemical studies were non-contributory, including negativity for SOX10, CD34, desmin, keratins, EMA, and melanocytic markers. A differential including sarcoma, epithelioid vascular neoplasm, and melanoma was considered. Excision revealed a sharply demarcated dermal neoplasm with lobulated architecture and abundant myxoid stroma, consistent with a pure myxoid morphology. The atypical cells seen in the biopsy were interpreted as part of fibrous bands separating the tumor lobules. Immunohistochemistry on the excision showed weak S100 positivity, strong NKI-C3 expression, and negativity for SOX10, MITF, CD68, PU.1, and other lineage markers. Taken together, findings were most consistent with a pure myxoid variant of cellular neurothekeoma. This case highlights a critical pitfall in the diagnosis of myxoid dermal neoplasms: limited biopsy may sample only fibrous stromal elements, mimicking sarcoma and obscuring the true lobulated architecture. Additionally, the immunophenotype underscores the evolving spectrum of cellular neurothekeoma and the overlap with nerve sheath myxoma. Awareness of these pitfalls can prevent overtreatment and misclassification of rare cutaneous tumors.