Track: Case Reports

Abstract:

Superficial ALK-rearranged myxoid spindle cell neoplasm (SAMS) is a recently described kinase fusion-positive mesenchymal tumor that shares clinicopathologic similarities to epithelioid fibrous histiocytoma (EFH), and the term “myxoid spindle cell variant of EFH” has been proposed. We present two additional cases, one of which shows moderate atypia, multinucleated giant cells, and harbors a novel fusion. We report two tumors found in an 81-year-old female’s back and a 61-year-old male’s thigh. The first involves the dermis and subcutis, while the second involves the dermis. Both have epidermal collarette and consist of spindle cells arranged in whorls, fascicles, and cords within a myxoid to collagenous stroma. The first tumor exhibits multinodular growth, focal infiltrative borders, and no atypia. The second tumor shows similar morphology but with moderate atypia, rare multinucleated giant cells, and a focus with EFH-like morphology. Mitotic activity is absent in the first tumor, while slightly elevated in the second (2/10 hpf). Immunohistochemically, tumor cells are positive for ALK (2/2), S100 (2/2), and CD34 (1/2). In the second tumor, the EFH-like area exhibits a different immunoprofile, showing stronger ALK and CD34 expression, and without S100 labeling. RNA sequencing identified an FXR1::ALK fusion in the first tumor and a novel LRRFIP1::ALK fusion in the second. A 3-month follow-up of the first case revealed no evidence of disease, while follow-up data for the second case were unavailable. Our study supports that SAMS likely represents a histological variant of EFH. To date, there has been only one case showing moderate atypia.

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Sintawat Wangsiricharoen