Abstract

Acral FibroChondroMyxoid Tumor (AFCMT) is a newly recognized benign acral soft-tissue neoplasm with distinctive histopathologic features and associated gene fusion (THBS1-ADGRF5). Thus far, only eleven cases have been described in the literature. Here we present two additional cases of this diagnostically challenging tumor. Both patients were otherwise healthy, middle-aged adults (a 43 year old female and a 49 year old male) and the tumors clinically appeared as slow-growing nodules on an acral surface (thumb and third toe). Histologically, both specimens demonstrated a multilobulated, well-demarcated nodule based in the deep dermis and composed of abundant stroma with admixed round to bluntly spindled tumor cells with increased cellularity at the periphery. The stroma is variably fibrous, chondroid, and myxoid. The tumor cells demonstrated oval to reniform nuclei, fine chromatin, and scant eosinophilic cytoplasm. No mitoses or atypia was present. By immununohistochemistry, both were positive for CD34 (2/2) and ERG (2/2) while negative for AE1/3 (2/2) and CAMTA-1 (2/2). Staining for S100 was negative in one case and demonstrated patchy positivity in the other. Staining was negative for SMA (1/1), EMA (1/1), and CD31 (1/1). Based on the histopathologic features and immunophenotype, the diagnosis of AFCMT was rendered. Recognition of the characteristic morphology and immunophenotype allows distinction from such entities as soft tissue chondroma, myoepithelioma, EWSR1-SMAD3 rearranged fibroblastic tumor, and epithelioid hemangioendothelioma.

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