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Abstract

Superficial fibromyxoid tumors represent a heterogenous diverse group of soft tissue neoplasms, ranging from benign to malignant, containing variable portions banal spindle cells with fibromyxoid stroma. A significant number of these entities are characterized by immunohistochemical and specific reciprocal molecular genetic profiles that aid in their diagnosis. Despite their morphologically banal nature, accurate diagnosis is critical for appropriate patient treatment and prognostication. SLX4 is a multidomain scaffold protein interacting with three distinct nucleases SLX1, ERCC4/XPF-ERCC1, and MUS81-EME1. Mutations in SLX4 gene are well described in Fanconi anemia but not yet previously reported in any solid tumor or soft tissue neoplam. Herein we present a superficial low-grade angiofibromyxoid neoplasm in a male with a novel SLX4 gene inactivating mutation that has not been previously characterized.   An 18-year-old male presented with a 2-year-history of right thigh mass of the thigh. Imaging revealed a 5.5 cm mass involving the right subcutaneous anterior thigh, radiologically worrisome for sarcoma. Initial core-biopsy was non-definitive thus prompting surgical resection. The histomorphology bears semblance to angiofibroma of soft tissue, exhibits a well circumscribed silhouette with prominent fibrous capsule containing prominent proliferating vessels. Electron microscopy showed prominent widening of interstitial space, spindled morphology and cytoskeleton composed predominantly of actin filaments, endoplasmic reticulum and mitochondria. Comprehensive genomic profiling revealed a heterozygous SLX4 inactivating variant. In summary we present superficial low-grade angiofibromyxoid neoplasm in the lower extremity, morphologically resembling angiofibroma of soft tissue but hitherto molecular profile of SLX4 gene inactivating mutation.