Round-to-Epithelioid Mesenchymal Tumor Harboring an MN1::BEND2 Fusion: Expanding the Morphologic Spectrum

Abstract

MN1::BEND2 fusion is well established in CNS tumors but exceptionally rare in extracranial mesenchymal neoplasms. We report a subcutaneous upper-back tumor in a 41-year-old woman, composed of bland round-to-epithelioid cells with branching capillary vasculature and focal infiltrative growth. The tumor recurred 42 months after excision. Targeted RNA sequencing identified an in-frame MN1 exon 1::BEND2 exon 9 fusion, supported by Sanger sequencing and MN1 break-apart FISH; Nuclear BEND2 immunoreactivity was an ancillary finding. Unlike two previously reported extracranial MN1::BEND2-harboring tumors with high-grade spindle-cell morphology and myofibroblastic differentiation, this tumor lacked overt high-grade nuclear features and myoid differentiation. This case broadens the morphologic spectrum of MN1::BEND2 mesenchymal neoplasms. Limited reported cases, no meaningful follow-up after re-excision, and unsuccessful methylation profiling preclude conclusions regarding biologic behavior. BEND2 immunohistochemistry may serve an ancillary screen marker, but its sensitivity and specificity remain to be established, and molecular confirmation is required.