Soft-tissue sarcoma with MN1-BEND2 fusion: second reported case and comparative analysis

MN1-BEND2 fusion is a defining molecular alteration in astroblastoma, MN1-altered, and is rarely identified in extracranial tumors. To date, only one soft-tissue sarcoma harboring this fusion has been reported. We present the second known case, arising in the deep pelvis of a 34-year-old woman with osseous and neural foraminal invasion. Imaging demonstrated a destructive pelvic mass, and biopsy revealed a spindle cell sarcoma with a myofibroblastic immunophenotype. The patient underwent neoadjuvant radiotherapy followed by radical resection. Histologic evaluation showed heterogeneous morphology, and molecular testing confirmed MN1-BEND2 fusion. The patient remains without evidence of recurrence on short-term follow-up. This case expands the clinicopathologic spectrum of MN1-BEND2-associated soft-tissue sarcomas and highlights the importance of molecular testing in undifferentiated spindle-cell tumors to ensure accurate diagnosis and classification.