A Long-Term Disease-Free Case of Biphenotypic Sinonasal Sarcoma with Intracranial and Intraorbital Extension Initially Misdiagnosed as Synovial Sarcoma

Biphenotypic sinonasal sarcoma (BSNS) is a very rare, locally aggressive sarcoma arising in the sinonasal region, initially recognized as low-grade sinonasal sarcoma with neural and myogenic differentiation. Here, we report a case of BSNS extending into the intracranial and intraorbital regions, finally diagnosed by a break-apart fluorescence in situ hybridization (FISH) assay for rearrangements of <i>PAX3</i>. A 50-year-old woman presented with left diplopia and exophthalmos. CT and MRI revealed a large ethmoidal mass with intracranial and intraorbital extension. Since preoperative biopsy suggested a benign tumor, endoscopic endonasal resection was performed while preserving the anterior skull base and intraorbital structures. Postoperative histopathological diagnosis indicated synovial sarcoma, and proton beam therapy with adjuvant chemotherapy was subsequently administered. After treatment, FISH demonstrated rearrangements of <i>PAX3</i> and <i>MAML3</i> genes, leading to a revised diagnosis of BSNS, which typically does not require chemotherapy due to its non-metastatic behavior. Eleven years after treatment, the patient remains free of recurrence. Understanding BSNS is essential to avoid excessive intervention, and confirmation of <i>PAX3</i> rearrangement by FISH or equivalent molecular testing is crucial for accurate diagnosis.