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

## TL;DR

A rare case of biphenotypic sinonasal sarcoma was correctly diagnosed after initial misdiagnosis, leading to a long-term disease-free outcome.

## Contribution

This case highlights the importance of molecular testing for accurate diagnosis of BSNS and avoiding unnecessary chemotherapy.

## Key findings

- Molecular testing confirmed the diagnosis of BSNS after initial misdiagnosis as synovial sarcoma.
- The patient remained disease-free for 11 years without requiring chemotherapy.
- BSNS typically does not metastasize, supporting the need for precise molecular diagnosis.

## Abstract

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 PAX3. 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 PAX3 and MAML3 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 PAX3 rearrangement by FISH or equivalent molecular testing is crucial for accurate diagnosis.