Sinonasal extraosseous Ewing's sarcoma of the nasal cavity with EWSR1: FLI1 fusion - a rare case report from Nepal.

Introduction: Ewing's sarcoma is a rare, aggressive malignancy primarily originating in bones. Sinonasal extraosseous Ewing sarcoma (EES) displays morphologic heterogeneity that often correlates with underlying genotypes, most commonly the EWSR1::FLI1 fusion. EES, a soft tissue variant, constitutes less than 15% of all Ewing's sarcoma cases and is exceptionally rare in the nasal cavity. Nasal EES presents with nonspecific symptoms such as nasal obstruction or bleeding, often leading to misdiagnosis and delayed treatment. Timely diagnosis through histopathological, immunohistochemical, and molecular testing is essential for initiating appropriate therapy and improving prognosis. Case Presentation: A 30-year-old female from Nepal presented with a 2-month history of a progressively enlarging mass in her left nasal cavity, accompanied by intermittent epistaxis and nasal obstruction. Physical examination revealed a firm, non-tender mass occupying the left nasal passage. Imaging showed a soft tissue lesion without bony erosion. Biopsy followed by histopathological analysis confirmed a diagnosis of sinonasal EES with demonstration of the EWSR1::FLI1 fusion by fluorescence Discussion: Nasal EES is exceedingly rare, often mimicking benign lesions, which contributes to diagnostic delays. While CD99 immunoreactivity supports the diagnosis, its lack of specificity mandates molecular confirmation. Comprehensive differential diagnosis, including exclusion of other CD99-positive tumors, is critical. Multimodal treatment, including chemotherapy, surgery, and radiotherapy, is the standard approach. Early intervention is critical due to the tumor's aggressive nature. Conclusion: This case emphasizes the need to consider EES in the differential diagnosis of nasal masses and highlights the necessity of molecular testing for EWSR1 rearrangements to confirm the diagnosis and guide therapy. Increased awareness and reporting are vital to enhancing diagnosis and management of this rare entity.