Novel EWSR1::HOXB8 fusion identified in a rare spindle cell sarcoma of a young woman

Spindle cell mesenchymal neoplasms comprise a heterogeneous group of tumors, with recent molecular advances uncovering novel pathogenic mechanisms. We report the case of a 22-year-old woman presenting with progressive dyspnea. A computed tomography (CT) scan revealed a solid mass located in the right cardiophrenic angle. Histological examination showed a spindle cell neoplasm composed of medium-sized, elongated cells with scant eosinophilic cytoplasm, arranged in storiform and whorled patterns. Immunohistochemical staining was negative for lineage-specific markers. Fluorescence in situ hybridization (FISH) detected an EWSR1 gene rearrangement in 15% of the neoplastic cells, and RNA sequencing demonstrated a novel EWSR1::HOXB8 fusion. This case highlights the relevance of EWSR1-driven fusions in spindle cell mesenchymal neoplasms and emphasizes the need for further exploration into their clinical and biological behavior. After 5 months of chemotherapy, the patient showed moderate clinical improvement.