345 An Unusual Presentation of EWSR1-PATZ1 Fusion Sarcoma with PAX8 Expression and Aggressive Clinical Behavior

Introduction/Objective EWSR1-PATZ1 fusion sarcomas have heterogenous morphology and immunophenotype with frequent expression of neural, myogenic and myoepithelial markers. These rare tumors are challenging to diagnose, have a wide age range, predilection for thoracoabdominal wall primary site, and variable clinical course. Methods/Case Report A 44-year-old woman presented with new onset left-sided weakness, and MRI brain revealed a right frontoparietal hemorrhagic mass. CT chest showed innumerable bilateral pulmonary nodules. Lung core biopsy showed a malignant small round cell neoplasm comprising relatively monotonous cells forming nests within a fibromyxoid stroma. Immunohistochemistry demonstrated PAX8 positivity, faint synaptophysin, and absence of lineage-specific markers. Sarcoma Targeted Gene Fusion Panel (138 genes) detected EWSR1-PATZ1 rearrangement. Whole body PET/CT scan subsequently found a large hypermetabolic mass in the left thigh compatible with the primary site. The right brain lesion was resected and was consistent with metastatic sarcoma. She was treated with VAC chemotherapy followed by pazopanib TKI with mixed response and passed away 12 months after diagnosis. Results NA Conclusion Pathologic diagnosis of EWSR1-PATZ1 fusion sarcomas is challenging and requires molecular testing. This case lacked previously described markers and instead showed PAX8 expression. Further studies are needed to determine whether immunophenotype or other tumor characteristics could be associated with more aggressive behavior.