Clinical Implications of Coexisting EWSR1 Rearrangements in Ewing Sarcoma: A Database Analysis and Representative Cases

Background Ewing sarcoma (ES) is a highly aggressive malignancy with a poor prognosis, particularly in metastatic disease. While localized disease treated with multimodal therapy achieves favorable survival rates, metastatic disease at presentation has a significantly poorer prognosis. The molecular hallmark of ES is the presence of EWSR1-ETS family fusion genes, with EWSR1-FLI1 being the most common gene. Objective To investigate the impact of additional EWSR1 rearrangements on treatment outcomes in ES by analyzing a national cancer genomic database. Methods We conducted an exploratory retrospective analysis of the cancer genomic status and treatment response using the Center for Cancer Genomics and Advanced Therapeutics (C-CAT) database. Patients and Materials This study analyzed clinical genomic testing results from the C-CAT database, focusing on bone and soft tissue tumors identified as ES (n=90). Representative cases were selected to illustrate the clinical implications of the molecular findings. Results An analysis of the C-CAT database revealed that a subset of ES cases showed additional EWSR1 rearrangements beyond the primary fusion gene. Cases with coexisting rearrangements demonstrated higher rates of disease progression with first-line chemotherapy than those with isolated fusions. This finding was exemplified in our institutional experience, where ES with an isolated EWSR1-FLI1 fusion showed a good response to standard chemotherapy, whereas a case with an additional ETS1-EWSR1 rearrangement exhibited primary resistance to multiple lines of therapy. Conclusion These findings suggest that coexisting EWSR1 rearrangements may have important prognostic implications and warrant further investigation to optimize treatment strategies in resistant cases.