DSRCT typically presents as a large abdominopelvic mass with widespread peritoneal involvement predominantly in young males. Familiarity with its radiological features can help guide diagnosis and treatment. Functional imaging with PET/CT offers advantage over anatomical imaging for accurate disease staging.

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Purpose Ewing Sarcoma (ES) and Desmoplastic Small Round Cell Tumors (DSRCT) are aggressive sarcomas molecularly characterized by EWSR1 gene fusions. As pathognomonic genomic events in these respective tumor types, EWSR1 fusions represent robust potential biomarkers for disease monitoring. Patients and Methods To investigate the feasibility of identifying EWSR1 fusions in plasma derived cell-free DNA (cfDNA) from ES and DSRCT patients, we evaluated two complementary approaches in samples from 17 patients with radiographic evidence of disease. The first approach involved identification of patient-specific genomic EWSR1 fusion breakpoints in formalin-fixed, paraffin-embedded tumor DNA using a broad, hybridization capture-based next generation sequencing (NGS) panel, followed by design of patient-specific droplet digital PCR (ddPCR) assays for plasma cfDNA interrogation . The second approach employed a disease-tailored targeted hybridization capture-based NGS panel applied directly to cfDNA which included EWSR1 as well as several other genes with potential prognostic utility. Results EWSR1 fusions were identified in 11/11 (100%) ES and 5/6 (83%) DSRCT samples by ddPCR, while 10/11 (91%) and 4/6 (67%) were identified by NGS. The ddPCR approach had higher sensitivity, ranging between 0.009–0.018% sensitivity. However, the hybrid capture-based NGS assay identified the precise fusion breakpoints in the majority of cfDNA samples, as well as mutations in TP53 and STAG2, two other recurrent, clinically significant alterations in ES, all without prior knowledge of the tumor sequencing results. Conclusion These results provide a compelling rationale for an integrated approach utilizing both NGS and ddPCR for plasma cfDNA-based biomarker evaluations in prospective cooperative group studies.

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