PROJECT SUMMARY/ABSTRACT Advances in our understanding of disease biology and development of new technologies have made individualized treatment a reality for many patients with cancer. Such improvements have been elusive for adolescents and young adults (AYAs) with sarcomas. Treatments for these diverse sarcomas often include intensive multiagent chemotherapy, surgery and radiation, yet many patients still suffer relapse and die of disease. Survivors are left with debilitating late effects including infertility, heart disease, and secondary cancers. Outcomes in other cancers have been vastly improved using biomarkers to delineate disease sub-groups, evaluate response to therapy, and detect relapse, maximizing chance of cure and minimizing late effects. In contrast, there are no validated biomarkers in AYA sarcomas to facilitate testing of risk-stratified therapy. This scientific gap presents a major barrier to improving outcomes. My long-term objective is to improve outcomes for AYAs with sarcomas through biomarker-informed clinical trials. Circulating tumor DNA (ctDNA) is an exciting candidate biomarker for this purpose and holds the potential for broad application across the majority of AYAs with sarcomas. I have demonstrated that elevated ctDNA burden at diagnosis is associated with an increased risk of disease relapse and death for patients with the two most common bone sarcomas affecting this population: Ewing sarcoma (EWS) and osteosarcoma. Broadly, all high-grade sarcomas can be divided into fusion-positive sarcomas like EWS or fusion-negative sarcomas with a high level of aneuploidy like osteosarcoma. ctDNA holds the potential to be paradigm shifting for the treatment of AYA sarcomas by leveraging these genomic characteristics to inform risk-stratification. We have assembled large patient cohorts (n>700) of two fusion-positive sarcomas (EWS and synovial sarcoma) and two fusion- negative sarcomas [osteosarcoma and undifferentiated pleomorphic sarcoma (UPS)] to perform validation studies to establish ctDNA prognostic thresholds at diagnosis and following initial therapy. To assess the clinical impact of return of results on clinicians, I have launched a pilot study using commercial liquid biopsy testing for fusion detection during disease surveillance for patients with EWS. I will evaluate the central hypothesis that ctDNA burden is clinically informative in sarcomas affecting AYAs and can inform trials of risk-adapted therapy in the near future by addressing the following aims: (1) Determine ctDNA-based prognostic thresholds for risk of relapse in fusion-negative sarcomas at baseline; (2) Determine ctDNA-based prognostic thresholds for risk of relapse in fusion-positive sarcomas at baseline and following initial treatment; and (3) Evaluate the clinical impact of ctDNA testing for disease surveillance in EWS. At the conclusion of this project, we will have validated ctDNA as a prognostic biomarker in multiple clinical settings and position...