Liquid Biopsy in Ewing Sarcoma and Osteosarcoma as a Prognostic And Response Diagnostic: LEOPARD

Inclusion Criteria: * For Part A, subjects must meet all of the following eligibility criteria. * Age: ≥ 12 months of age at time of study enrollment to 50 years of age * Diagnosis: Patients with histologic diagnosis (by institutional pathologist) of newly diagnosed, localized or regionally disseminated Ewing sarcoma or peripheral primitive neuroectodermal tumor (PNET) of bone or soft tissue or; Patients with histologic diagnosis (by institutional pathologist) of newly diagnosed, non-pelvic, localized or regionally disseminated high-grade osteosarcoma. NOTE: Staging will be assessed according to standard of care at the treating center. * Prior Therapy: * Patients should have only previously had a biopsy, and not had prior attempt at tumor resection. * Not yet started chemotherapy or radiation therapy OR patient has started chemotherapy or radiation therapy, but an appropriate pre-treatment baseline sample was collected and processed for ctDNA under a local banking study in DFCI Pediatrics and is available to use for this study. * Planned to receive chemotherapy as follows: \-- VDC/IE as per COG protocols AEWS0031, AEWS1031 or AEWS1221 (for patients with Ewing sarcoma or PNET); or MAP as per COG protocol AOST0331 (for patients with osteosarcoma). * For Part B subjects must meet all of the following eligibility criteria. * Age: ≥ 12 months of age at time of study enrollment * Diagnosis: Patients with histologic diagnosis (by institutional pathologist) of newly diagnose…