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 diagnosed Ewing sarcoma or peripheral primitive neuroectodermal tumor (PNET) of bone or soft tissue * Prior Therapy: * Patients should have had only frontline therapy as per institutional standard, and maintenance therapy if given (no relapse therapy). * If frontline systemic therapy already completed (not including maintenance or metastatic site radiation), therapy completed within 6 months of enrollment to Part B. * Subjects must have a willing physician provider supporting their participation in Part B. * For Part B, providers are eligible to receive the provider survey if they are listed as the primary provider for the patient at the study site. Exclusion Criteria: * For Part A, subjects must not meet any of the following exclusion criteria. * Patients with distant metastatic disease. * Patients with known Ewing-like sarcoma (e.g., BCOR-CCNB3 or CIC-DUX4 translocated small round cell sarcomas) are not eligible. * Patients who are enrolled with an initial diagnosis of Ewing sarcoma and subsequently found to have Ewing-like sarcoma will be replaced. Samples obtained prior to removal from study will be analyzed and reported descriptively. Patients with Ewing-like tumors may continue to provide samples and clinical data until they meet off-study criteria per protocol. * Patients weighing \< 5 kg at time of diagnosis * Patients with a second malignant neoplasm * Patients without detectable tumor at the time of study enrollment (ie, complete tumor resection prior to study enrollment) * Patients already receiving tumor-directed therapy at the time of study enrollment except when a pre-treatment baseline sample has already been obtained under a local banking study in DFCI Pediatrics that would be eligible for analysis under this study. * Patients with osteosarcoma with a pelvic primary tumor site Pregnancy * For Part B, subjects must not meet any of the following exclusion criteria. * Patients with known Ewing-like sarcoma (e.g., BCOR-CCNB3 or CIC-DUX4 translocated small round cell sarcomas) are not eligible. * Samples obtained prior to removal from study will be analyzed and reported descriptively. Patients with Ewing-like tumors may continue to provide samples and clinical data until they meet off-study criteria per protocol * Patients weighing \< 5 kg at time of enrollment * Patients diagnosed with relapsed disease and/or having started therapy directed at disease relapse * Pregnancy * Resides outside of the United States * For Part B, providers at non-study centers will not be eligible to receive the provider survey.