NRSTS2021, A Risk Adapted Study Evaluating Maintenance Pazopanib, Limited Margin, Dose-Escalated Radiation Therapy and Selinexor in Non-Rhabdomyosarcoma Soft Tissue Sarcoma (NRSTS)

The study participant has been diagnosed with non-rhabdomyosarcoma (NRSTS). Primary Objectives Intermediate-Risk * To estimate the 3-year event-free survival for intermediate-risk patients treated with ifosfamide, doxorubicin, pazopanib, surgery, and maintenance pazopanib, with or without RT. * To characterize the pharmacokinetics of pazopanib and doxorubicin in combination with ifosfamide in intermediate-risk participants, to assess potential covariates to explain the inter- and intra-individual pharmacokinetic variability, and to explore associations between clinical effects and pazopanib and doxorubicin pharmacokinetics. High-Risk * To estimate the maximum tolerated dose (MTD) and/or the recommended phase 2 dosage (RP2D) of selinexor in combination with ifosfamide, doxorubicin, pazopanib, and maintenance pazopanib in high-risk participants. * To characterize the pharmacokinetics of selinexor, pazopanib and doxorubicin in combination with ifosfamide in high-risk participants, to assess potential covariates to explain the inter- and intra-individual pharmacokinetic variability, and to explore associations between clinical effects and selinexor, pazopanib and doxorubicin pharmacokinetics. Secondary Objectives * To estimate the cumulative incidence of primary site local failure and distant metastasis-free, disease-free, event-free, and overall survival in participants treated on the risk-based treatment strategy defined in this protocol. * To define and describe the CTCAE Grade 3 or higher toxicities, and specific grade 1-2 toxicities, in low- and intermediate-risk participants. * To study the association between radiation dosimetry in participants receiving radiation therapy and the incidence and type of dosimetric local failure, normal adjacent tissue exposure, and musculoskeletal toxicity. * To evaluate the objective response rate (complete and partial response) after 3 cycles for high-risk patients receiving the combination of selinexor with ifosfamide, do