The CATSINDO Trial - Clinical and Translational Study in Newly Diagnosed Osteosarcoma

The goal of this study is to learn whether children, adolescents, and young adults with newly diagnosed high-grade osteosarcoma can be safely discharged from the hospital at slightly higher methotrexate blood levels after receiving standard high-dose methotrexate chemotherapy. Participants are 22 years old or younger and are receiving standard MAP (high-dose methotrexate with doxorubicin and cisplatin) chemotherapy as part of their routine cancer treatment. The main questions this study aims to answer are: * Is hospital discharge at higher methotrexate levels safe, based on side effects or hospital re-admission within 7 days? * Can patient-derived osteosarcoma tumor organoids be successfully generated across multiple centers? Researchers will compare safety outcomes and hospital length of stay to historical patient data discharged at lower methotrexate levels. Participants will receive standard chemotherapy, meet study-defined discharge criteria, be monitored for side effects, and have the option to provide tumor and blood samples for future research.