PROJECT SUMMARY Radiation therapy (RT) is a standard-of-care oncological treatment for the central nervous system (CNS). However, during cranial RT, normal brain tissue adjacent to the tumors is inevitably irradiated, causing transient reversible abnormalities as well as progressive irreversible late toxicities. Approximately 100,000 patients with primary and metastatic brain tumors per year in the United States survive long enough (>6 months) to develop radiation-induced brain injury, including cognitive impairment and/or neurological sequelae, significantly impeding the quality of life. These symptoms occur in 50-90% of adult patients after treatment and can be seen without clinical and radiographic evidence of histological changes. In this proposal, we aim to establish a novel, robust, cell-based 3D brain organoid model that closely mimics the complexity of the human brain microenvironment, serving as a platform to study pathophysiological processes and neuroinflammation induced by RT in the normal brain tissue. Further, leveraging the established UCLA portfolio of radiation mitigators, the brain organoid model could be a promising screening and testing platform that recapitulates the human brain responses to therapeutic agents. Overall, this proposal will not only characterize the real-time and dose-dependent physiological responses of normal brain tissue to ionizing radiation, but will also provide proof-of-principle evidence to support the use of human brain organoid model as a platform for screening and testing radiation mitigators, enabling thus the development of adequate countermeasures against radiation. Considering that patient-derived iPSCs may be leveraged to develop personalized testing models, our novel biomimetic model holds significant promise in the screening of patient-specific radiation mitigators in the era of precision medicine in the near future.