ABSTRACT: Radiation therapy is a necessary component of current treatment for many brain tumors. Radiation therapy leverages a therapeutic ratio in which tumor cells suffer radiation damage and die, while normal brain cells repair radiation damage and are less negatively affected. Improvements in the therapeutic ratio are badly needed both to improve brain tumor cure probability and limit risk for radiation effects on the normal brain such as radiation necrosis and neurocognitive dysfunction. Efforts to widen the therapeutic window for radiation therapy of brain tumors have included chemical approaches to combine RT with drugs intended to increase tumor cell radiosensitivity, or normal cell radioresistance, however, to date, few effective therapies have emerged. Recently, advances in physics have produced “ultra-high” rates of radiation dose delivery (termed “FLASH-RT”). Compelling reports point to normal tissue protection with FLASH-RT dose delivery, however the molecular mechanism of FLASH-RT is largely unknown. Interestingly, ferroptosis cell death mechanisms have been implicated. Our multidisciplinary collaborative team has leveraged complementary expertise to develop resources and preliminary data to improve the brain tumor radiation therapy therapeutic window through both chemical and physical means. We have created a unique experimental platform with two novel components: 1) A highly tunable, very high dose-rate electron FLASH- RT beam provided by the High Intensity Gamma-ray Source (HIGS) at the Triangle Universities Nuclear Laboratory (TUNL) and 2) an Organotypic brain slice culture (OBSC) assay platform that allows interrogation of both tumor and normal brain tissue cells in an elevated-throughput dry-culture system highly amenable to experiments with both standard RT and our unique FLASH-RT beam. Encouragingly, preliminary data in our OBSC system shows that ATM kinase inhibition seems to protect neurons from ferroptotic death. To achieve our long-term goal to widen the therapeutic window, we propose two hypotheses: 1) ATM inhibition can widen the radiation therapeutic window for brain tumors by sensitizing tumor cells and protecting neurons especially under FLASH-RT conditions, and 2) Cancer cells depend on adaptations to maintain rapid growth in the brain, and these adaptations can be exploited to enhance tumor cell killing. We will test these hypotheses with the following aims: 1) Optimize HIGS-FLASH beam parameters (dose, dose-rate, targeting) for enhanced metastatic brain tumor cell kill with maximal protection of normal brain under normal and ATM inhibition conditions. 2) Test genetic targets for radiation sensitization in glioblastoma cells under standard and FLASH- RT dose rates in the OSBC system. We expect that this project will elucidate cytotoxic mechanisms of both standard and FLASH-RT and generate highly-translatable drugs and drug targets to widen the therapeutic window for brain tumors. If successful, this project will e...