PROJECT SUMMARY/ABSTRACT Flash radiotherapy, the delivery of high radiation dose (10 – 30 Gy) at ultra-high dose rates (40 – 200 Gy/s), has recently been shown to reduce significantly normal tissue toxicity compared to conventional irradiation, while maintaining tumor control probability at similar level. Great excitement has since ensued about the transformative potential of FLASH radiotherapy. However, the biological mechanisms of FLASH irradiation (FLASH effect) are not well understood. The clinical translation of FLASH irradiation necessitates comprehensive laboratory studies to elucidate the biological effects as well as pertinent technological and physical requirements. At present, FLASH research employs complex accelerator technologies of limited accessibilities. We propose to develop a novel self-shielded x-ray irradiation cabinet system, as an enabling technology to greatly enhance the preclinical research capabilities of the radiation research community. The system employs two commercially available high capacity 150 kV fluoroscopy x-ray sources with rotating anode technology in a parallel-opposed arrangement. For a medium less than 2 cm in thickness, the system can deliver both FLASH and conventional dose-rate radiations to support a broad range of laboratory radiation research. We submit our proposal as an academic-industrial partnership (AIP) to design and construct the first FLASH kilo-voltage x-ray cabinet system for preclinical laboratory research. The AIP consists of Johns Hopkins University (JHU) with the expertise in radiation physics, dosimetry, Monte Carlo simulation, robotics, and preclinical radiation research, Xstrahl to manufacture and commercialize the FLASH cabinet system for preclinical research, and University of Pennsylvania (UPenn) to support in-field validation of the novel system. A JHU patent application is currently under review. Our specific aim for the 4-year research efforts are: (1) Design a new self-shielded pre-clinical radiation research system based on in-depth characterization of the dosimetric properties of the x-ray beam for both FLASH and conventional radiations, and the mechanical requirements of the system to support small and large radiation fields. (2) Develop a Monte-Carlo based dose calculation system to provide information on the delivered dose, dose rate, and LET distributions in the irradiated target, pertinent to FLASH research. (3) Conduct in-field validation, at JHU and UPenn, of the system dosimetric performance, and demonstrate the system capability for FLASH and conventional irradiation of in-vivo models. The successful development of the system by the AIP will make available transformative FLASH capabilities for the laboratory researchers, and significantly enhance mechanistic and translational research on FLASH irradiation.