Simulation was performed with the GEANT4 Monte-Carlo platform. 
GEANT4
Ultrahigh dose-rate (FLASH) x-ray irradiator for pre-clinical laboratory research.FLASH irradiation has been shown to reduce significantly normal tissue toxicity compared to conventional irradiation, while maintaining tumor control probability at similar level.  Clinical translation of FLASH irradiation necessitates comprehensive laboratory studies to elucidate biological effects as well as pertinent technological and physical requirements.  At present, FLASH research employs complex accelerator technologies of limited accessibilities.  Simulated dosimetric properties of the x-ray beam for both FLASH and conventional dose-rate irradiations were characterized.  The rotating anode x-ray source can be used to deliver both FLASH and conventional dose-rate irradiations with the field dimensions well suitable for small animal and cell-culture irradiations.  For FLASH irradiation using parallel-opposed sources, entrance and exit doses can be higher by 30% than the dose at the phantom center. 
FLASH