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Simple Summary: The ability of ultra-high dose rate FLASH radiation therapy (RT) to reduce normal tissue toxicity without affecting tumor response relative to conventional dose rate radiation therapy could fundamentally change the way we treat cancer. However, this field is still in its early stages, and the magnitude of the sparing effect between treatment centers differs greatly for reasons as yet unknown, which has put the robustness of the effect into question. In this study, we show that when similar irradiation beam parameter settings are used, the induced sparing effect is robust and reproducible across institutions. These settings should serve as a reference for further optimization of the FLASH effect. FLASH radiation therapy (RT) is a promising new paradigm in radiation oncology. However, a major question that remains is the robustness and reproducibility of the FLASH effect when different irradiators are used on animals or patients with different genetic backgrounds, diets, and microbiomes, all of which can influence the effects of radiation on normal tissues. To address questions of rigor and reproducibility across different centers, we analyzed independent data sets from The University of Texas MD Anderson Cancer Center and from Lausanne University (CHUV). Both centers investigated acute effects after total abdominal irradiation to C57BL/6 animals delivered by the FLASH Mobetron system. The two centers used similar beam parameters but otherwise conducted the studies independently. The FLASH-enabled animal survival and intestinal crypt regeneration after irradiation were comparable between the two centers. These findings, together with previously published data using a converted linear accelerator, show that a robust and reproducible FLASH effect can be induced as long as the same set of irradiation parameters are used. [ABSTRACT FROM AUTHOR]