This thesis investigates the tissue sparing effect of FLASH (>40 Gy/s) radiation, as opposed to CONV (conventional, dose rates typically between 0.01­0.1 Gy/s) radiation. We irradiated zebrafish embryos (4 days past fertilisation) with 116 MeV protons. The aim was (1) to measure the effect, and (2) if the effect were significant, see whether it depended on the oxygen concentration in the tissue, as the oxygen depletion hypothesis (a popular theory on the underlying mechanics of the FLASH effect) predicts. We irradiated embryos with either FLASH or CONV, where a possible FLASH effect would reduce toxicity of the FLASH radiation. We did the same for zebrafish which were deliberately put in a hypoxic condition prior to irradiation. In that case, the depletion hypothesis would predict that the difference between FLASH and CONV disappears. Our biomarkers for radiobiological damage were the survival rate and γ­H2AX foci formation. In our experimental conditions, the radiation effect on the survival rate was
eclipsed by other factors which could not be isolated. We did confirm the possibility of using γ­H2AX foci formation as a marker for radiobiological damage in full­body irradiated zebrafish embryos. There were individual samples that showed clear and localised specific γ­H2AX signal, but these were too
scarce and the signal was too inconsistent across samples to gather meaningful statistics. This was most often caused by limited antibody penetration in the embryo. We were therefore unable to draw conclusions about the FLASH effect. Better and more consistent antibody penetration, e.g. by longer digestion in collagenase before antibody staining, could change this in the future. We further custom­built and validated a hypoxic aquarium to produce hypoxic zebrafish tissue, as well as a computational model of the irradiation setup to simulate the dose distribution in the zebrafish container. We found the dose distribution to be sufficiently homogeneous for our experiment, at least 91.47% uniform for CONV and 90.72% uniform for FLASH.