BACKGROUND AND PURPOSE: Continuing recent experiments at the research electron accelerator ELBE at the Helmholtz-Zentrum Dresden-Rossendorf the influence of beam pulse structure on the Flash effect was investigated.

MATERIALS AND METHODS: The proton beam pulse structure of an isochronous cyclotron (UHDR iso) and a synchrocyclotron (UHDR synchro) was mimicked at ELBE by quasi-continuous electron bunches at 13 MHz delivering mean dose rates of 287 Gy/s and 177 Gy/s and bunch dose rates of 10 6Gy/s and 10 9 Gy/s, respectively. For UHDR synchro, 40 ms macro pulses at a frequency of 25 Hz superimposed the bunch delivery. For comparison, a maximum beam intensity (2.5 × 10 5 Gy/s mean and ∼10 9 Gy/s bunch dose rate) and a reference irradiation (of ∼8 Gy/min mean dose rate) were applied. Radiation induced changes were assessed in zebrafish embryos over four days post irradiation.

RESULTS: Relative to the reference a significant protecting Flash effect was observed for all electron beam pulse regimes with less severe damage the higher the mean dose rate of the electron beam. Accordingly, the macro pulsing induced prolongation of treatment time at UHDR synchro regime reduces the protecting effect compared to the maximum regime delivered at same bunch but higher mean dose rate. The Flash effect of the UHDR iso regime was confirmed at a clinical isochronous cyclotron comparing the damage induced by proton beams delivered at 300 Gy/s and ∼9 Gy/min.

CONCLUSION: The recent findings indicate that the mean dose rate or treatment time are decisive for the normal tissue protecting Flash effect in zebrafish embryo.