Background and Aims

To investigate the impact of temporal beam structure on physico-chemical events following exposure to ultra-high (UHDR) and conventional dose rates, non-buffered water and pBR322 plasmids were irradiated using: eRT6/Oriatron/CHUV 5.5MeV electron beam at 0.1Gy/s (conventional) and ≥555Gy/s (UHDR); Gantry 1/COMET cyclotron/PSI 235MeV proton beam in transmission mode at 1Gy/s (conventional) and 1400Gy/s (UHDR); and CLEAR/CERN 220MeV electron beam at 0.2Gy/s (conventional) and 10⁹ Gy/s (UHDR).

Methods

G°(H₂O₂) were estimated with scavenging methods while G(H₂O₂) were measured minutes after water irradiation. DNA damage was quantified after plasmid irradiation in presence or absence of hydroxyl radicals’ scavenger (DMSO) by gel electrophoresis (AGE).

Results

When CONV and UHDR-irradiation were compared, similar G°(H₂O₂) were found with electron, proton and VHEE. However, electron produced the highest primary yield of H₂O₂ (0.8±0.01 and 0.75±0.02 molecules/100eV), that was reduced with proton (0.66±0.02 and 0.67±0.01 molecules/100eV) and even lower with VHEE (0.56±0.02 and 0.52±0.02 molecules/100eV). G(H₂O₂) was significantly lower after UHDR irradiation compared to CONV-RT. The reduction was similar for all 3 beams (31% for 5.5MeV electrons; 35% for protons and 37% for VHEE). No difference in plasmid DNA damage was measured after UHDR vs CONV for any of the beams when plasmids were irradiated in 21%O₂. Investigation of scavenger’s effect is ongoing.

Conclusions

These experiments are the first systematic investigations involving various beams able to operate at UHDR with different temporal structure. The results show that initial physico-chemistry events post-irradiation are dose rate independent, whereas UHDR irradiation lowers G(H₂O₂) values at later time point, suggesting modification of recombination/diffusion processes at UHDR. This does not impact DNA damages in plasmids but needs to be investigated in biological systems.

Acknowledgement: The study is funded by FNS Synergia grant (FNS CRS II5_186369).