Session Item

The delivery of radiation at ultra-high dose rates (UHDR) reduces the formation of Hydrogen Peroxide (H2O2) by both electron and proton beams compared to conventional dose-rate irradiation (CONV). Those previous results were obtained with a single beam whereas clinical treatments are mainly delivered with multiple beams. Consequently, the present study aims to evaluate the concentration of H2O2 in function of the number of beams of a treatment plan.

Using the 68 MeV proton beam of the ARRONAX facility, the UHDR radiation chemistry by the measurement of H2O2 produced from water radiolysis was measured after irradiation. Thirty Gy were delivered to a water sample using a single macro-pulse of 0.18 Gy/s (dt = 167 s) and 8600 Gy/s (dt = 3.5 ms), respectively for CONV and UHDR modes. The impact of the number of the beams was evaluated in the UHDR mode by delivering N times the same pulsed beam every 20 s but with a reduced duration of dt/N, with N varying from 1 to 10. This delivery mode allowed to mimic a multiple beam plan. H2O2 concentrations were determined with the Ghormley triiodide method. For quality control of the experimentations, online dose monitoring was performed by measuring the nitrogen fluorescence produced by the beam-air interaction and delivered dose was confirmed by analyzing orthochromic films placed at the beam entrance. Experiments were performed two times on two different dates.

Online dose monitoring and film dosimetry confirmed proper irradiations. For UHDR irradiation with a single beam, the H2O2 concentration was measured to a significantly lower value than CONV irradiation, in good agreement with the literature (-34%). H2O2 concentration increased when the number of beams increased. Although the CONV H2O2 concentration was 7.7x10-6 mol L 1, the UHDR H2O2 concentration increased from 5.1x10-6 mol L 1 (N = 1) to 6.7x10-6 mol L 1 (N = 10). The difference in H2O2 concentrations between CONV and UHDR modes was reduced when the number of beams increased. Difference was -34%, -24%, -21%, -17% and -12% respectively for N equal to 1, 2, 3, 5 and 10. The longer time required to deliver 30 Gy with multiple beams or the smaller dose per beam lowered the radical recombination responsible for the decrease in the production of H2O2.

Our study investigated the impact of the number of beams on the concentration of Hydrogen Peroxide after UHDR proton irradiation. The decrease of the H2O2 concentration was more important with a single beam. However, it appears possible to obtain a differential effect between CONV and UHDR irradiations with multiple beams, but with a lower amplitude. These results showed that, in addition to the dose rate, the observed effect depends also on the dose per beam or the total duration of the irradiation. Next experiments will study the time delay between two beams to deepen this finding.