Jay-Gerin, Jean-PaulAlanazi, Ahmed Mohammed Abar2023-05-032023-05-032023https://hdl.handle.net/20.500.14154/67954Radiation therapy is an important part of the care of cancer patients. Theoretically, cancer could be cured by high doses of ionizing radiation. However, its practical application at high doses causes undesirable side effects in normal tissues. For this reason, most therapeutic improvements have focused on reducing side effects on healthy tissues. In 2014, a novel irradiation technique called "FLASH-RT" was proposed to more effectively kill cancer cells while protecting healthy tissue. Since little is known about the physicochemical mechanism underlying the effects of FLASH, e.g., the early events that occur after energy deposition, the aim of this project was to find out how the early physical and physicochemical stages along the radiation tracks are affected by high dose rates. To verify our approach, we first used our Monte Carlo code to study the effect of low dose rates of protons on radiolysis of water in the 150 keV-500 MeV energy range. The good agreement between the experimental data and our simulation results (our yield calculations for the primary radiolytically generated species) at low dose rates shows that we can use our code to study the effects of high dose rates on proton irradiation-induced radiolysis of water. As a second step, we were able to determine the critical point in time when the interaction between tracks starts in the track stage of radiolysis. The "onset" of dose-rate effects is shown to be inversely proportional to the dose rate, as demonstrated by our simulation results using our cylinder model. Based on a comparison with experiments/models using pulsed electrons, it appears that the geometry of the irradiation volume significantly affects both the time period over which dose-rate effects develop and the radiolytic yields. Finally, we extended our previous work to study the effect of linear energy transfer on oxygen depletion with protons at high dose rates. We found that in contrast to what is observed with low LET irradiation, the transient O2 consumption that occurs with high LET irradiation is quite significant. Taken together, our modeling demonstrates its suitability to study the effects of ultra-high dose rates on the initial physicochemical stages of water radiolysis.182enFast protonsFLASH radiotherapywater radiolysislinear energy transferMonte Carlo simulationsultra-high dose rates.Thesis