Experimental investigation of oxygen diffusion in the peak and valley region of minibeam patterns during X-Ray irradiation

BackgroundMinibeam radiotherapy has demonstrated its potential to reduce normal tissue toxicity, while maintaining tumor control. However, the underlying mechanisms behind this phenomenon remain unknown. Recent theoretical studies suggest a dose surrogate by diffusion of H2O2 into the valley regions. PurposeThe aim of this study is to experimentally investigate oxygen depletion and diffusion upon minibeam (MB) irradiation. MethodsA 3D-printed water phantom with four sensors was developed to enable the real-time, simultaneous measurement of oxygen concentration in the peak and valley. Water with 0 % - 11 % O2 and 0.1 % / 5 % CO2 was irradiated with broad beam (BB) and MB characterized by peak and valley widths of 2 mm x 2 mm and 0.5 mm x 2 mm. The depletion was further compared in other chemical environments. ResultsThe oxygen depletion rates per dose in hypoxic water in the valley regions were found to be 3 to 7 times higher compared to the peaks or BB. This observation was found to be independent of oxygen concentration above 2 %, indicating oxygen depletion saturation. For MB, diffusion between peaks and valleys was observed. After a certain period, an equilibrium between diffusion and dose rate differences was established. Glutathione and HEPES as medium increased the depletion further and distinguished MB from BB. ConclusionsA novel way of simultaneously measuring oxygen in the peak and valley of the MB dose pattern was introduced. The observed oxygen depletion saturation and diffusion between the peaks and valleys suggest the importance of oxygen in spatially fractionated radiotherapy studies, which is even greater for 5 mM glutathione compared to water.