Importance of track density for radiolytic reactions: changes in yields of OH radical molecular probe under protons around the Bragg peak

The aim of the present study is to elucidate the mechanisms of sparing effects of tissues observed under ultra-high-dose rate (UHDR: > 40 Gy/s) irradiations. To do so, under Bragg peak energy protons, we evaluate changes in OH radicals, that efficiently react with DNA and proteins, consequently, they govern the indirect action, by varying the spatial distribution of proton tracks, while maintaining absorbed dose rate. By applying beam with 2 μm × 2 μm (“micro” beam), yields of OH radicals decrease by − 38 ± 7%, compared to that with 100 μm × 100 μm (“macro beam). The present results show that radical–radical reactions between neighboring tracks occur by UHDR irradiations even around the Bragg peak energy. Additionally, radical–radical reactions, leading the reduction of yields of OH radicals, are one of the mechanisms of sparing effects seen by UHDR irradiations.