Radiotherapy performed at ultra-high dose rate (UHDR: > 40 Gy/s), so called FLASH, has attracted great interests due to its great advantage, for instance, the maintenance of killing effects to tumors while the damage to surrounding normal tissues are minimized as compared to the conventional dose rate (CONV: 0.1 Gy/s) irradiation. The sparing effect has been observed with biological experiments using mice and living cells.
However, the mechanism of the sparing effect is not clarified. To elucidate the mechanism of FLASH radiotherapy from a point of view of radiation chemistry, we address to evaluate dose rate dependence of water radiolysis species (*OH, eaq– and H2O2) under proton and C ion beams. Yields of 7-hydroxy-coumarin-3-carboxylic acid (7OH-C3CA), which is produced by reactions of *OH with coumarin-3-carboxylic acid (C3CA), decrease monotonically with increasing dose rates. Above 10 Gy/s, they become constant. Possible interpretations of the reduction of the yields of 7OH-C3CA with dose rates would be (1) the reduction of oxygen concentration and (2) the reduction of the amount of *OH reacting with C3CA due to radical-radical reactions (e.g., *OH + *OH → H2O2) between neighboring tracks. In the presentation, we will discuss this point by presenting data sets, regarding direct measurements of oxygen concentration during irradiations and evaluation of dose rate dependence of yields of eaq– and H2O2.
National Institute for Quantum Science and Technology, Japan