FLASH radiotherapy

The radiotherapy for cancer that is performed at an ultra-high dose rate (>40 Gy/s), so-called “FLASH” radiotherapy, has attracted great interest due to its therapeutic advantage. Unlike the conventional dose rate of <0.1 Gy/s, the tumor cell-killing effect of FLASH radiotherapy is maintained while the damage to surrounding normal tissues is minimized, as the ultra-high dose rate irradiation spares healthy tissues. FLASH radiotherapy thus improves patients’ quality of life. The effects of FLASH radiotherapy have been actively investigated in radiobiological studies using electrons, protons, and heavy ions. One of the key points in the mechanisms underlying FLASH radiotherapy regards the chemical reactions that occur in tissues. The sparing effect is related to the yields of water radiolysis products, which contributes to the indirect action toward DNA strand breaks. In this chapter, we overview the effects of FLASH radiotherapy that have been observed in biology and chemistry studies, and we discuss the applications for therapy.