New modalities for ultra-high dose rate irradiation and FLASH experiments

In vitro radiobiological experiments using ultra-high dose rate (UHDR) irradiation were performed already during the 1960s to 1980s, with several promising studies demonstrating normal tissue sparing [1–5]. In the last decade, UHDR irradiation has again gained significant momentum within the radiotherapy community. UHDR irradiation has demonstrated increased normal tissue tolerance across organ systems and endpoints, using diverse delivery techniques. Intriguingly, the sparing effect appears to preferentially benefit normal tissue, suggesting a widening of the therapeutic window - a phenomenon now widely referred to as the FLASH effect [6–9].