Radiation exposures at ultra-high dose rates (UHDR) at several orders of magnitude greater than in current clinical radiotherapy have been shown to manifest differential radiobiological responses compared to conventional dose rates (CONV). This has led to studies investigating the application of UHDR for therapeutic advantage (FLASH-RT) which have gained significant interest since the initial discovery in 2014 that demonstrated reduced lung toxicity with equivalent levels of tumour control compared with conventional dose-rate radiotherapy. Many subsequent studies have demonstrated the potential protective role of FLASH-RT in normal tissues, yet the underlying molecular and cellular mechanisms of the FLASH effect remain to be fully elucidated. Here, we summarise the current evidence of the FLASH effect and review FLASH-RT studies performed in preclinical models of normal tissue response. To critically examine the underlying biological mechanisms of responses to UHDR radiation exposures, we evaluate in vitro studies performed with normal and tumour cells. Differential responses to UHDR vs CONV irradiation recurrently involve reduced inflammatory processes and differential expression of pro-and anti-inflammatory genes. In addition, frequently reduced levels of DNA damage or misrepair products are seen after UHDR irradiation. So far, it is not clear what signal elicits these differential responses, but there are indications for involvement of reactive species. Different susceptibility to FLASH effects observed between normal and tumour cells may result from altered metabolic and detoxification pathways and/or repair pathways used by tumour cells. We summarize the current theories that may explain the FLASH effect and highlight This article is protected by copyright. All rights reserved.3 important research questions which are key to a better mechanistic understanding and, thus, the future implementation of FLASH-RT in the clinic.
-INTRODUCTIONRadiation therapy (RT) remains a critical part of clinical cancer care prescribed to > 50% of patients in high-income countries and contributes to more than 30% of all long-term cancer survivors 1,2 . Technological advances in imaging and RT delivery techniques have resulted in major improvements in patient survival through improved precision and ability to conform dose to the tumour targets whilst minimising dose to surrounding organs at risk (OARs). In addition to improvements in technical radiotherapy, major research efforts have been made to exploit the unique radiobiological responses that occur at ultra-high dose rates (UHDR). In comparison to conventional clinical dose rates (CONV) in the region of 0.01-0.40 Gy/s, UHDR radiotherapy was originally established using microsecond pulses of 5 MeV electrons with intra-pulse dose rate in the range 10 6 -10 7 Gy/s, time-averaged dose rate > 40 Gy/s and