FLASH radiotherapy (FLASH RT) has gained attention in radiation therapy research, because it has been observed that when the dose is delivered at ultra-high dose rate (UHDR), a biological effect called FLASH effect appears. It is sparing normal tissues whereas the tumor control remains the same compared to conventional irradiations. The FLASH effect was observed with typical irradiations of less than 100ms and a mean dose rate of at least 100Gy/s, in different animal species (fish eggs, mice, cats and pigs) and a first patient was treated in 2019. Most of the experiments were performed with UHDR electron beams, but also with photons and protons. The redundant observations of FLASH effect on animals, makes it relevant for clinical transfer and clinical protocols have been recently started.
However, there are important remaining questions that have to be answered. The biological cause of FLASH effect is not yet fully understood and the physical parameters of the beams that trigger FLASH effect are not fully defined. The metrological traceability of the dose at UHDR is not yet established and most of the time redundant dosimetry is used to characterize the delivered dose. The type of beams to be used, particularly related to the field size, is still a question to be debated, and which one between very high energy electron (VHEE), protons or photons beams are best suited for the treatment of deep-seated tumors remains an open question. Finally, a safe and reliable use of FLASH RT is mandatory for the clinical transfer and the short duration of the irradiations need new control systems that still have to be developed.
FLASH RT is a highly promising way for the treatment of cancer in radiation therapy, but important questions remain to be solved before a large dissemination of that treatment technique will become possible in clinical practice.