## 3 Control and Monitoring
In this study, the authors could measure more than 200 Gy/s dose-rate beams produced by a clinical synchro-cyclotron with a commercially available plane-parallel ionization chamber; however, the measured value was about 5% off the expectation, an uncertainty which might be due to several reasons. In particular, the standard dosimetry protocols used to calculate the recombination corrections resulted in correction values that disagreed by up to 4% at the highest dose rates. Common correction approaches show strong limitations when applied in ultra-high dose rates beams. Other online beam current/dose monitors, developed for high power accelerators, do not show dose-rate dependence and are usually capable of coping with large beam currents. However, the short times associated with the FLASH delivery pose a challenge to the usage of these detectors in a clinical FLASH setting. The secondary emission monitors and ionization chambers in use at the PROSCAN facility at PSI [36] provide reliable current measurements, with limited saturation and recombination effects even at currents of 500 nA; however, the presence of temporary drops in the signal (for about 20 s after beam-on time) and microphonic noise limits their direct applicability in FLASH experiments. In addition, resonator detectors [16] could easily cope with such high currents but usually have a relatively long integration time. To be usable as FLASH monitors, they should provide a reliable signal within only about 100 us; investigations using these detectors are still ongoing. Detectors are also linked to important tasks of delivering the treatment safely. With increasing beam current, the time to react to beam delivery instabilities or failures of hardware devices reduces. This makes an active control of the beam delivery rather challenging. Additionally, this time also needs to be estimated very precisely in order to assess how much dose could be erroneously delivered to the patients in case of delivery errors before the safety elements switch off the beam. If we assume an irradiation scenario of about 10 Gy delivered dose, and 50 Gy/s dose rate, to make sure the additional dose delivered to the patient is 1% or less of the total dose, the reaction time of the safety system must be below 2 ms. For higher dose rates, even shorter reaction times are needed (please note that instantaneous dose rate rather than average dose rate should be considered in these calculations, if the beam structure is not continuous). Such reaction times seem within reach of currently available safety systems; for example, at PSI the first level of safety is able to interrupt the irradiation within 300 us. However, if higher instantaneous dose rates will be required in future FLASH applications, even faster-reacting systems might be needed. Several aspects could be exploited to improve the reaction times; at the PSI facility cited above, switching from a magnetic deflector to an electrostatic deflector to stop the beam in case of errors reduces the reaction time by a factor 6 [37]. However, since most of the reaction time of the safety system is needed to actually detect the error and communicate the beam-off command, a further reduction of reaction times will also require faster detectors (for example, based on scintillation), as well as fast logic and communication. Therefore, achieving faster monitorswill be one of the main steps towards realizing FLASH in the clinic for both dose control and safety.