## 3 Control and Monitoring  
In the early days of proton therapy, accelerators had a less stable output. The nowadays widely used step-and-shoot PBS delivery technique was invented at Paul Scherrer Institut (PSI) [28] and at GSI Helmholtzzentrum fur Schwerionenforschung (GSI) [29] to realize a very precise delivery despite possible beam intensity variations from the particle source [30]. The technique relies on the integration of a certain amount of dose and is as such completely independent of beam current. However, the precision of the detector used for the dose measurement, which triggers the switch from one spot to the next, is of fundamental importance to ensure that FLASH irradiations could be delivered correctly--and safely--in a clinical setting. Ionization chambers vented to air are commonly used in radiotherapy and are the recommended dosimeter for dosimetry in radiotherapy. As such, they are often integrated in the delivery or monitoring system of proton therapy facilities, with the absolute dose monitoring system relying on them. Therefore, their ability to work at a higher dose rate needs to be assessed before such systems can be released as 'FLASH-ready' for treatments. Pencil beam scanning typically employs large dose rates along the beam axis; therefore, studies about dose-rate dependent detector effects have been performed, even before FLASH became an important topic. [PERSON] et al. [31] found small variation in recombination visible already at clinical dose rates; in particular, they found that recombination corrections estimated at an isochronous-cyclotron facility increase as a function of dose-rate. Such recombination correction variations are usually neglected in clinical practice, either because of their size (less than 0.5% in a clinical volume) or because the variations in dose rates are compensated by the accelerator system by extracting lower currents at higher energies, therefore keeping the dose rate constant at the patient. Beam monitors used for controlling the dose delivery are usually placed close to the patient treatment position, at the end of the beamline and, therefore, will not require dose-rate dependent corrections in this case. In a first attempt at controlling the delivery in dose-driven mode, [PERSON] et al. [32] have realized a dose-rate dependent efficiency drop correction to the ionization chamber placed at the end of the beamline and used it to control the beam, delivered in PBS spot-scanning mode [30]. The measured efficiency drop of an ionization chamber as a function of beam current was calibrated against the number of protons collected by a Faraday cup, which does not show any dose-rate dependency [33]. In the study, below 5 nA no efficiency drop was visible; the maximum drop observed was below 1% up to 10 nA. for higher currents, though, recombination increases dramatically, causing an efficiency drop up to 20% at the highest current measured. The proposed correction uses as input the current measured more upstream with a dose-rate independent detector; therefore, it could be easily implemented as an online-correction, applied during the delivery. Such a correction strategy could be an easy way to adapt the dosimetry and control systems of already existing clinical proton gantries to FLASH dose-rates, as they usually rely on ionization chambers to decide when to move the beam to the next irradiation position. Such effects are not unexpected; the Boag theory [34] predicts non-linearity effects depending on the dose per pulse rather than the (average) dose rate, an effect confirmed also in photon and electron studies [35]. Therefore, the beam structure might be very important in realizing a good correction strategy for recombination effects in a proton facility. The challenge in obtaining a high-precision dose monitoring with ionization chambers was highlighted by [PERSON] et al. [21].