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Purpose Key factors in FLASH treatments are the ultra‐high dose rate (UHDR) and the time structure of the beam delivery. Measurement of the time structure in pencil beam scanning (PBS) proton FLASH treatments is challenging for many types of detectors since high temporal resolution is needed. In this study, a fast scintillator detector system was developed and used to measure the individual spot durations as well as the time when the beam moves between two positions (transition duration) during PBS proton FLASH and UHDR treatments. The spot durations were compared with machine log‐file recordings. Methods A detector system based on inorganic scintillating crystals was developed. The system consisted of four detector probes made of a sub‐millimeter ZnSe:O crystal that was coupled via an optical fiber to an optical reader with 50 kHz sampling rate. The detector system was used in two experiments, both performed with a PBS proton beam with 250 MeV beam energy and 215 nA requested nozzle beam current. The sampling rate enabled multiple measurements during each spot delivery and during the beam transition between spots. First, the detector was tested in a phantom experiment, where a total of 305 scan sequences were delivered to the four detectors. The number of spots delivered without beam interruption in a single scan sequence ranged from one to 35. The spot duration and transition duration were measured for each individual spot. Secondly, the detector system was used in vivo in preclinical experiments with FLASH irradiation of mouse legs placed in the entrance plateau of the beam. A single detector was placed 1 cm downstream of the irradiated mouse leg. The mouse dose ranged from 30.5 to 44.2 Gy and the field consisted of 35 spots. The spot durations as well as the mean dose rate (field dose divided by the measured field duration) for each mouse were determined using the detector and then compared with the corresponding log files. Results The phantom experiment showed that the logged total duration of an uninterrupted spot sequence was consistently shorter than the measured duration with a difference of −0.252 ms (95% confidence interval: [‐0.255, −0.249 ms]). This corresponded to 0.05%–0.07% of the spot sequence duration in the mice experiments. For individual spots, the mean ± 1SD difference between logged and measured spot duration was −0.39 ± 0.05 ms for the first spot in a sequence, 0.13 ± 0.04 ms for the last spot in a sequence, and −0.0017 ± 0.09 ms for the intermediate spots in a sequence. The measured spot transition durations were 0.20 ± 0.04 ms (5.1 mm horizontal steps) and 0.50 ± 0.04 ms (5.0 mm vertical steps). For the mouse experiments, the mean dose rate calculated from the measured field duration was 84.1–92.5 Gy/s. It agreed with log files with a root mean square difference of 0.02 Gy/s. Conclusions Fiber‐coupled scintillator detectors were designed with sufficient temporal resolution to measure the spot and transition duration during PBS proton UHDR deliveries. Their small volume makes the...

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