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We aim to investigate the feasibility of online monitoring of irradiationtime (IRT) and scan time for FLASH radiotherapy using a pixelated semiconductordetector. Measurements of the time structure of FLASH irradiations wereperformed using fast, pixelated spectral detectors, AdvaPIX-TPX3 andMinipix-TPX3. The latter has a fraction of its sensor coated with a neutronsensitive material. With little or no dead time and an ability to resolveevents that are closely spaced in time (tens of ns), both detectors canaccurately determine IRTs as long as pile-ups are avoided. To avoid pile-ups,we placed the detectors beyond the Bragg peak or at a large scattering angle.We acquired prompt gamma rays and secondary neutrons and calculated IRTs basedon timestamps of the first (beam-on) and the last (beam-off) charged species.We also measured scan times in x, y, and diagonal directions. We performedthese measurements for a single spot, a small animal field, a patient field,and a ridge filter optimized field to demonstrate in vivo online monitoring ofIRT. All measurements were compared to vendor log files. Differences betweenmeasurements and log files for a single spot, a small animal field, and apatient field were within 1%, 0.3% and 1%, respectively. In vivo monitoring ofIRTs was accurate within 0.1% for AdvaPIX-TPX3 and within 6.1% forMinipix-TPX3. The scan times in x, y, and diagonal directions were 4.0, 3.4,and 4.0 ms, respectively. Overall, the AdvaPIX-TPX3 can measure FLASH IRTswithin 1% accuracy, indicating that prompt gamma rays are a good surrogate forprimary protons. The Minipix-TPX3 showed a higher discrepancy, suggesting aneed for further investigation. The scan times (3.4 \pm 0.05 ms) in the 60-mmdistance of y-direction were less than (4.0 \pm 0.06 ms) in the 24-mm distanceof x-direction, confirming the much faster scanning speed of the Y magnets thanthat of X.

Measurement of the time structure of FLASH beams using prompt gamma rays and secondary neutrons as surrogates