Purpose: Spatiotemporal pulse to pulse beam profile for an ultra-high dose rate (UHDR, >40Gy/s) electron FLASH (eFLASH) irradiator was characterized via Cherenkov emission and radioluminescence imaging using iCMOS cameras.
Methods: Cameras time-gated to the delivered clinical LINAC pulses, imaged radioluminescence and Cherenkov emission signals incited by single pulses of 10MeV eFLASH irradiation at the treatment room isocenter. The two cameras imaged the entrance Cherenkov emission from a solid water phantom surface or scintillation from a screen on top of the phantom and the projected depth profile from a tank filled with water or quinine sulfate solution. A 450nm peak bandpass filter was considered for improving imaged depth profiles. The optical results were compared to lateral profiles measured by Gafchromic film at varying depths including the surface.
Results: The per pulse beam output, from Cherenkov imaging, agreed with the photomultiplier tube (PMT) Cherenkov output to within 3%, after the first ~5-7 ramp-up pulses. In comparison to film measurements, Cherenkov emission and scintillation images were dose rate independent to >300Gy/s and linear with dose (R²=0.987, 0.995, respectively). The surface dose profiles from film agreed more with scintillation than Cherenkov emission imaging (3%/3mm gamma pass rate of 98.9% and 88.8% respectively). A 450nm bandpass filter improved the quinine sulfate-based water images of the projected depth optical profiles to match projected film dose within 5%.
Conclusion: The cameras resolved dose profiles of the UHDR beam with a single pulse (60Hz) temporal and 1mm spatial resolution. The agreement of scintillation screen imaging with film measured dose suggests it can accurately confirm the consistency of the beam’s parameters for quality assurance. The cross validation of Cherenkov images with PMT temporal profile of each pulse suggest it can monitor beam output but required its optical filtering to acquire accurate dose via quinine sulfate-based water imaging.