  Purpose: Ultra-fast scintillation imaging has been shown to provide a unique
tool for spatio-temporal dosimetry of conventional cyclotron and
synchrocyclotron pencil beam scanning (PBS) deliveries, indicating the
potential use for characterization of ultra-UHDR PBS proton beams. The goal of
this work is to introduce this novel concept and demonstrate its capabilities
in recording complex dose rate maps at FLASH-capable proton beam currents, as
compared to log-based dose rate calculation, internally developed UHDR beam
simulation, and a fast point detector (EDGE diode). Methods: The light response
of a scintillator sheet located at isocenter and irradiated by pencil beam
scanning proton fields (40-210 nA, 250 MeV) was imaged by an ultra-fast iCMOS
camera at 4.5-12 kHz sampling frequency. Image intensity was calibrated to dose
with film and the recorded spatio-temporal data was compared to a PPC05 ion
chamber, log-based reconstruction, and EDGE diode. Calculation of full-field
mean and PBS dose rate maps were used to highlight the importance of high
resolution, full-field information in UHDR studies. Results: Camera response
was linear with dose (R2 = 0.997) and current (R2 = 0.98) in the range from
2-22 Gy and 40-210 nA, respectively, when compared to ion chamber readings.
Planned and delivered spot positions agreed within 0.2+/-0.1 mm and total
irradiation time agreed within 0.2+/-0.2 ms when compared with the log files,
indicating the high concurrent spatial and temporal resolution. For all
deliveries, the PBS dose rate measured at the diode location agreed between the
imaging and the diode within 3+/-2% and with the simulation within 5+/-3%.
Conclusion: The high linearity and various spatiotemporal metric reporting
capabilities confirm the continued use of this camera system for UHDR beam
characterization, especially for spatially resolved dose rate information.
