Recently, shoot-through proton FLASH has been proposed where the highest
energy is extracted from the cyclotron to maximize the dose rate (DR). Even
though our proton pencil beam scanning system can deliver 250 MeV (the highest
energy), it is not typical to use 250 MeV protons for routine clinical
treatments and as such 250 MeV may not have been characterized in the
commissioning. In this study, we aim to characterize 250 MeV protons from
Varian ProBeam system for FLASH RT as well as assess the ability of clinical
monitoring ionization chamber (MIC) for FLASH-readiness. We measured data
needed for beam commissioning: integral depth dose (IDD) curve, spot sigma, and
absolute dose calibration. To evaluate MIC, we measured output as a function of
beam current. To characterize a 250 MeV FLASH beam, we measured: (1) central
axis DR as a function of current and spot spacing and arrangement, (2) for a
fixed spot spacing, the maximum field size that still achieves FLASH DR (i.e.,
> 40 Gy/s), (3) DR reproducibility. All FLASH DR measurements were performed
using ion chamber for the absolute dose and irradiation times were obtained
from log files. We verified dose measurements using EBT-XD films and
irradiation times using a fast, pixelated spectral detector. R90 and R80 from
IDD were 37.58 and 37.69 cm, and spot sigma at isocenter were {\sigma}x=3.336
and {\sigma}y=3.332 mm, respectively. The absolute dose output was measured as
0.377 GyE*mm2/MU for the commissioning conditions. Output was stable for beam
currents up to 15 nA, and it gradually increased to 12-fold for 115 nA. DR
depended on beam current, spot spacing and arrangement and could be reproduced
within 4.2% variations. Even though FLASH was achieved and the largest field
size that delivers FLASH DR was determined as 35x35 mm2, current MIC has DR
dependence and users should measure DR each time for their FLASH applications.Comment: 11 pages, 6 figure