  Purpose: The purpose of this work was to provide a flexible platform for
FLASH research with protons by adapting a former clinical pencil beam scanning
gantry to irradiations with ultrahigh dose rates.
  Methods: PSI Gantry 1 treated patients until December 2018. We optimized the
beamline parameters to transport the 250 MeV beam extracted from the PSI COMET
accelerator to the treatment room, maximizing the transmission of beam
intensity to the sample. We characterized a dose monitor on the gantry to
ensure good control of the dose, delivered in spot-scanning mode. We
characterized the beam for different dose rates and field sizes for
transmission irradiations. We explored scanning possibilities in order to
enable conformal irradiations or transmission irradiations of large targets
(with transverse scanning). Results: We achieved a transmission of 86 % from
the cyclotron to the treatment room. We reached a peak dose rate of 9000 Gy/s
at 3 mm water equivalent depth, along the central axis of a single pencil beam.
Field sizes of up to 5x5 mm$^{2}$ were achieved for single spot FLASH
irradiations. Fast transverse scanning allowed to cover a field of 16x1.2
cm$^{2}$. With the use of a nozzle-mounted range shifter we are able to span
depths in water ranging from 19.6 to 37.9 cm. Various dose levels were
delivered with a precision within less than 1 %. Conclusions: We have realized
a proton FLASH irradiation setup able to investigate continuously a wide dose
rate spectrum, from 1 to 9000 Gy/s in a single spot irradiation as well as in
the pencil beam scanning mode. As such, we have developed a versatile test
bench for FLASH research.
