The first PET glimpse of a proton FLASH beam.

We demonstrate the first ever recorded PET imaging and dosimetry of a FLASH proton beam at the Proton Center of the MD~Anderson Cancer Center. Two scintillating LYSO crystal arrays, read out by silicon photomultipliers, were configured with a partial field of view of a~cylindrical PMMA phantom irradiated by a FLASH proton beam. The proton beam had a kinetic energy of 75.8\,MeV and an intensity of about $3.5\times10^{10}$ protons that were extracted over 101.5\,ms-long spills. The radiation environment was characterized by CZT and plastic scintillator counters. Preliminary results indicate that the PET technology used in our tests can efficiently record FLASH beam events. The instrument yielded informative and quantitative imaging and dosimetry of beam-activated isotopes in a PMMA phantoms, as supported by Monte Carlo simulations. These studies open a new PET modality that can lead to improved imaging and monitoring of FLASH proton therapy.