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 ultra-high 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 5 × 5 mm 2 were achieved for single-spot FLASH irradiations. Fast transverse scanning allowed to cover a field of 16 × 1.2 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 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 single-spot irradiation as well as in the pencil beam scanning mode. As such, we have developed a versatile test bench for FLASH research.

Recently, proton therapy treatments delivered with ultra-high dose rates have 19 been of high scientific interest, and the Faraday cup is a promising dosimetry 20 tool for such experiments. Different institutes use different Faraday cup designs, 21 and either a high voltage guard ring, or the combination of an electric and a 22 magnetic field is employed to minimize the effect of secondary electrons. The 23 authors first investigate these different approaches for beam energies of 70 MeV, 24 150 MeV, 230 MeV and 250 MeV, magnetic fields between 0 mT and 24 mT 25and voltages between -1000V to 1000V. When applying a magnetic field, the 26 measured signal is independent of the guard ring voltage, indicating that this 27 setting minimizes the effect of secondary electrons on the reading of the Faraday 28 cup. Without magnetic field, applying the negative voltage however decreases 29 the signal by an energy dependent factor up to 1.3% for the lowest energy tested 30 and 0.4% for the highest energy, showing an energy dependent response. Next, 31 the study demonstrates the application of the Faraday cup up to ultra-high dose 32 rates. Faraday cup measurements with cyclotron currents up to 800nA (dose 33 rates of up to approximately 1000 Gy/s) show that the Faraday cup is indeed 34 dose rate independent. Then, the Faraday cup is applied to commission the 35 primary gantry monitor for high dose rates. Finally, short-term reproducibility 36 of the monitor calibration is quantified within single days, showing a standard 37 deviation of 0.1% (one sigma). In conclusion, the Faraday cup is a promising, 38 dose rate independent tool for dosimetry up to ultra-high dose rates. Caution is 39 however necessary when using a Faraday cup without magnetic field, as a guard 40 ring with high voltage alone can introduce an energy dependent signal offset.

The response of Al2O3:C optically stimulated luminescence detectors (OSLDs) was investigated in a 250 MeV pencil proton beam. The OSLD response was mapped for a wide range of average dose rates up to 9000 Gy s−1, corresponding to a ∼150 kGy s−1 instantaneous dose rate in each pulse. Two setups for ultra-high dose rate (FLASH) experiments are presented, which enable OSLDs or biological samples to be irradiated in either water-filled vials or cylinders. The OSLDs were found to be dose rate independent for all dose rates, with an average deviation <1% relative to the nominal dose for average dose rates of (1–1000) Gy s−1 when irradiated in the two setups. A third setup for irradiations in a 9000 Gy s−1 pencil beam is presented, where OSLDs are distributed in a 3 × 4 grid. Calculations of the signal averaging of the beam over the OSLDs were in agreement with the measured response at 9000 Gy s−1. Furthermore, a new method was presented to extract the beam spot size of narrow pencil beams, which is in agreement within a standard deviation with results derived from radiochromic films. The Al2O3:C OSLDs were found applicable to support radiobiological experiments in proton beams at ultra-high dose rates.

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