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.

No abstract

No abstract

No abstract

The objective of this study was to improve the precision of linear energy transfer (LET) measurements using $$\text {Al}_2\text {O}_3\text {:C}$$ Al 2 O 3 :C optically stimulated luminescence detectors (OSLDs) in proton beams, and, with that, improve OSL dosimetry by correcting the readout for the LET-dependent ionization quenching. The OSLDs were irradiated in spot-scanning proton beams at different doses for fluence-averaged LET values in the (0.4–6.5) $$\hbox {keV}\, \upmu \hbox {m}^{-1}$$ keV μ m - 1 range (in water). A commercial automated OSL reader with a built-in beta source was used for the readouts, which enabled a reference irradiation and readout of each OSLD to establish individual corrections. Pulsed OSL was used to separately measure the blue (F-center) and UV ($$F^+$$ F + -center) emission bands of $$\text {Al}_2\text {O}_3\text {:C}$$ Al 2 O 3 :C and the ratio between them (UV/blue signal) was used for the LET measurements. The average deviation between the simulated and measured LET values along the central beam axis amounts to 5.5% if both the dose and LET are varied, but the average deviation is reduced to 3.5% if the OSLDs are irradiated with the same doses. With the measurement procedure and automated equipment used here, the variation in the signals used for LET estimates and quenching-corrections is reduced from 0.9 to 0.6%. The quenching-corrected OSLD doses are in agreement with ionization chamber measurements within the uncertainties. The automated OSLD corrections are demonstrated to improve the LET estimates and the ionization quenching-corrections in proton dosimetry for a clinically relevant energy range up to 230 MeV. It is also for the first time demonstrated how the LET can be estimated for different doses.

Purpose Energy changes in pencil beam scanning proton therapy can be a limiting factor in delivery time, hence, limiting patient throughput and the effectiveness of motion mitigation techniques requiring fast irradiation. In this study, we investigate the feasibility of performing fast and continuous energy modulation within the momentum acceptance of a clinical beamline for proton therapy. Methods The alternative use of a local beam degrader at the gantry coupling point has been compared with a more common upstream regulation. Focusing on clinically relevant parameters, a complete beam properties characterization has been carried out. In particular, the acquired empirical data allowed to model and parametrize the errors in range and beam current to deliver clinical treatment plans. Results For both options, the local and upstream degrader, depth‐dose curves measured in water for off‐momentum beams were only marginally distorted (γ(1%, 1 mm) > 90%) and the errors in the spot position were within the clinical tolerance, even though increasing at the boundaries of the investigated scan range. The impact on the beam size was limited for the upstream degrader, while dedicated strategies could be required to tackle the beam broadening through the local degrader. Range correction models were investigated for the upstream regulation. The impaired beam transport required a dedicated strategy for fine range control and compensation of beam intensity losses. Our current parameterization based on empirical data allowed energy modulation within acceptance with range errors (median 0.05 mm) and transmission (median –14%) compatible with clinical operation and remarkably low average 27 ms dead time for small energy changes. The technique, tested for the delivery of a skull glioma treatment, resulted in high gamma pass rates at 1%, 1 mm compared to conventional deliveries in experimental measurements with about 45% reduction of the energy switching time when regulation could be performed within acceptance. Conclusions Fast energy modulation within beamline acceptance has potential for clinical applications and, when realized with an upstream degrader, does not require modification in the beamline hardware, therefore, being potentially applicable in any running facility. Centers with slow energy switching time can particularly profit from such a technique for reducing dead time during treatment delivery.