Characterisation of the UK high energy proton research beamline for high and ultra-high dose rate (FLASH) irradiation.

Objective . This work sets out the capabilities of the high energy proton research beamline developed in the Christie proton therapy centre for Ultra-High Dose Rate (UHDR) irradiation and FLASH experiments. It also characterises the lower limits of UHDR operation for this Pencil Beam Scanning (PBS) proton hardware. Approach . Energy dependent nozzle transmission was measured using a Faraday Cup beam collector. Spot size was measured at the reference plane using a 2D scintillation detector. Integrated depth doses (IDDs) were measured. EBT3 Gafchromic film was used to compare UHDR and conventional dose rate spots. Our beam monitor calibration methodolgy for UHDR is described. A microDiamond detector was used to determine dose rates at z _ref . Instantaneous depth dose rates were calculated for 70-245 MeV. PBS dose rate distributions were calculated using Folkerts and Van der Water definitions. Main results . Transmission of 7.05 ± 0.1% is achieveable corresponding to a peak instantaneous dose rate of 112.7 Gy s ^-1 . Beam parameters are comparable in conventional and UHDR mode with a spot size of σ _x = 4.6 mm, σ _y = 6.6 mm. Dead time in the beam monitoring electonics warrants a beam current dependent MU correction in the present configuration. Fast beam scanning of 26.4 m s ^-1 (X) and 12.1 m s ^-1 (Y) allows PBS dose rates of the order tens of Grays per second. Significance . UHDR delivery is possible for small field sizes and high energies enabling research into the FLASH effect with PBS protons at our facility. To our knowledge this is also the first thorough characterisation of UHDR irradiation using the hardware of this clinical accelerator at energies less than 250 MeV. The data set out in this publication can be used for designing experiments at this UK research facility and inform the possible future clinical translation of UHDR PBS proton therapy.
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