Evidence indicates that ultra-high dose rate (UHDR) irradiation in radiotherapy can induce a normal tissue sparing effect without compromising effectiveness against tumour cells, known as the FLASH effect. This has prompted active research into clinical proton FLASH therapy. A key step is developing a clinically safe and predictable proton FLASH beam, which is being pursued through the commissioning of the ProBeam gantry in FLASH mode at HollandPTC. As part of this process, this thesis aims to characterise the gantry-based 250 MeV ultra-high dose rate continuous scanning proton beam, currently only intended for preclinical research.

Four distinct dosimetric aspects have been investigated, in accordance with the AAPM TG-224 report and machine quality assurance guidelines for UHDR proton beams in transmission mode. The first three, which are also part of conventional characterisations, include lateral and longitudinal relative dosimetry, along with absolute dosimetry measurements. These encompass the spot shape, spot position, integral depth dose (IDD) curve and output measurements. The fourth category includes temporal dosimetry, an essential aspect for FLASH characterisations since delivery time has now become an important aspect. For this, the scanning speed, spot dwell time and dose rate (constancy) have been determined. The temporal measurements were conducted using the FlashQ detector, a 2D strip ionisation chamber with a temporal resolution of 1 ms, which also served as a reference monitor chamber to enable the full spatiotemporal reconstruction of each irradiation. Measurements were conducted at nominal nozzle currents from 2 to 215 nA to explore potential correlations between the measured parameters and nozzle current. The suitability of all other detectors used in this work for FLASH measurements has also been assessed.

The spot shape did not show a clinically significant dependency on the nozzle current, and the average Gaussian parameters were determined to be σx = 3.39 ± 0.06 mm, σy = 3.86 ± 0.02 mm and θ = -16.6 ± 4.5 degrees. Spot position accuracy was within 0.15 mm, complying with AAPM TG-224 standards. The R80-value from the integral depth dose (IDD) measured 37.9 ± 0.1 cm, aligning with IDD results from other ProBeam facilities. Absolute dose per monitor unit (MU) varied significantly with nozzle current, from 0.0041 Gy/MU at 10 nA to 0.0466 Gy/MU at 215 nA. In terms of temporal aspects, the gantry scanning speed was found to depend on the spot spacing but converged to 7.8 ± 0.1 m/s and 29.4 ± 0.9 m/s in the x- and y-directions respectively for large spot spacings (> 50 mm). The nozzle monitor chamber reached saturation at a nominal nozzle current of 18.4 nA, resulting in fixed spot dwell times. This saturation caused significant dose rate fluctuations, both day-to-day and beam-to-beam. Over six measurement sessions in a four-month period, deviations ranged from -13.9% to -23.2% compared to planned dose rates, with an average intraday fluctuation of 3.9%. These fluctuations were measured with the FlashQ, which has been verified as a suitable reference detector.

Ultimately, the gantry-based 250 MeV UHDR continuous scanning proton beam has been successfully characterised at HollandPTC. All conventional parameters met the AAPM TG-224 standards or aligned with findings from other ProBeam institutes. Using the FlashQ as a reference monitor chamber enables the reconstruction and simulation of dose delivery in both space and time through calibration.