Time-resolved point dosimetry for spread-out Bragg-peak proton FLASH using fibre-coupled scintillators

In FLASH radiotherapy, the dose is delivered using ultra-high dose rates (UHDR), which are approximately 100 times higher than those used for conventional (CONV) treatments. This has shown promise in sparing normal tissue while maintaining tumour control. Proton beams, particularly in the spread-out Bragg peak (SOBP), offer a favourable depth-dose profile for sparing healthy tissue. However, quality assurance for FLASH requires time-resolved dosimetry to capture the temporal structure of pencil beam scanning delivery. Fibre-coupled scintillating detectors have been used both in electron and photon UHDR beams, and have been applied in the proton beam entrance plateau. Extending their use to the SOBP requires careful calibration to address quenching and water-inequivalent response near the Bragg peak.

To calibrate and validate a fibre-coupled inorganic scintillator detector system for accurate, time-resolved point dosimetry in the SOBP for UHDR proton beams, which will enable preclinical and in-vivo FLASH studies with robust dosimetric and geometric verification.

Experiments were conducted using a clinical proton PBS beam line. A 2D range modulator generated a 5 cm SOBP from a mono-energetic beam. Four ZnSe:O scintillator probes coupled to optical fibres were read out by silicon photomultipliers at 50 kHz. An ionisation chamber provided reference dose measurements. The calibration included determining a signal dependent saturation factor of the silicon photomultiplier $k_{\text{sat}}$ , measuring the absolute calibration factor $N_{D,w,15}$ , and characterising the correction for the depth-dependent under-response $k_Q$ . A calibration validation was performed in the SOBP across a range of UHDR beam currents, evaluating both dosimetric accuracy and probe positional stability. The calibrated system was then used to characterise SOBP beam spot profiles, in terms of full width at half-maximum and dose rate variation with depth.

A saturation multiplier of up to 55% was observed across all four probes. The depth-dependent under-response reached up to 12% at the distal SOBP edge. Both effects were successfully corrected for through fitting simple functions. The validation in the SOBP demonstrated that the calibration achieved positional stability within $\pm$ 0.1 mm and agreement between the measured and absolute doses within 0.5% for all probes. Beam characterisation revealed full-width at half-maximum broadening from 8.3 mm at shallow depth to 21.5 mm near the range end, with spot profiles comprising two Gaussian cores and a Lorentzian tail. The maximum instantaneous dose rate in the UHDR beam fell from 800 Gy/s in the entrance plateau to 280 Gy/s in the SOBP.

The developed calibration method enables accurate, time-resolved dosimetry in UHDR proton SOBP beams, allowing for the separation of saturation and quenching corrections. The fibre-coupled scintillator system demonstrated high precision in both dose and geometry, making it suitable for quality assurance in preclinical FLASH studies. This approach streamlines recalibration, reducing beam time requirements, and supports routine monitoring of PBS-delivered proton FLASH treatments in complex depth-dose scenarios.