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

Area of Science:

• Medical Physics
• Radiotherapy Physics
• Radiation Detection

Background:

• FLASH radiotherapy utilizes ultra-high dose rates (UHDR) for potential normal tissue sparing and maintained tumor control.
• Proton beams with spread-out Bragg peaks (SOBP) offer favorable depth-dose profiles.
• Accurate quality assurance for FLASH requires time-resolved dosimetry, especially in the SOBP, necessitating calibration for detector responses like quenching.

Purpose of the Study:

• To calibrate and validate a fiber-coupled inorganic scintillator detector system for time-resolved point dosimetry in UHDR proton SOBP beams.
• To enable robust dosimetric and geometric verification for preclinical and in-vivo FLASH studies.

Main Methods:

• Utilized a clinical proton pencil beam scanning (PBS) line with a 2D range modulator to create a 5 cm SOBP.
• Employed ZnSe:O scintillator probes coupled to optical fibers, read out by silicon photomultipliers at 50 kHz, with ionization chambers for reference dose.
• Calibrated for signal-dependent silicon photomultiplier saturation ($k_{\text{sat}}$), absolute dose ($N_{D,w,15}$), and depth-dependent under-response ($k_Q$).

Main Results:

• Observed and corrected for signal saturation (up to 55%) and depth-dependent under-response (up to 12%) in the SOBP.
• Achieved positional stability within ±0.1 mm and dose agreement within 0.5% in the SOBP validation.
• Characterized SOBP beam spot profiles, showing FWHM broadening and dose rate reduction from 800 Gy/s to 280 Gy/s.

Conclusions:

• The developed calibration method enables accurate, time-resolved dosimetry for UHDR proton SOBP beams, separating saturation and quenching effects.
• The scintillator system offers high precision for dose and geometry, suitable for quality assurance in preclinical FLASH studies.
• This approach streamlines recalibration and supports routine monitoring of PBS-delivered proton FLASH treatments.