Spot Scanning Proton Beam Characterization By Plastic Scintillator Detector
Abstract
Purpose
To investigate the response of a plastic scintillator dosimeter in quantifying depth-dose distributions and beam spot characteristics in a clinical proton therapy setting, using both Monte Carlo modeling and experimental comparison.
Methods
Spot scanning beams of 100 and 200 MeV were delivered to a water phantom equipped with a plastic scintillator dosimeter. Depth doses and lateral spot profiles were measured at millimeter and sub-milimeter intervals respectively using the PSD aided with motion control software and a time-resolved acquisition system. Identical beams and detector geometry, as well as scanning protocols, were modeled in TOPAS Monte Carlo model. The simulation scored dose in water, dose in scintillator and proton linear energy transfer in the scintillator across all measurement positions. LET-dependent ionization quenching was characterized using Birks’ law, enabling correction factors to be applied to the experimental data for direct comparison with simulation results.
Results
For 100 MeV protons, the experimental and simulation both showed the Bragg peak at approximately 7.5 cm, with sharp dose falloff beyond the peak. LET values increased steeply up to 17.5 MeV/mm/(g·cm⁻³) at the distal edge, correlating with the region of greatest quenching correction. For 200 MeV, the measured Bragg peak was at 26.4 cm, again closely matching simulation i.e. 25.6 cm, and dose falloff was similarly steep. Although LET values peaked lower at 12 MeV/mm/(g·cm⁻³) for 200 MeV, the trend of increasing LET at the distal edge persisted. Across both energies, agreement between measured and simulated PDDs was high following correction, with the largest discrepancies confined to high-LET regions at the distal edge.
Conclusion
Plastic scintillator dosimeters, when combined with LET-based quenching corrections, accurately reproduce percent depth dose distributions for spot scanning clinical proton beams across different energies. This approach enables reliable and precise dosimetric verification for both shallow and deep-seated Bragg peaks in proton therapy.