Radiation Chemistry–Based Characterization of Proton Beam Quality Using Hydroxyl Selective Fluorescence and Microscopic Monte Carlo Modeling
Abstract
Purpose
Proton beam quality varies with depth in both monoenergetic and spread-out Bragg peak (SOBP) beams due to changes in track structure and local energy deposition. This study investigates hydroxyl radical-sensitive fluorescence as an intrinsic method to quantify depth-dependent beam-quality variations and employs microscopic Monte Carlo (MC) track-structure simulations to mechanistically interpret the observed chemical response.
Methods
Hydroxyphenyl fluorescein (HPF) was used as a chemical probe to quantify hydroxyl radical production at multiple depth locations in 142.4 MeV monoenergetic proton beams and a 10-cm modulated SOBP with a nominal range of 20 cm. HPF is initially non-fluorescent and becomes fluorescent upon selective oxidation by hydroxyl radicals. Fluorescence response was quantified using a background-normalized relative fluorescence unit (RFU). Dose-response measurements were performed with varying HPF concentrations at different depths along the proton beams. Microscopic MC track-structure simulations were conducted at corresponding depths to model relative changes in HPF response.
Results
RFU increased linearly with absorbed dose at both entrance and distal-edge positions for both 3 μM and 10 μM HPF solutions. At 2 Gy, RFU decreased by 78% and 88% when comparing distal-edge to entrance regions for monoenergetic and SOBP beams, respectively, indicating reduced detectable hydroxyl radical yield in regions of increased radiation quality. Within the SOBP, RFU varied by only around 9% at positions 2 cm from the midpoint, suggesting relatively uniform chemical response across the central SOBP region. Microscopic MC simulations predicted an ~80% reduction in RFU between distal-edge and entrance positions for monoenergetic beams, in quantitative agreement with experimental measurements.
Conclusion
These results demonstrate that radiation chemistry can serve as a method to visualize and quantify proton beam-quality variations in both monoenergetic and SOBP configurations. This chemistry-based approach, combined with microscopic modeling, provides an alternative framework for characterizing therapeutic proton beams beyond conventional physical dosimetric metrics.