Author: Michael Caulfield, Ye Chen, Yoshihiro Ishi, Yasutoshi Kuriyama, Taeko Matsuura, Naoki Miyamoto, Taichi Murakami, Tomonori Uesugi, Kohei Yokokawa 👨🔬
Affiliation: Hokkaido University, Kyoto University, University of Alberta 🌍
Purpose: Protoacoustics has the potential for real-time and direct imaging of the Bragg peak. This study investigated the accuracy of the Bragg peak imaging based on the water phantom experiment at a clinical dose.
Methods: A 100-MeV proton pencil beam emitted from a fixed-field alternating gradient accelerator was irradiated onto (A) a homogeneous water phantom and (B) a water phantom with a half-sized or (C) full-sized silicone plate downstream of the Bragg peak (BP) or (D) a bone plate that covered half of the beam cross-section in the beam path. The BP dose per pulse was 0.240.34 Gy in homogeneous water. An optical hydrophone was scanned laterally to the beam axis downstream of the BP over a length of 70 mm at a pitch of 5 mm and the protoacoustic waveforms were recorded. After deconvolving the waveforms to remove the detector echoes, the dose distribution was reconstructed using a modified Delay and Sum method. The beam range, lateral position, and size were compared with the Geant4 Monte Carlo simulations, and their precision was evaluated by the standard deviations over 1000 images.
Results: The beam ranges were successfully localized with a difference of 0.13±0.03, 0.93±0.03 mm, +0.07±0.02 mm, and 0.26±0.02 mm for geometries (A-D), respectively. The lateral positions were localized to -0.84±0.26 mm and -0.85±0.07 mm for geometries (A) and (B), while the accuracy was slightly reduced to -2.34±0.17 mm and -2.45±0.19 mm for geometries (C) and (D), respectively. The deviations of the beam sizes (FWHM) at the Bragg peak were 4.02±0.38 mm, 3.50±0.44, -2.33±0.17 mm for geometries (A-C), which is less than 20% of the beam size derived by Monte Carlo simulations (19.87 mm).
Conclusion: This work demonstrates the accurate reconstruction of the BP in water phantoms using protoacoustic waves recorded by an optical hydrophone at a clinical dose.