Abstract
Abstract
Objective: In-beam positron emission tomography (PET) is a promising technique for monitoring beam range and activity distribution in carbon-ion radiotherapy. However, spread-out Bragg peak (SOBP) delivery introduces a challenge because sequentially delivered energy layers experience different decay times in in-beam and off-beam PET acquisitions. This study developed a decay-time correction method to reduce temporal bias in measured PET activity distributions under SOBP irradiation.
Approach: A dual-panel LYSO-SiPM PET system in a 4 × 1 module arrangement, with an effective detection area of 21.28 × 5.32 cm², was used to acquire in-beam and off-beam PET images during homogeneous solid-water phantom irradiation. The beam was delivered in scanning mode using sequential high-to-low energy layers, with up to 11 layers ranging from 216.50 to 263.23 MeV/u. Four irradiation schemes were evaluated: physical-dose, RBE-weighted dose (RWD), full-opposing-field RWD, and half-opposing-field RWD SOBP. A Monte Carlo-based decay-time correction framework was established by incorporating beam delivery timing, radionuclide decay characteristics, and activity accumulation. Corrected PET images were compared with Monte Carlo-simulated activity distributions generated using a platform consistent with the experimental setup.
Main Results: Sequential energy-layer delivery and radionuclide decay affected measured PET activity distributions under SOBP irradiation. After correction, experimental PET profiles showed improved agreement with corresponding Monte Carlo profiles across all four SOBP schemes. Depending on the irradiation scheme and acquisition mode, the correction reduced root-mean-square error by approximately 7.7%–53.7%, reduced SOBP-region mean absolute deviation by approximately 20.8%–71.4%, and increased the Pearson correlation coefficient from 0.860–0.973 before correction to 0.941–0.978 after correction.
Significance: This study addressed temporal bias in PET activity distributions caused by decay-time differences among sequentially delivered energy layers during carbon-ion SOBP irradiation. The proposed correction improved the reliability and interpretability of dual-panel in-beam PET under sequential multi-energy carbon-ion irradiation, extending planar PET monitoring toward more treatment-relevant verification scenarios.