Feasibility study of LYSO-SiPM in-beam TOF-PET for dose monitoring in carbon-ion radiotherapy

Objective: In-beam PET is a promising approach for dose monitoring in carbon-ion radiotherapy (CIRT). This study evaluated the feasibility of a dual-head PET system using LYSO scintillators and SiPM detectors incorporating time-of-flight(TOF) technology to monitor carbon-ion beam dose distribution. Approach: The dual-head PET system includes four modules per head, each comprising a 12 × 12 array of LYSO crystals (4.14 mm × 4.14 mm × 20 mm) coupled with SiPM detectors. TOF information was integrated into image reconstruction using a TOF-MLEM algorithm to enhance spatial resolution and reduce noise, especially along the panel-to-panel (X-axis) direction. Two configurations were used: (1) a 4 × 1 module for single-energy beams (120.12–280.56 MeV/u) and multi-energy beams (178.87, 190.19, 201.03 MeV/u), and (2) a 2 × 2 module for 20 mm × 20 mm field measurements using a 151.32 MeV/u beam. During the 5-second spill-off in-beam phase, the system detected 511 keV γ-rays from positron annihilation while suppressing prompt γ-ray interference. Analysis included 48 in-beam periods of 261.03 MeV/u carbon ions, divided into eight groups, and Monitor Units (MU) estimates were based on the PET image peak intensity. Main Results: In-beam PET imaging showed peak depth increased with beam energy, from 29.9 mm (120.12 MeV/u) to 132.0 mm (280.56 MeV/u). For multi-energy beams, three distinct peaks corresponded to different energies (178.87, 190.19, and 201.03 MeV/u). The 20 mm × 20 mm field measurements showed FWHM) values of 20.42 mm (X-axis) and 19.76 mm (Y-axis), with a 45.6 mm peak depth (Z-axis) consistent with the single-energy results. PET-based MU estimation with a 261.03 MeV/u beam showed decreasing relative error with more beam periods, reaching a stable value around 1% after 12 periods.. Significance: In-beam PET offers a robust solution for dose monitoring and verification in CIRT, supporting precise dose delivery and better patient outcomes.