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

Objective.In-beam positron emission tomography (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 lutetium-yttrium oxyorthosilicate (LYSO) scintillators and silicon photomultiplier (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-maximum likelihood expectation-maximization 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^-1) and multi-energy beams (178.87, 190.19, 201.03 MeV u^-1), and (2) a 2 × 2 module for 20 mm × 20 mm field measurements using a 151.32 MeV u^-1beam. During the 5 s spill-off in-beam phase, the system detected 511 keVγ-rays from positron annihilation while suppressing promptγ-ray interference. Analysis included 48 in-beam periods of 261.03 MeV u^-1carbon ions, divided into eight groups, and monitor units (MUs) 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^-1) to 132.0 mm (280.56 MeV u^-1). For multi-energy beams, three distinct peaks corresponded to different energies (178.87, 190.19, and 201.03 MeV u^-1). The 20 mm × 20 mm field measurements showed full width at half maximum 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^-1beam 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.