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

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.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 (-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 ubeam. During the 5 s spill-off in-beam phase, the system detected 511 keV-rays from positron annihilation while suppressing prompt-ray interference. Analysis included 48 in-beam periods of 261.03 MeV ucarbon ions, divided into eight groups, and monitor units (MUs) estimates were based on the PET image peak intensity.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 full width at half maximum values of 20.42 mm (-axis) and 19.76 mm (-axis), with a 45.6 mm peak depth (-axis) consistent with the single-energy results. PET-based MU estimation with a 261.03 MeV ubeam showed decreasing relative error with more beam periods, reaching a stable value around 1% after 12 periods.In-beam PET offers a robust solution for dose monitoring and verification in CIRT, supporting precise dose delivery and better patient outcomes.