Motion mitigation in positron-emission tomography guided radiotherapy delivered on a magnetic resonance imaging-linear accelerator

Background and Purpose: The precision of Positron Emission Tomography (PET)-guided radiotherapy may be compromised by motion. To address this, we demonstrated the feasibility of a motion-robust PET-to-magnetic resonance linear accelerator (MR-linac) pipeline. Materials and Methods: For comparison, PET images of a phantom with a target in a movable insert were acquired under a patient-derived breathing waveform and static conditions. Motion correction using the PET Maximum-Likelihood Motion and Activity (MLMA) reconstruction was compared with static reference and motion-blurred images. Targets were delineated using thresholding based on maximum standardized uptake value, and Intensity-Modulated Radiation Therapy (IMRT) plans were generated. Plans were delivered on an MR-linac with and without gating or Multi Leaf Collimator (MLC) tracking. Dosimetry was assessed using film measurements, evaluating dose differences and 3%/3 mm gamma pass rates. The pipeline was further evaluated using clinical PET data, delivered with and without gating and tracking after motion correction. Dosimetric evaluation was performed using a cylindrical multi-diode phantom. Results: Motion-blurred PET produced larger gross target volumes (GTV) (GTV: 58 cm3, GTV_boost: 14 cm3) than motion-corrected PET (GTV: 52 cm3, GTV_boost: 8 cm3), the latter matching the static reference (GTV: 52 cm3, GTV_boost: 9 cm3). Motion correction reduced dose discrepancies and improved gamma pass rates (72%), with further gains from gating (90.3%) and MLC tracking (98.3%). The measurements of patient simulations showed higher gamma pass rates for the motion-corrected plan (static-delivery: 81.8%, gating: 99.2%, MLC-tracked: 100%) compared to motion-blurred plan (static-delivery: 49.4%, gating: 65.6%, MLC-tracked: 63.6%). Conclusions: A motion-robust PET-to-MR-linac pipeline using motion-corrected PET imaging and MR-guided motion mitigation is technically feasible and enhances dose precision.