Purpose: Respiratory motion remains a major challenge in scanning proton therapy for liver cancer because interplay between pencil beam delivery and patient-specific breathing degrades dose distributions.Phantombased studies cannot fully represent clinical respiratory complexity.This study aimed to determine clinically relevant residual motion thresholds for respiratory-gated scanning proton therapy using four-dimensional dynamic dose (4DDD) analysis integrating 4D computed tomography (4DCT), respiratory waveforms, and accelerator time structure.Methods: Twelve liver cancer patients were retrospectively analyzed.Nominal plans were generated with gating windows of 10%-100% for two dose prescriptions (3.3 and 6.6 GyRBE per fraction) and two rescanning settings (four and eight rescans) using 4D robust optimization.A custom TPS-based tool calculated 4DDD by assigning individual spots to 4DCT phases based on recorded respiratory waveforms, accounting for variations in starting phase, breathing cycle length, and accelerator spill timing.Residual motion, beam delivery time, and CTV D98% (difference between 4DDD and nominal D98%) were evaluated.Motion thresholds were derived using quantile regression with a clinical criterion of mean CTV D98% 2%.Results: Beam delivery time and CTV D98% showed a trade-off with increasing residual motion.Increasing rescans reduced D98% without prolonging delivery time.With eight rescans, residual motion of approximately 6 mm for 3.3 GyRBE/fraction and 7 mm for 6.6 GyRBE/fraction maintained the 5th percentile of CTV D98% within 2%.Conclusion: 4DDD analysis integrating patient-specific respiratory data and accelerator timing enables derivation of clinically relevant motion thresholds, providing practical guidance for optimizing gating windows and rescanning strategies in scanning proton therapy.
Wakisaka et al. (Mon,) studied this question.