Investigate gantry position error effect on dynamic spot-scanning proton arc (SPArc) therapy

Spot-scanning proton arc therapy (SPArc) enables dynamic dose delivery through continuous gantry rotation, offering potential improvements in dose conformity for complex targets. However, dynamic delivery introduces susceptibility to gantry position errors (GPEs), which may degrade the treatment accuracy, particularly in highly precise and conformal techniques. This study aims to quantitatively assess the dosimetric impact of systematic and random GPEs during the dynamic SPArc treatment. Method: Four clinical cases, including brain stereotactic radiosurgery (SRS), lung, liver, and prostate cancer, were evaluated under simulated systematic (±0.5°-2° per 180° arc) and random (0-2° per 180° arc) GPEs. SPArc plans were generated using a published algorithm implemented in RayStation through scripting and simulated the treatment via a validated IBA DynamicARC® system model. Virtual machine logfiles reconstructed control points and spot positions, incorporating GPEs through modified gantry velocities. Target coverage (D98) and gamma passing rates (GPR) under 3%/3mm and 2%/2mm criteria were analyzed. Results: Under the 3%/3 mm criterion, GPR exceeded 95% for all cases; liver, lung, and prostate plans remained >99% across GPE magnitudes. Brain SRS represented greater sensitivity, with GPR dropping to 95.6% at a 2° systematic error. At 2%/2 mm, brain SRS GPR fell to 85.3% at a 2° systematic error due to smaller target volumes, larger arc range and steep dose gradient, while liver plan maintained >95%, and lung and prostate plans remained >92%. Significance: This study provides the first comprehensive quantitative evaluation of dose perturbation caused by GPE in the dynamic SPArc treatment delivery. Systematic GPEs introduce more significant dose deviations compared to random errors, with the impact varying by disease sites and error magnitude. These findings could guide machine quality assurance protocols and site-specific error tolerance thresholds for future clinical implementation. .