2156 - Quantitative Analysis of Dose Perturbations from Systematic and Random Gantry Rotational Errors in Dynamic Spot-Scanning Proton Arc Therapy (SPArc) Delivery
Presenter(s)
P. Liu1, X. Cong1, L. Zhao2, G. Liu3, X. Xu1, and X. Ding1; 1Department of Radiation Oncology, Corewell Health William Beaumont University Hospital, Royal Oak, MI, 2Department of Radiation Oncology,Stanford University, Palo Alto, CA, 3Cancer Center, Union Hospital, Tongji Medical College, Huazhong University of Science and Technology, Wuhan, China
Purpose/Objective(s): 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 plan quality, particularly in highly precise treatments. This project aims to quantitatively assess the dosimetric impact of systematic and random GPEs during dynamic Spot-Scanning Proton Arc Therapy (SPArc) treatment.
Materials/Methods: Four clinical cases, 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 a treatment planning system 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%/3mm gamma criterion, GPR remained above 95% across all cases, indicating excellent robustness of SPArc against minor gantry errors. Liver, lung, and prostate treatments were the most robust with GPR exceeding 99% across all GPE magnitudes. Brain SRS, however, demonstrated higher sensitivity to systematic errors due to smaller target volumes and steep dose gradients, with GPR dropping to 95.43% at a 2° systematic error. Under the stricter 2%/2mm criterion, the impact of GPEs became more pronounced. The brain SRS case showed the largest drop in GPR, decreasing to 79.84% (systematic) and 90.63% (random) at a 2° error, highlighting the increased vulnerability of high-gradient dose distributions. In contrast, the liver case remained the most robust, consistently maintaining GPR above 97%. Lung and prostate cases showed moderate sensitivity, with GPR values slightly above 91% at 2° systematic error.
Conclusion: This study provides the first comprehensive quantitative evaluation of dose perturbation caused by GPE in SPArc delivery. Systematic GPEs introduce more significant dose deviations compared to random errors, with the impact varying by disease sites and error magnitude. These findings guide machine QA protocols and site-specific error tolerance thresholds for future SPArc implementation.