Evaluation of Spot-Wise Monitor Unit Delivery Errors In Spot-Scanning Proton Arc Therapy
Abstract
Purpose
To evaluate the robustness of spot-scanning proton arc therapy (SPArc) to stochastic spot-wise monitor unit (MU) delivery errors.
Methods
Four representative clinical cases from distinct disease sites—brain stereotactic radiosurgery (SRS), lung, liver, and prostate—were selected to encompass a range of target sizes and anatomical locations. For each case, a clinically optimized SPArc treatment plan was generated. Spot-wise independent MU delivery errors were simulated using a Gaussian distribution with standard deviations of 1%, 2%, and 3%, representing realistic random delivery uncertainties. Dynamic delivery was simulated by reconstructing machine-specific control-point sequencing. Perturbed dose distributions were recalculated and compared with nominal plans using target coverage and organ-at-risk (OAR) dosimetric metrics. Three-dimensional (3D) gamma analysis was also performed for evaluation.
Results
Across all four disease sites (brain SRS, lung, liver, and prostate) and simulated spot-wise MU error magnitudes of 1–3%, dosimetric deviations relative to nominal SPArc plans remained minimal. The largest target coverage difference (3 cGyE <0.2%) was observed in the brain SRS case under 3% spot-wise MU perturbation, while the lung, liver, and prostate cases demonstrated smaller variations across all error levels. OAR dosimetric metrics represented similarly limited sensitivity across all disease sites, with absolute and relative differences remaining below 1 unit and 0.2%, respectively. 3D gamma analysis demonstrated 100% passing rates for all disease sites and MU error scenarios, indicating excellent agreement between perturbed and nominal dose distributions.
Conclusion
SPArc demonstrates strong robustness to random spot-wise MU delivery errors across diverse anatomical sites. Dosimetric perturbations remain minimal, with complete gamma agreement observed under all tested stochastic conditions. These results characterize the baseline delivery robustness of SPArc with respect to spot-level MU uncertainty and provide quantitative evidence to inform future proton arc therapy quality assurance design.