2353 - A First Quantitative Evaluation of Plan Quality Effects from Gantry Rotational Error on Spot-Scanning Arc Therapy (SPArc)

P. M. Liu¹, L. Zhao², G. Liu³, X. Cong¹, X. Li¹, 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): To quantitatively assess the plan quality impact from the random gantry rotational error during the treatment delivery of spot-scanning arc (SPArc) therapy. Materials/

Methods: To simulate random gantry rotational error and its associated dosimetric perturbations, the gantry angle of each control point from the nominal SPArc treatment plans was modified using an in-house script. Twelve cases from different disease sites, including brain, lung, liver, and prostate, were selected in this study. The original SPArc plans (SPArc_orig) were developed through iterative processes, including control point re-sampling, redistribution of control points & energy layers, energy layer filtration, and re-sampling. Considering unpredictable limitations, rotation errors of 1deg and 2deg were randomly applied to all SPArc_orig nominal plans to generate the perturbed dose, simulating the random gantry delivery error during the dynamic treatment. Two kinds of random error were introduced in the gantry error with (SPArc_GS) and without Gaussian distribution (SPArc_R). The effects on SPArc plans were evaluated using dosimetric parameters and the gamma-index passing ratio (GPR).
Results: GPR results are presented in Table 1. For randomized rotational error of 1deg and 2deg, SPArc plans demonstrated GPRs exceeding 96% for both 3%/3mm and 2%/2mm measurement criteria. Remarkably, SPArc plans maintained a GPR of 93.01%±6.12% even under the strictest measurement criteria of 1%/1mm for 1deg error. For rotation errors of 1deg and 2deg with Gaussian distribution, SPArc plans achieved acceptable outcomes, with GPRs of 99.99%±0.02%/99.97%±0.05% and 99.78%±0.33%/98.91%± 0.73% for 1mm/1%, 2mm/2% criteria, respectively. Besides, the dose coverage differences of target between SPArc_orig and SPArc_GS1mm/2mm are 0.78%±0.83%, 3.2%±1.4%.
Conclusion: This is the first quantitative study investigating the robustness of SPArc therapy treatment against gantry rotational error. The data provides a valuable reference for future investigation of quality assurance and measurement in the clinical implementation of SPArc therapy. Abstract 2353 – Table 1: GPR results between original SPArc plan and SPArc with gantry rotational error

Measurement criteria	SPArcR1deg GPR (%)	SPArcR2degGPR (%)	SPArcGS1deg GPR (%)	SPArcGS2deg GPR (%)
3mm/3%	99.99±0.02	99.94±0.07	99.99±0.02	99.97±0.05
2mm/2%	99.67±0.42	98.02±1.32	99.78±0.33	98.91±0.73
1mm/1%	93.01±6.12	75.39±12.80	95.25±4.33	82.06±11.29