The first investigation of the dosimetric perturbations from the spot position errors in spot-scanning arc therapy (SPArc)

To quantitatively investigate the impact of spot position error (PE) on the dose distribution in (Spot-scanning arc therapy) SPArc plans compared to Intensity-Modulated Proton Therapy (IMPT). Approach: Four representative disease sites, including brain, lung, liver, and prostate cancers, were retrospectively selected. Spot PEs were simulated during dynamic SPArc treatment delivery. Two types of errors were generated, including random error and systematic error. Two different probability distributions of random errors were used (1) Gaussian distribution (PEran-GS) (2) uniform distribution (PEran-UN). In PEran-UN, four sub-scenarios were considered: 25%, 50%, 75%, and 100% spots were randomly selected to in various directions in the scale of 0-1 mm or 0-2mm of PE. Additionally, systematic error was simulated by shifting all the spot uniformly by 1 or 2mm in various directions (PEsys). Gamma Passing Rate (GPR) is applied to assess the dosimetric perturbation. Main results: For PEran-GS in 1mm scenario, both SPArc and IMPT are comparable with a GPR exceeding 95%. However, for PEran-GS in 2mm scenario, SPArc could provide better GPR. As PEran-UN of 2mm, SPArc plans have a much better GPR compared to IMPT plans: SPArc's GPR is 99.40±0.74%, 93.66±4.75% and 61.53±10.30% for 3mm/3%, 2mm/2% and 1mm/1% criteria compared to IMPT with 98.18±2.11%, 86.12±5.58% and 39.74±7.71%. Besides, SPArc has shown its advantage in dosimetric sparing and robustness in the Organ-at-risks. For example, the brain case of 2mm PEsys, SPArc significantly reduced the dosimetric perturbation in the maximum dose to the brainstem (109cGy) and the mean dose to the left cochlea (32cGy) compared to IMPT (296cGy, 146cGy). Significance: Compared to IMPT, SPArc shows better dosimetric robustness in spot PEs. This study presents the first simulation results and the methodology that serves as a reference to guide future investigations into the accuracy and quality assurance of SPArc treatment delivery&#xD.