2354 - Investigating the Influence of Spot Error on Plan Quality: A Comparative Study of Spot-Scanning Arc Therapy (SPArc) and Intensity-Modulated Proton Therapy (IMPT)

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 impact of spot position errors (PEs) on plan quality in Spot-Scanning Arc Therapy (SPArc) plans compared with Intensity-Modulated Proton Therapy (IMPT). Materials/

Methods: Four representative disease sites, including brain, lung, liver, and prostate cancers, were retrospectively selected with. Clinical IMPT plan and dynamic SPArc plan were used as the original plan (PE_orig). Two types of errors were generated, including randomized error and systematic error. For each random error scenario, they were further examined across four sub-scenarios (25%, 50%, 75%, and 100% of spots affected) at 1 mm and 2 mm deviations. The randomized errors used two categories with and without Gaussian distribution (PE_ran-GS, PE_ran-UN). Systematic errors were simulated by uniformly adjusting all spot positions by 2 mm in varying directions. The Gamma-index Passing Ratio (GPR) was utilized to evaluate dose perturbations relative to nominal plans.
Results: For PE_ran-GS in 1mm scenario, both SPArc and IMPT are comparable with a GPR exceeding 95%. However, for PE_ran-GS in 2mm scenario, SPArc could provide better GPR. As PE_ran-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. In the prostate case, SPArc could provide smaller dose difference for the target coverage (D95: 0.46% vs 1.48%) and rectum (mean dose: 0.41% vs 1.33%) compared to IMPT (table1) with similar differences for other OARs.
Conclusion: SPArc shows better dosimetric robustness to spot PEs than IMPT, suggesting its enhanced capability for achieving precise dose distribution. This study presents the first simulation results and the methodology that serves as a reference to guide future investigations into quality assurance of SPArc treatment delivery. Abstract 2354 – Table 1: Dosimetric outcome comparison of nominal plan and spot error plan for IMPT and SPArc plan in 2mm PE_sys scenario (Prostate case)

Dosimetric parameter	IMPT	SPArc
?D98 of target(absolute/relative) (cGy/%)	93.00(1.33)	75.00(1.09)
?Dmean_Bladder (absolute/relative) (cGy/%)	105.00(7.07)	6.00(0.61)
?Dmean_Rectum (absolute/relative) (cGy/%)	343.00(17.50)	16.00(1.08)