ESTRO 2025 - Abstract Book

S3520

Physics - Optimisation, algorithms and applications for ion beam treatment planning

ESTRO 2025

3269

Digital Poster Impact of Gantry Position Errors on Spot-Scanning Arc Therapy (SPArc) Peilin Liu 1 , Xiaoda Cong 1 , Lewei zhao 2 , Gang Liu 3 , Xuanfeng Ding 1 1 Radiation Oncology, Corewellhealth William Beaumont University hosptial, royal oak, USA. 2 Radiation Oncology, Stanford University, Palo Alto, USA. 3 Radiation Oncology, Tongji Medical College, Wuhan, China Purpose/Objective: To investigate the impact of gantry position errors (GPE) on dose distribution in Spot-scanning arc therapy (SPArc) plans during dynamic delivery. Material/Methods: Four representative patient cases (brain SRS, lung, liver, and prostate) were selected to evaluate the dosimetric effects of GPE on SPArc plans during dynamic delivery. Initial SPArc plans were generated using an in-house script in RayStation, considering different arc ranges based on target geometry. Dynamic SPArc plans were then reconstructed using timing information from virtual log files to re-sample control points and spot positions, accurately representing the delivery process. GPEs were simulated to assess their impact on treatment delivery, with GPEs involving constant gantry speed deviations (±0.5°, ±1°, ±1.5°, and ±2° per 180° arc). Dosimetric differences between nominal and error-affected plans were evaluated based on target coverage (D98), and 3D gamma-index pass ratios (GPR) were assessed using 3%/3mm and 2%/2mm criteria with a 10% dose threshold. Results: With 3%/3mm gamma criteria, all anatomical sites exhibited GPR exceeding 95%, indicating robustness to moderate GPEs. Liver, lung, and prostate plans maintained GPR above 99%, while brain SRS showed greater sensitivity, with GPR dropping to 95.43% at 2° deviation. Under stricter 2%/2mm criteria, GPR decreased, particularly for brain SRS, which had the lowest GPR (79.84% at 2°). Liver plans consistently showed high robustness, maintaining GPR above 97%, while lung and prostate plans had GPR values exceeding 91% (Table1).