ESTRO 2025 - Abstract Book

S3200

Physics - Intra-fraction motion management and real-time adaptive radiotherapy

ESTRO 2025

across patients immobilised with either closed or open-face masks, using both cone-beam computed tomography (CBCT) and surface-guided radiation therapy (SGRT).

Material/Methods: Before full trial recruitment, a secondary objective analysis was conducted on the first 56 patients enrolled in the OPEN trial as a safety checkpoint for intrafraction motion. These patients were randomised into three arms: closed facemask, 3-point open facemask, or 5-point open facemask. Intrafraction motion was assessed using CBCT and SGRT. CBCT provided deviations in translational and rotational dimensions based on bony alignment, while SGRT offered continuous monitoring of surface motion. Intrafraction motion metrics—including the average, standard deviation, absolute maximum, and the 95th percentile of recorded surface motion during treatment—were recorded for each patient using the continuous SGRT data to fully quantify motion variation during treatment. The 95th percentile of deviations was used for direct comparison with CBCT motion data. Bayesian analysis was conducted to determine the equivalence of motion across mask types and measurement techniques. Results: Mean CBCT deviations were less than 0.4 mm and 0.2 degrees, while SGRT recorded 95th percentile deviations of 0.39 mm and 0.78 degrees. SGRT detected transient maximum deviations not captured by CBCT, particularly in the yaw axis. However, these differences were transient. Bayesian analysis showed that the likely differences in intrafraction deviations between mask types or measurement methods were not clinically significant. No correlation was found between SGRT and CBCT measured motion within the small range of motion recorded. Based on CBCT measured intrafraction motion, margins of 1.8 mm, 1.7 mm, and 1.3 mm were calculated for the vertical (VRT), lateral (LAT), and longitudinal (LNG) directions, respectively, to account for intrafraction motion for all mask types, with SGRT confirming that patient motion during treatment remained within these margins.