S2953
RTT- RTT operational practice and workflow innovations
ESTRO 2026
1 Radiation Oncology, St. Luke’s Radiation Oncology Network, Dublin, Ireland. 2 Department of Radiotherapy, Erasmus MC Cancer Institute, Rotterdam, Netherlands Purpose/Objective: Surface Guided Radiotherapy (SGRT) relies on Regions of Interest (ROIs) to track patient position and motion. However, ROI selection is typically subjective, introducing variability in tracking performance. A recent feasibility study [1] demonstrated that geography-derived topographic metrics—slope (0– 90°), aspect (0–360°), Vector Ruggedness Measure (VRM), Terrain Ruggedness Index (TRI), and Topographic Position Index (TPI)—can quantitatively describe ROI surface features. This study aims to evaluate, under controlled conditions, whether ROIs with higher topographic measures deliver more accurate and stable SGRT tracking. Material/Methods: Measurements were performed using AlignRT’s breast phantom, first in a stationary setup and then on a motion platform. The phantom's surface mesh was analysed using an in-house Python application to calculate slope, aspect, VRM, TRI, and TPI. Kernel sizes of 5 mm, 15 mm, and 50 mm were applied for VRM, TRI, and TPI respectively to capture fine, local, and regional features. For all metrics, higher values with a greater spread represent greater surface variation (more topographically complex), while lower values indicate flatter regions. Multiple ROIs with differing topographic profiles were defined, and ROIs were selected that represented the most and least topographically complex surfaces. Stationary tracking stability was quantified using the mean and standard deviation of the Real-Time Delta (RTD) magnitude with the static phantom, representing ROI “flicker”. Motion tracking accuracy was evaluated by comparing the measured vertical displacement to the actual displacement (7 mm) during simulated sinusoidal motion.
Results: ROIs displayed distinct topographic profiles, with the breast ROI showing greater slope (17.1 ± 12.2°, 45.6 ± 17.8°) and aspect (62 ± 45°, 307.5 ± 70°) variability, along with a higher TRI (0.3 ± 0.1, 0.6 ± 0.2), and TPI (0.1 ± 1, 6.2 ± 3.5) values compared to the flat ROI (Fig.1). The breast ROI produced more stable static tracking (RTD = 0.8 ± 0.1 mm) compared with the flat ROI (2.4 ± 1.2 mm) (Fig.2), indicating reduced flickering of the ROI and a steadier on-screen display. During sinusoidal vertical motion (7 ± 0.5 mm), the breast ROI accurately tracked motion (7.2 ± 0.1 mm), whereas the flat ROI overestimated displacement (9.7 ± 0.9 mm).
Conclusion: ROIs demonstrating accurate and stable SGRT tracking were topographically distinct from those that performed poorly. This controlled evaluation confirms that surface topography underlies variability in ROI performance, supporting the use of quantitative surface metrics as objective indicators of ROI suitability. Incorporating these metrics into clinical workflows may facilitate standardised, data-driven ROI selection for more consistent SGRT performance. References: [1] Malone C, Ryan S, Nicholson J, Brennan S, McArdle O, Woods R, MacGairbhith A, Waldron J, Callagh C, Harwood R, McClean B, Duane F, Hanna GG. From
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