S2894
RTT - RTT education, training, and advanced practice
ESTRO 2026
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.
second, prospective software was then implemented, integrating patient- and treatment-specific data (age, anatomical site, number of fractions, CBCT mode). This tool provides recommendations on CBCT frequency, allowing adjustment of the imaging schedule to maintain dose optimization while ensuring image quality and set-up accuracy. Results: A total of 41 pediatric patients were enrolled: 23 retrospectively and 18 prospectively. The retrospective analysis revealed a high degree of variability in CBCT acquisition parameters, mainly related to operator- dependent choices, resulting in a non-homogeneous IGRT workflow (Figure 1). The implementation of the Image Gently low-dose protocol was therefore adopted as a shared standard, enabling uniform and standardized imaging practice among radiation therapists (Figure 2). The prospective use of the predictive software further optimized CBCT scheduling, maintaining the cumulative imaging dose below the predefined threshold without compromising image quality or set-up accuracy. In some cases, the number of CBCTs was intentionally reduced compared with the total number of treatment sessions (Figure 3), while remaining fully compliant with international reference protocols.Overall, the Image Gently protocol reduced the mean effective dose by 92.5% compared with standard protocols, with an additional 1.1% reduction achieved through predictive planning. Conclusion: The integration of these software tools has become an essential part of the daily workflow for radiation therapists, ensuring operator-independent, standardized, and optimized IGRT management. This approach allows for personalized yet homogeneous radiotherapy delivery, significantly reducing the additional dose contribution from IGRT and enhancing safety, quality, and sustainability in pediatric treatments. Keywords: Pediatric Radiotherapy, CBCT, Gentle IGRT Digital Poster 4863 Phantom-Based Evaluation of Topographic Metrics for SGRT ROI Optimisation Paul O Connor 1 , Serena O'Keeffe 1 , Ciaran Malone 1,2 1 Radiation Oncology, St. Luke’s Radiation Oncology
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
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,
Made with FlippingBook - Share PDF online