ESTRO 2025 - Abstract Book

S3231

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

ESTRO 2025

1521

Digital Poster Comparative Stability of Two CBCT Devices for Adaptive Radiotherapy Applications Marco Fusella, Mattia Nicoletto, Marco Ciro Schifano, Erick Maffi, Claudio Antonelli, Martina Ometto, Isabella Alessi, Fabio Busato, Davide Fiorentin, Alessandro Testolin, Luigi Corti, Francesca Dusi Radiation Oncology, Policlinico Abano Terme, Abano Terme, Italy Purpose/Objective: Accurate Hounsfield unit (HU) values in cone-beam computed tomography (CBCT) are essential for precise dose calculations and treatment replanning in adaptive radiotherapy workflows. This study evaluates and compares the stability of HU values from two CBCT systems mounted on different linear accelerator (Linac) configurations: a standard CBCT (TrueBeam) and an advanced system with enhanced reconstruction algorithms (Halcyon). The analysis focuses on the reliability of these systems in maintaining stable HU values over time for structures critical to dose calculations. Material/Methods: Mean HU values for bone structures (femoral heads, ischium, and ilium) and bladder were retrospectively analyzed in 20 prostate cancer patients (10 per CBCT system), each undergoing 20 daily CBCT scans over a 4-month period. HU variability was assessed by calculating the standard deviation (SD) of HU values across all sessions for each patient using a validated deep learning-based contouring system (LimbusAI). The mean SD was chosen as a robust indicator of HU stability across daily imaging sessions, providing insight into intersystem performance variability. The results for each system were compared using independent t-tests to evaluate statistical differences in stability. The comparison emphasized the implications of variability in HU values for accurate dose calculation and treatment adaptation. Results: The Halcyon CBCT system demonstrated superior stability compared to the TrueBeam system. For bone structures, Halcyon achieved a mean SD of 5.0, significantly lower than TrueBeam’s 11.0 (t = 14.83, p < 0.001). For bladder, Halcyon also exhibited reduced variability, with a mean SD of 4.2 compared to TrueBeam’s 7.7 (t = 2.75, p = 0.007). the TrueBeam system, particularly for bones, which are crucial for accurate dose calculations in adaptive radiotherapy of pelvis patients. The advanced reconstruction algorithms integrated into the Halcyon system contribute significantly to its enhanced stability and reliability. This stability, when paired with an accurate analysis of dose calculation accuracy against CT scans, makes Halcyon Hypersight a more suitable option for both offline and online adaptive workflows, enabling reliable treatment replanning with minimal variability. By leveraging advanced reconstruction technology, the Halcyon system enhances clinical accuracy and overall treatment quality, reinforcing its role as a preferred choice for adaptive radiotherapy applications. Conclusion: The advanced reconstruction algorithms of the Halcyon CBCT system significantly improve HU stability compared to

Keywords: CBCT, ART, Quality Assurance

1610

Digital Poster The gating accuracy of the MR-linac system: the effect of tracking margins and target velocity Pauliina Petrow 1 , Arttu Cowell 2 , Antti Kulmala 3 , Laura Tuomikoski 1 , Timo Hakkarainen 4 , Mikko Tenhunen 1