S2240
Physics - Intra-fraction motion management and real-time adaptive radiotherapy
ESTRO 2026
1 Department of Radiotherapy, UZ Brussel, Brussels, Belgium. 2 Research Centre for Digital Medicine, Vrije Universiteit Brussel, Brussels, Belgium Purpose/Objective: Stereotactic body radiotherapy (SBRT) offers high local control for lung lesions but is limited by respiratory motion. Traditional methods using internal target volumes (ITV) or fiducial markers can be invasive or unsuitable. The ExacTrac Dynamic system (Brainlab, Munich, Germany) combines thermal surface imaging and stereoscopic x-rays enabling markerless deep- inspiration breath-hold (DIBH)-SBRTwith real-time monitoring and verification. This prospective observational feasibility study reports the first clinical experience using markerless DIBH-SBRT for lung lesions in a real-world setting employing anatomical surrogates termed to as Correlation Objects- for lesion localization. Material/Methods: Twenty-four consecutive patients (26 lesions) were included. Simulation involved both 4DCT and DIBH-CT acquisitions for target definition and motion characterization to generate anatomical surrogates. The latter uses a dedicated software solution developed by Brainlab SE designed to create Correlation Objects for lung lesions and to fuse them with X-ray images for lesion localization, supporting clinical implementation. Several Correlation Objects were created each representing a different degree of motion similarity with the lesion to have varying size for registration purpose and positioning identification. Treatment was delivered on a Varian TrueBeam with ExacTrac Dynamic. Surface-guided (SGRT) prepositioning, and stereoscopic x-rays acquired in DIBH were registered to Correlation Objects. This markerless DIBH workflow was compared to the standard gated CBCT setup to evaluate feasibility, workflow integration, DIBH reproducibility, and intrafraction motion control. The primary endpoint was feasibility, successful treatment completion under breath-hold with accurate and reproducible targeting. Results: All 123 fractions across 24 patients were successfully completed under DIBH without complications and patient were eligible to perform multiple DIBH. Mean lesion volume was 3.4cc±6.4cc compared to 5.5cc±14.7cc showing a reduction of 54% in volume due to the DIBH-technique. Average inter and intra- breath-hold stability was 1.9±0.7mm. The mean and standard deviation of the absolute value of the residual setup errors after stereoscopic x-rays registered with the Correlation Object, verified with gated CBCT were 2.8±4.9mm, 3.1±4.5mm, and 2.9±3.4mm, for vertical, longitudinal, and lateral directions, respectively. The yaw, pitch, and roll were not significantly different and were neglected for the
TVSYNC improved coverage by ~2.5–3.5% with largest gains in patients with poor ITV coverage (P3: +8.7%, P10: +11.8%). TVSYNC maintained or slightly increased the mean dose (~0.7 Gy). Lung dose analysis confirmed reduced mean dose by 8–12% and decreasing lung exposure in 81.9% of cases (Figure 2).
Figure 2. Left plot:Thedose (Gy) in 10% of the lung volume.Right plot: The mean lung dose with the 8 Gy reference line. Conclusion: Real-time adaptive delivery with the Synchrony® system reduced treatment margins and produced smaller, more homogeneous tracking volumes. Dosimetric uniformity and normal-tissue sparing improved, with consistently lower mean lung doses compared to ITV-based plans. DAA planning confirmed accurate dose delivery across respiratory phases. These results support the clinical adoption of motion- synchronized adaptive therapy to enhance precision and safety in lung SBRT. References: Keall PJ, Mageras GS, Balter JM, et al. The management of respiratory motion in radiation oncology report of AAPM Task Group 76. Med Phys. 2006;33(10):3874- 3900.William SF, Michael WK, Wesley SC, et al. Evaluation of radixact motion synchrony for 3D respiratory motion: Modeling accuracy and dosimetric fidelity. Radiation Oncology Physics. 19 June 2020. Dang HQ, Zhang Y, Wang Z, et al. Four- dimensional dose accumulation for lung cancer patients using deformable image registration: a comparative study. Radiother Oncol. 2021; 161:169– 175. https://doi.org/10.1016/j.radonc.2021.06.015. Keywords: Synchrony-based adaptive, Phase-resolved dosimetry Mini-Oral 2935 First Clinical Experience with Markerless Breath- Hold SBRT for Lung Lesions Using ExacTrac Dynamic: A Prospective Observational Study Thierry Gevaert 1,2 , Marlies Boussaer 1,2 , Cristina Teixeira 1,2 , Anette Bretz 1,2 , Selma Ben Mustapha 1,2 , Anne-Sophie Bom 1,2 , Sven Van Laere 1,2 , Guy Soete 1,2 , Mark De Ridder 1,2
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