S2219
Physics - Intra-fraction motion management and real-time adaptive radiotherapy
ESTRO 2026
Material/Methods: 40 patients with spine and non-spine bone metastases treated in five fractions were analysed retrospectively. Prescriptions varied across anatomical sites, typically 5x6Gy on the 80% isodose line. Doses to the spinal canal and cord were limited to 28 Gy and 27 Gy, respectively. Margins of 1.5mm were added to the organs at risk and 3mm to the target volumes.Patients were pre-positioned using SGRT then positioned using XR and a volume-of-interest fusion with digitally- reconstructed-radiographs (DRRs). After sequentially reducing residual errors with XR, CBCT was acquired. During beam delivery, XR at cardinal positions of the gantry were used to monitor the patient and triggered repositioning when out of tolerance (1.5mm/1.5°). SGRT data was acquired throughout the treatment and triggered XR acquisition when out of tolerance (2mm/2°). Figure 1 - Treatment workflow Results: XR residuals were compared to CBCT-measured errors to evaluate positioning accuracy. Residuals for combined translational errors were 0.47 ± 0.37 mm for cervical, 0.79 ± 0.68 mm for thoracic, 0.58 ± 0.39 mm for lumbar and 0.26 ± 0.15 mm for sacral spine metastases. Residuals were 0.60 ± 0.56 mm for hips and legs, 1.77 ± 2.31 mm for metastases in arms and shoulders, and 2.50 ± 2.81 mm for ribs.Intra-fraction positioning errors from XR monitoring data were analyzed by treatment location (see Figure 2). For 95% of fractions, mean translational and rotational errors remained below 1.5 mm and 1.5° in all directions. Exceptions were observed for targets in the ribs, arms, and shoulders, due to uncorrected over-threshold deviations.Spearman correlation between SGRT and XR monitoring errors was computed for coinciding timepoints. Very strong correlations (>0.8) were observed for lateral and roll, strong correlations (>0.6) for longitudinal, vertical, and yaw, and weak correlation for pitch.
Conclusion: Cardiac blurring seems to have only a small effect. Both respiratory tracking and a margin accounting for the effect of surrogate-to-target mismatch are necessary to achieve adequate coverage of the target. References: [1] R. R. F. Stevens et al., “Stereotactic Arrhythmia Radioablation (STAR): Assessment of cardiac and respiratory heart motion in ventricular tachycardia patients - A STOPSTORM.eu consortium review,” Radiother Oncol, 2023.[2] L. Knybel et al., “Real-time measurement of ICD lead motion during stereotactic body radiotherapy of ventricular tachycardia,” Rep Pract Oncol Radiother, 2021.[3] T. Bortfeld, S. B. Jiang, and E. Rietzel, “Effects of motion on the total dose distribution,” Seminars in Radiation Oncology, 2004.[4] S. H. Benedict et al., “Stereotactic body radiation therapy: The report of AAPM Task Group 101,” Medical Physics, 2010. Keywords: Stereotactic Arrhythmia Radioablation, CyberKnife Beyond CBCT: real-time motion management in spine and non-spine bone SBRT using stereoscopic X-rays and SGRT. Adrián Gutiérrez, Kyra Dierinck, Tim Everaert, Selma Ben Mustapha, Guy Soete, Mark De Ridder, Thierry Gevaert Radiotherapy, UZ Brussel, Brussels, Belgium Purpose/Objective: Stereotactic body radiotherapy (SBRT) achieves durable local control and symptom relief in spine and non-spine bone metastases. Delivering high doses in few fractions introduces challenges due to proximity to critical structures such as the spinal cord. Digital Poster Highlight 1262 Conventional cone-beam CT (CBCT) remains common, but image guidance techniques like stereoscopic X- rays (XR) in combination with surface-guided radiotherapy (SGRT) enable continuous, precise monitoring. This study evaluated XR and SGRT for positioning and real-time motion management of bone metastases.
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