S2223
Physics - Intra-fraction motion management and real-time adaptive radiotherapy
ESTRO 2026
overdosage. This necessitates real-time monitoring of the spine during treatment. In this work, we share our initial validation of RapidTrack, a markerless real-time kV tracking system, for radiosurgery. We also present its first clinical study analysis for spinal nerve-root ablation for chronic thoracoabdominal wall pain. Material/Methods: RapidTrack uses real-time template matching to localize the vertebra position in 3D from planar fluoroscopic images acquired during treatment delivery using the LINAC’s on-board imager (1). We performed pre-clinical evaluations with an anthropomorphic spine phantom and a programmable motion platform. The tracking performance during dynamic arc delivery was evaluated at incremental phantom offsets and varying arc lengths and compared to the programmed offset, as well as estimated shifts from CBCT and ExacTrac imaging. Moreover, the tracking accuracy was clinically validated in 12 spine SBRT patients using CBCT projections acquired as part of routine care. Finally, we implemented the system in a clinical setting as part of an IRB-approved nerve-root ablation protocol. Two patients were treated with frameless single-fraction stereotactic radiosurgery delivering 70 Gy to the spinal nerve dorsal root ganglia using 16 static-MLC partial arcs arranged in two sectors (Figure 2A). We evaluated overall tracking performance and compared the detected shifts in vertebral position with ExacTrac. Results: The phantom study demonstrated an average RapidTrack accuracy of 0.2±0.1mm (maximum: 0.4mm) in the detected spine position for offsets up to 4 mm (Figure 1). No substantial impact of the arc length was observed, and the agreement with CBCT and ExacTrac was within 0.2mm. The clinical validation with CBCT data showed an average agreement better than 0.4mm in all directions, with maximum deviations of 0.8mm. RapidTrack was successfully used for spinal nerve radiosurgery and detected a shift exceeding 1mm during a treatment arc (Figure 2B, arrow). The median (±interquartile range) difference in the detected vertebra position compared to ExacTrac was -0.2(±0.6)mm, 0.1(±0.4)mm, and -0.1(±0.4)mm in vertical, lateral, and longitudinal direction.
stereotactic body radiotherapy (SBRT) across three clinical platforms: Varian TrueBeam with AutoBeam Hold (ABH), Varian TrueBeam with Brainlab ExacTrac Dynamic (ETD), and Accuray Radixact with Synchrony. Material/Methods: An anthropomorphic pelvis phantom containing four gold fiducial markers was used. For each system, the lowest kilovoltage exposure parameters allowing stable automatic fiducial localization were identified. The corresponding kerma-in-air per image was then measured at isocenter using a calibrated dosimetry system. Per-image values were converted to per- fraction and full-course imaging doses based on clinically representative intrafraction monitoring frequencies for prostate SBRT. Results: The mean kerma-in-air per image was 1.47 mGy for TrueBeam ABH, 0.034 mGy for ETD, and 0.15 mGy for Radixact Synchrony. When extrapolated to a five- fraction SBRT regimen, cumulative imaging dose was 557.1 mGy for ABH, 13.7 mGy for ETD, and 179.6 mGy for Synchrony. Relative to ETD, ABH delivered approximately 41 times more imaging dose, and Synchrony approximately 13 times more. Conclusion: Under harmonized conditions and equivalent fiducial localization performance, ETD resulted in the lowest intrafraction imaging dose, Synchrony an intermediate dose, and ABH the highest. These differences reflect intrinsic imaging geometry and acquisition design. Although kerma-in-air does not directly represent organ dose, the results emphasize that imaging exposure should be considered when selecting intrafraction monitoring strategies for prostate SBRT. Keywords: Intra-fraction, prostate tracking, kV dose Poster Discussion 1672 Real-time 3-dimensional kV spine tracking during dorsal spinal nerve radiosurgery Sebastian Meyer 1 , Shu Xing 1 , Lei Zhang 1 , Yunjie Yang 1 , Liangjia Zhu 2 , Kristijan Macek 2 , Yoshiya Yamada 3 , Seng- Boh Lim 1 1 Department of Medical Physics, Memorial Sloan Kettering Cancer Center, New York, USA. 2 Radiotherapy Solutions, Varian Medical System, Palo Alto, USA. 3 Department of Radiation Oncology, Memorial Sloan Kettering Cancer Center, New York, USA Purpose/Objective: Monitoring intrafractional motion is crucial for high- dose single-fraction functional radiosurgery, especially when targeting spinal nerves. Due to the steep dose fall-off and close proximity of the spinal cord, even small shifts during beam delivery can result in
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