S2226
Physics - Intra-fraction motion management and real-time adaptive radiotherapy
ESTRO 2026
Mini-Oral 1980
Beam-view tracking is safe, feasible and reduces imaging dose for fiducial based prostate SABR patients Jeremy Booth 1,2 , Danielle Chrystall 1,2 , Maegan Stewart 1 , Alannah Kejda 1 , Chandrima Sengupta 3 , Freeman Jin 3 , Thomas Eade 1 , Andrew Kneebone 1 , George Hruby 1 , Paul Keall 3 1 Radiation Oncology, Royal North Shore Hospital, Sydney, Australia. 2 Institute of Medical Physics, School of Physics, University of Sydney, Sydney, Australia. 3 Image-X Institute, University of Sydney, Sydney, Australia Purpose/Objective: Beam-view tracking uses the treatment field to monitor intra-fraction motion of implanted fiducials without additional imaging dose. Poor megavoltage image quality and occlusion of fiducials during modulated VMAT delivery have been challenges to implementation.1-3 A deep-learning approach to detect implanted gold fiducials in cine MV imaging was developed and implemented inside a real-time framework that estimates the in-plane depth for 3D tracking.4 We clinically evaluate the first in-human implementation of real-time beam-view tracking for prostate stereotactic ablative body radiotherapy (SABR). Material/Methods: Fourteen prostate cancer patients receiving SABR with simultaneous integrated focal boost to intraprostatic lesion(s) were recruited to this single institution, ethically approved motion monitoring trial (NCT01742403) between March and September 2025. Figure 1 shows the study schematic. Beam-view fiducial tracking using cine MV imaging was integrated into the clinical workflow, enabling real-time 3D image- guided radiotherapy. A 2 mm gating threshold prompted radiation therapists to interrupt the treatment beam and reposition the patient when a time tolerance (5-20 s) was exceeded. Feasibility was assessed as the successful delivery with beam-view tracking workflow and geometric accuracy <2mm compared against triangulated kV/MV ground truth. Dosimetric performance of beam-view tracking was determined using a motion-encoded dose estimation compared against standard of care passive (no beam- view gating) treatment and the planned dose distribution. Safety was assessed as early patient outcomes, collected for the first twelve patients at 3 months post-treatment, and acute genitourinary (GU) and gastrointestinal (GI) toxicities analysed.
In a 2D-3D match, the fiducials on the 3D-DRR appear blurry. The TrueBeam software can automatically match the fiducials. However, when reviewing the matching, it is hard to match the fiducials acquired from the kV imaging to the 3D-DRR, often relying on the fiducials contour to verify the automatic matching.From the results, triggered imaging acquired at cardinal angles provided only limited information on positional deviations, particularly when the displacement occurred along the direction of the kV beam. Additionally, roll rotations could not be accurately determined using a 2D-3D registration approach.ExacTrac stereoscopic X-rays calculated shifts closely match CBCT shifts in all six directions. Conclusion: This study demonstrated strong agreement between CBCT and ExacTrac stereoscopic X-ray-derived shifts for intrafraction motion assessment. While triggered imaging may serve as a supplementary method for fiducial tracking, the detected deviation should be interpreted with caution. Although stereoscopic X-rays do not provide volumetric data, once CBCT-based corrections are applied, they remain a reliable and efficient tool for ongoing fiducial-based motion monitoring. Keywords: fiducials tracking
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