S2264
Physics - Intra-fraction motion management and real-time adaptive radiotherapy
ESTRO 2026
Purpose/Objective: The Radixact Synchrony tomotherapy system achieves radiotherapy target tracking by modifying the positioning of binary MLCs and dynamic jaws in response to changes in the location of the treatment target, represented by fiducial markers (in fiducial tracking mode) or lung tumours (in respiratory tracking mode) [1]. This study developed and evaluated in-house software to meet the challenge of verifying the suitability of Synchrony adaptations, for these real-time tracking deliveries that exhibit frequent and intentional deviations from their treatment plans. Material/Methods: In-house software has been developed to process treatment planning and delivery data obtained from the Radixact system using the Accuray Patient Data Extractor application. The developed software maps fractional treatment target motion, and shows sinograms (leaf-open times (LOT) for each of the MLC leaves vs distance in mm). The lateral displacement of the target from planned position in the beams-eye view is calculated using the IEC-X and IEC-Z target positions and gantry angle. The resulting transverse displacement of each target at each treatment time point is then plotted against a reported 4.3 mm threshold for MLC leaf adaptation [2], and illustrated alongside the corresponding planned and delivered sinogams, to allow qualitative verification of the adaptation. Results: Example results are shown in Figures 1 and 2 for prostate Synchrony treatment fractions. In Figure 1, the target displacement was not sufficient to trigger any leaf adaptation. In Figure 2, an initial left-right target displacement of 2 mm combined with continuous anterior-posterior target drift has resulted in leaf adaptation during the latter half of the treatment.
Figure 2 Results as defined in caption to figure 1, shown for a sample prostate treatment delivery where adaptation commenced mid-way through the fraction delivery. Conclusion: The developed in-house Synchrony verification tool shows promise, to confirm that the system has adapted (or not adapted) as required, after each treatment fraction. Further work is required to establish the suitability of the 4.3 mm MLC adaptation threshold for multiple treatment geometries, include exit detector data in the analysis [3], refine the handling of re-planned and paused treatment fractions, and add reporting of quantitative results to the tool. References: [1] Kairn T, Yu L, Crowe, SB. A System for Verifying Radiation Therapy Treatments That Use Multiple Modes of Real-Time Motion Adaptation. Cureus 2025;17(4):e83023. https://doi.org/10.7759/cureus.83023. [2] Miura M, Sasaki K, Shiota Y, Inoue K. Investigation of the Response of Binary Multileaf Collimator Compensation to Target Setup Errors in the Radixact Synchrony System: A Phantom Study. Cureus 2025;17(6):e85305. https://doi.org/10.7759/cureus.85305.[3] Han MC, Chang KH, Kim J, Han SC, Park K, Kim DW, Kim H, Kim JS. TomoEQA: Dose verification for patient-specific quality assurance in helical tomotherapy using an exit detector. Physica Medica 2021;82:1-6.
https://doi.org/10.1016/j.ejmp.2020.12.021. Keywords: quality assurance, MLC tracking, tomotherapy
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The performance of zero-shot and fine-tuned Medical Segment Anything 2 compared to dedicated trained nn-UNet for real-time MR-guided tumour tracking Pia A.W. Görts 1,2 , Rob H.N. Tijssen 3,2 , Marcel Breeuwer 2,4 , Coen W. Hurkmans 1,4 1 Department of Radiation Oncology, Catharina Hospital Eindhoven, Eindhoven, Netherlands. 2 Department of Biomedical Engineering, Technical
Figure 1 Planned and LOT sinograms, shown alongside the result of subtracting the delivered telemetry from the planned LOT and a plot of the detected target displacement (with pink bars showing the nominal 4.3 mm adaptation threshold), at each time point (projection), shown for a sample prostate treatment delivery that required no adaptation.
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