S2236
Physics - Intra-fraction motion management and real-time adaptive radiotherapy
ESTRO 2026
Digital Poster 2728
being derived from a more recent dataset with less time on-bed.
Correlations between target size, displacement, envelope margin, and VOICE threshold in MR-Linac radiotherapy with Comprehensive Motion Management Pak-Hang Nam, Chi-To Yung, Dejun Zhou, Chi-Wah Kong, Kin-Yin Cheung, Siu-Ki Yu, Bin Yang Medical Physics Department, Hong Kong Sanatorium & Hospital, Hong Kong, Hong Kong Purpose/Objective: Magnetic-resonance-guided online-adaptive radiotherapy on the Elekta Unity (Elekta AB, Stockholm, Sweden) platform utilizes the Comprehensive Motion Management (CMM) system for real-time motion. The Volume Overlapping Criterion (VOICE) gating technique stops the beam when the percentage volume overlap between the target and a gating envelope falls below a threshold. While an appropriate envelope margin aims to minimize dose to normal tissue and maintain treatment efficiency, the quantitative relationships among target size, displacement, envelope margin, and VOICE threshold require further elucidation. This study investigates these correlations to inform margin and threshold selections. Material/Methods: Spherical (4-42 mm diameter at 2 mm interval) and cylindrical (10-42 mm diameter at 2 mm interval; short cylinder: length 16-48 slices at 2-mm interval; long cylinder: length 21-53 slices at 2-mm interval) virtual targets were created in MIM Version 7.3.7 (MIM Software Inc., Cleveland, OH, USA) with voxel size 1x1x1mm. Gating envelopes were generated by adding a 1-5 mm isotropic margin around the targets. To simulate intra-fraction motion, the targets were displaced 1–5 mm in 0.5 mm step along the lateral or 1 mm step along superior-inferior direction. Percentage volume overlap between displaced target and gating envelope was measured. [1] Maximum displacements corresponding to 95% or 98% volume overlap were determined via interpolation or extrapolation. Results: Volume overlap rapidly decreased as target size increased. Meanwhile, increases in margin size led to a non-linear rise in volume overlap. Larger margins increase the target displacement tolerance for achieving high volume overlap percentages, but the incremental tolerance diminished with increasing margin. The 98% VOICE threshold permitted 15–25% smaller displacements tolerance than 95% VOICE threshold. For a 16 mm diameter sphere target (Superior-inferior displacement), maximum displacement tolerance at 1 mm margin was 1.78 mm(95%) vs. 0.71 mm(98%); at 3 mm margin, this rose
The patient-specific margins provided adequate coverage for almost all fractions, as seen in Figure 2. Although there were some fractions where the bladder grew outside of the PTVPS, these are unlikely to be clinically significant given they only occur on 1- 2#.
Conclusion: This work shows that patient-specific PTV margins can be derived from pre and post-treatment images of the first 5 fractions. When used, these margins can reduce the volume of irradiated normal tissue for patients with less bladder filling whilst ensuring coverage for patients with more filling. Work is underway to extract dosimetric information. References: [1] Lalondrelle S et al. International Journal of Radiation Oncology*Biology*Physics 2011;79:705–12. https://doi.org/10.1016/j.ijrobp.2009.12.003.[2]
Kim HS et al. Med Phys 2012;39:6779–90. https://doi.org/10.1118/1.4754802.[3]
Chen S et
al. J Med Imaging Radiat Sci 2018;49:420–7. https://doi.org/10.1016/j.jmir.2018.08.002.[4] Carter H et al. Radiotherapy and Oncology 2025;206:S3236–8. https://doi.org/10.1016/S0167- 8140(25)00674-7. Keywords: bladder, margin, personalisation
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