S2203
Physics - Intra-fraction motion management and real-time adaptive radiotherapy
ESTRO 2026
baseline shifts (BLS). Efficiency (% beam-on) per fraction was assessed. Employing a previously described method[1], dosimetric benefit of IMM was assessed by performing dose accumulation for each fraction in two scenarios: using either only beam-on motion data (‘managed’), or all datapoints reversing any BLS to simulate treatment without IMM (‘non- managed’). GTV DVH parameters were extracted and compared between the planned (daily treatment plan), managed, and non-managed datasets. Results: Median dose delivery time was 12.7 min (range 5.5- 33.4 min). Median efficiency was 79.1% (range 34.0- 99.5%). Larger respiratory amplitude was associated with lower efficiency (R2=0.53; Figure 1). In 55/208 (26.4%) fractions, ≥ 1 BLS was performed. In 74.5% of these, the first BLS improved efficiency. The actually delivered dose could be reconstructed for 188 fractions. When accounting for application of IMM, median GTV V100% was significantly lower in the simulated non-managed scenario compared to the planned and managed scenario (97.5%, 100%, and 99.2%, respectively; p<0.001). In fractions with higher target coverage in the managed scenario compared to the non-managed scenario (n=110), median difference in GTV coverage was +1.8% (range 0.1-72.6%). A relation was observed with respiratory amplitude, as the difference in target coverage between the managed and non-managed scenario became more pronounced with larger amplitudes (Figure 2).
Keywords: Cervical cancer; Online adaptive EBRT; Lymph nodes
Mini-Oral 442
Treatment efficiency and dosimetric benefit of intrafraction motion management during MRI- guided SBRT for liver tumors Thomas Willigenburg, Guus Grimbergen, Gert J. Meijer, Lieke T.C. Meijers, Jonna K. van Vulpen, Martijn P.W. Intven Department of Radiation Oncology, University Medical Center Utrecht, Utrecht, Netherlands Purpose/Objective: MRI-guidance has enhanced precision of SBRT for upper abdominal tumors by providing superior soft- tissue contrast and enabling real-time online plan adaptation. Recently, intrafraction motion management (IMM) has become available on the 1.5 T MR-Linac, allowing real-time gating and intrafraction drift corrections, theoretically further enhancing accuracy. Since July 2024, IMM is used to treat liver tumors in our institute. We describe our first experiences with IMM, focused on treatment efficiency and dosimetric benefits. Material/Methods: Thirty-eight patients (208 fractions) with liver tumors (primary and metastases) treated on a 1.5 T MR-Linac using IMM and abdominal compression were included. GTV motion was tracked on vendor-supplied cine-MRI (balanced Turbo Field Echo sequence) using a surrogate structure (part of the liver). Beam delivery was automatically paused whenever overlap between the GTV and the static gating envelope (defined as PTV=GTV+3 mm or 5 mm) fell below 95%. Intrafraction drift resulting in reduced efficiency was corrected with
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