S2251
Physics - Intra-fraction motion management and real-time adaptive radiotherapy
ESTRO 2026
volumes, potentially improving the therapeutic ratio. However, treatment success depends on selecting an optimal margin strategy encompassing all the uncertainties involved. This study developed a Monte Carlo (MC) simulation framework to compare gating strategies and assess their population-level dosimetric performance. The tool was used to optimize planning and treatment parameters—such as CTV-PTV margins, target-surrogate, and gating envelope definitions—for exhale gating using CMM, based on measured dosimetric, geometric, and temporal uncertainties. Material/Methods: The MC framework incorporates uncertainty components (Table~I) evaluated during machine commissioning, applied at multiple stages: institution, equipment calibration, patient, fraction, and treatment delivery. Treatment delivery is modeled through time discretization of the respiratory cycle, where the simulated beam is triggered once the CMM-predicted target position lies fully within the gating envelope, accounting for sampled exhale latency and positional errors. CTV doses are accumulated over time using a simplified 6-surfaces one-dimensional dose model assuming 95% PTV coverage with realistic gradients. Fractional and cumulative CTV dose coverage and beam-on time statistics were compared across PTV margin and gating envelope strategies. Three breathing amplitudes (5/10/20mm), with and without ±2 mm superior-inferior baseline drifts, and three target/envelope/PTV approaches (defined in Table~II) were evaluated for a prescribed dose of 45 Gy in 5 fractions. Results: For a 10 mm breathing amplitude without baseline drift, a uniform 3 mm gating envelope yielded approximately 37% beam-on time. However, the minimum cumulative CTV coverage varied: the probability of achieving ≥ 95% of the prescribed dose (P(Dmin>95%)) ranged from 55.8% to 99.9%, depending on the PTV definition. When baseline drifts of ±2 mm were included, only the PTV approach with a setup margin added to the envelope maintained robustness, with P(Dmin>95%) > 90% across all breathing amplitudes. Conclusion: The proposed MC simulation framework predicts population-level CTV dose coverage under CMM-based exhale gating. Optimal results are achieved when an exhale defined target is enclosed in a uniform 3 mm gating envelope (5 mm inferior expansion for larger amplitudes), using a PTV defined as the convolution of this envelope with a 3 mm setup margin. This configuration provides CTV dose coverage that is robust against baseline drifts while maintaining acceptable beam-on times. Compared to an all-phases inclusive ITV approach, it also reduces treated volumes when the target motion amplitude is greater than the
Conclusion: The results indicate that AI markerless real-time IGRT meets the reliability but not the performance criteria for liver cancer SABR patients. The findings focus future research towards performance and patient selection prior to planned clinical deployment in the Real-time IGRT trial (NCT06708221). References: 1. Anastasi G, Patterns of practice for adaptive and real-time radiation therapy (POP-ART RT) part I Radiother Oncol.2. Kishan AU, MRI–guided vs CT– guided for prostate cancer: The MIRAGE randomized clinical trial. JAMA Oncol. 3. Gardner M, Deep learning-based real-time detection of head and neck tumors during radiation therapy. Phys Med Biol.4. Mylonas A, Patient-specific prostate segmentation in kilovoltage images for radiation therapy intrafraction monitoring via deep learning. Communications Medicine. 5. Couwenberg A, Master protocol trial design for technical feasibility of MR-guided radiotherapy. Radiother Oncol. Keywords: AI, liver cancer SABR, real-time IGRT Optimal SBRT treatment margin strategy for exhale gating of abdominal malignancies using CMM motion monitoring with Elekta MR-LINAC Luc Gingras 1,2 , Janelle Morrier 1 , Éric Poulin 1,2 , Magali Besnier 1 , Audrey Cantin 1 , Olivier Fillion 1 , Bernard Lachance 1 , Frédéric Lacroix 1,2 , Jason St-Hilaire 1 1 Service de Radio-Oncologie, CHU de Québec, Québec, Canada. 2 Département de physique, génie physique et d’optique, Université Laval, Québec, Canada Purpose/Objective: The use of Comprehensive Motion Management (CMM) online tumor tracking on the Elekta Unity MR- LINAC may enable substantial reductions in treated Digital Poster Highlight 3498
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