S2211
Physics - Intra-fraction motion management and real-time adaptive radiotherapy
ESTRO 2026
Digital Poster Highlight 908
A framework to estimate Planning Organ-At-Risk Volume margins for intra-fraction motion of the small bowel during MR-guided radiotherapy. Saskia L.C. Damen, Astrid L.H.M.W. van Lier, Cornel Zachiu, Martijn P.W. Intven, Bas W. Raaymakers Radiotherapy Department, UMC Utrecht, Utrecht, Netherlands Purpose/Objective: Stereotactic body radiation therapy (SBRT) is an effective treatment for abdominal malignancies. However, proximity to the radiosensitive bowels can limit the achievable dose coverage1. Bowel position during treatment is often estimated by the complete bowel bag, which overestimates true bowel volume and can limit dose unnecessarily. In contrast, delineating individual bowel loops each fraction, expanded with a planning organ-at-risk volume (PRV) margin to account for intra-fraction motion, can offer more flexibility in planning. Previously, margins for inter-fraction small bowel coverage were derived, which are expected to be larger than for intra-fraction coverage2. This research aims to develop a framework for automated PRV margin derivation for intra-fraction bowel motion. Material/Methods: A previously presented framework was used to image and automatically track the bowel3. Bowel tracking data (7.5 minutes, 1.8 s/frame) was obtained in 18 patients treated for abdominal lesions on a 1.5T MR- Linac, with abdominal compression. Per patient, an occupancy map was constructed, indicating the fraction of time each small bowel voxel occupied a given position. Subsequently, the original bowel loop delineation (on frame 1) was systematically expanded by various margins, and the amount of bowel occupation outside the expanded mask was determined. This way the margin needed to cover >x% of voxels that were occupied for >y% of time, was determined. The 90th percentile through the cohort determined the final margin4. The same analysis was performed on subsets of the full acquisition, using a sliding window approach to evaluate temporal consistency. Results: Figure 1 shows that applying a 4.5 mm isotropic margin around the bowel loops was sufficient to cover 95% of the voxels occupied >5% of the time in 90% of the patients. The volume of the delineation with this margin, lead to a median volume reduction of ~50% as compared to the bowel bag delineation. The precision analysis (Figure 2) shows that even for smaller subsets like 50 frames (2.5 min.), resulting margins vary only around 0.5 mm within a patient.
Imaging intervals were slightly shorter in the 6DOF group but showed only a weak correlation with residual displacement (r = 0.25).A positive correlation was observed between pitch corrections and longitudinal residuals (r = 0.31), and a negative correlation between roll and lateral residuals (r = – 0.66*).
For larger pitch angles (> 2°), small yet directionally consistent longitudinal shifts were seen, indicating possible patient sliding towards the foot-end. No systematic offsets were detected for other parameters. Conclusion: Full 6DOF couch corrections in thoracic radiotherapy do not appear to induce unintended motion or increase setup uncertainty. However, large pitch (> 2°) or roll (> 1.5°) corrections may introduce minor but biomechanically plausible residual shifts. These findings support the safe clinical use of rotational corrections, while emphasising the need for caution when applying high-magnitude adjustments. Keywords: 6DOF couch, IGRT, patient motion
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