KEY POINTS
- The study analysed 7.5-minute three-dimensional cine magnetic resonance imaging acquisitions from 18 patients receiving MR-guided radiotherapy for upper abdominal lesions under abdominal compression.
- Small-bowel loops were tracked across 246 frames using deformable image registration to generate voxel-level motion and occupancy maps.
- Motion was predominantly craniocaudal. Displacements exceeded 20 mm in one case, and approximately half of the bowel voxels experienced positional variations close to 10 mm.
- A 4.5 mm isotropic planning organ-at-risk volume margin covered 95% of bowel voxels occupied for at least 5% of the acquisition time in 90% of patients.
- At 4.5 mm, the expanded bowel-loop volume was approximately 20% smaller than the full bowel-bag contour. Margin estimates were already relatively stable with acquisitions of about 2.5 minutes.
CLINICAL TAKEAWAY
Motion-informed bowel-loop margins could provide more planning flexibility than contouring the entire bowel bag during online MR-guided treatment. The proposed 4.5 mm value should not be adopted as a universal constraint: it comes from a small, compressed upper-abdominal cohort and was not validated against delivered dose or gastrointestinal toxicity.