3060 - Quantification of Gastrointestinal Organ Motion during Abdominal Adaptive Radiotherapy under Deep Inspiration Breath Hold

Purpose/Objective(s): Adaptive radiotherapy workflows require on-table contouring and plan generation, during which abdominal organs may undergo geometric changes despite immobilization techniques. This study quantified gastrointestinal (GI) organ motion during abdominal adaptive radiotherapy under deep inspiration breath hold (DIBH), by comparing planning CBCT (pCBCT) and verification CBCT (vCBCT) acquired 30–60 minutes apart.

Materials/Methods: Five patients undergoing abdominal adaptive radiotherapy under DIBH were retrospectively analyzed across five treatment sessions each (25 total sessions). All patients were instructed to remain nil per os (NPO) for at least three hours prior to treatment and were positioned on a stretcher or treatment table for 20–30 minutes before pCBCT scan to allow stabilization of GI anatomy. For each session, GI lumen structures (small bowel, duodenum, and stomach) were contoured on both pCBCT and vCBCT. Surface-based geometric differences were quantified within a predefined anatomical ring generated from the planning target volume expanded by 3 cm in the transverse directions and 1.5 cm in the cranio-caudal direction. Voxel-boundary average symmetric surface distance (ASSD) and 95th percentile Hausdorff distance (HD95) were used to characterize geometric changes. Median, interquartile range (IQR), and maximum displacement were reported.

Results: Ten to fifteen evaluable sessions were available per organ. Small bowel demonstrated the greatest geometric variability. Median ASSD for small bowel was 3.1 mm (IQR 0.9–4.0 mm), compared to 1.0 mm (IQR 0.5–1.7 mm) for duodenum and 0.9 mm (IQR 0.5–1.4 mm) for stomach. Localized extreme displacement (HD95) was substantially larger in small bowel (median 10.0 mm, IQR 5.1–13.9 mm), with a maximum observed displacement of 26.4 mm. In contrast, duodenum and stomach demonstrated median HD95 values of 4.7 mm (IQR 3.5-6.5 mm) and 5.3 mm (4.3-7.5 mm), respectively.

Conclusion: During abdominal adaptive radiotherapy under DIBH, clinically meaningful GI lumen motion occurs within the 30-60 minute adaptive workflow interval. These findings provide quantitative insight into intra-fractional GI motion and may inform margin selection for organs-at-risk during abdominal adaptive treatment.