3164 - Respiratory and GI Motility-Driven Organ Encroachment into High-Dose Fall-Off Regions in Pancreatic SBRT

Purpose/Objective(s):

To quantify the probability of gastrointestinal (GI) organ-at-risk (OAR) encroachment into intermediate-to-high dose fall-off regions during pancreatic SBRT. While respiratory motion is routinely managed, independent GI motility remains under-characterized. Although a standard 5-mm planning margin may ensure target coverage, abdominal motion can shift GI OARs into the 50-80% dose fall-off region, where small positional changes produce large dose increases. We hypothesize that combined respiratory and GI motion increases the risk of OAR encroachment into this high-dose fall-off regions.

Materials/Methods:

The motion-inclusive geometry of nine patients receiving pancreatic SBRT was evaluated. A physiology-informed abdominal motion model was constructed, integrating 4DCT-derived respiratory kinematics with simulated GI motility events (peristalsis, rhythmic segmentation, and high-amplitude propagating contractions). GI motility amplitudes were derived from published physiologic measurements and cine-MRI observations. To estimate encroachment into high-dose regions, geometric intersection analysis was performed by computing the overlapping volumes (cc) between the distorted OARs and uniform GTV expansion margins (1 to 20 mm in 1 mm increments). Median (50th percentile), 95th percentile, and maximum overlaps were calculated across composite respiratory–motility cycles (3 minutes, 1 frame/s).

Results:

Geometric encroachment risk varied substantially by OAR and patient anatomy. As detailed in the table below, a 5 mm expansion (standard PTV margin) resulted in minimal overlap for the stomach and small bowel but revealed a potential vulnerability for the duodenum. Crucially, as expansion margins increased to 10 mm, pushing deeper into the intermediate-to-high dose fall-off region, the overlapping volumes increased markedly across all evaluated organs.

Conclusion:

Within this 9-patient cohort, a 5 mm GTV expansion posed minimal geometric overlap risk for the stomach and small bowel, but presented a localized risk to the duodenum. Expanding to 10 mm revealed that the broader dose fall-off region is highly susceptible to motion-induced overlapping across all three OARs. These findings suggest that static PRV margins may underestimate dynamic duodenal risk in pancreatic SBRT. Motion-inclusive geometric modeling may improve patient-specific margin assessment near GI OARs.

Organ	Margin (mm)	Mean 50% Overlap (cc)	Mean 95% Overlap (cc)	Worst-case Max Overlap (cc)
Duodenum	5	0.96	1.94	8.39
Duodenum	10	6.46	8.37	24.90
Stomach	5	0.07	0.22	1.53
Stomach	10	0.92	1.44	6.50
Small Bowel	5	0.00	0.02	0.14
Small Bowel	10	0.08	0.33	1.83