3050 - Deep Learning Auto-Segmentation for Quantification of Inter-Fraction GI Organ Positional Uncertainty on Serial Abdominal MRI
Presenter(s)
J. Lasington1, L. S. Mathew Lasington2, S. Umamaheshwaran3, and J. Thomas4; 1New York Medical College at St. Mary's and St. Clare's, Denville, NJ, 2Rutger's University, East Hanover, NJ, 3Personio, Manhattan, NY, 4Christian Medical College, Vellore, India
Purpose/Objective(s): To develop a deep learning auto-segmentation model for gastrointestinal organs at risk on abdominal MRI and to quantify the magnitude of inter-fraction organ positional uncertainty across serial scans in patients undergoing radiotherapy.
Materials/Methods: A U-Net with EfficientNet-B4 encoder was trained on the UW-Madison GI Tract dataset (107 patients, 64,368 MRI slices) with expert contours of stomach, small bowel, and large bowel. Segmentation was evaluated using Dice similarity coefficient (DSC) and 95th-percentile Hausdorff distance (HD95) on a held-out test set (193 scans). Centroid localization accuracy was assessed against expert contours on the same scans. Inter-fraction displacement was quantified using expert-contour 3D centroids across 96 patients with serial imaging (median 5 timepoints/patient), with directional decomposition (AP, LR, CC). Van Herk margins (M=2.5S+0.7s) were calculated and compared against published planning margins (RTOG 10mm, MR-Linac 3mm).
Results: Auto-segmentation achieved DSC of 0.91 (stomach), 0.88 (large bowel), 0.78 (small bowel), with centroid localization error of 4.4-11.4mm vs expert contours. Expert-contour inter-fraction 3D displacement was substantial: 41.0±26.9mm (large bowel), 37.4±27.0mm (stomach), 36.6±21.1mm (small bowel). At short intervals (=7 days), displacement remained 28-34mm. The CC axis contributed the largest component (25.9-30.5mm). 98% of patients (92% of fractions) exceeded 10mm displacement. Van Herk margins were 57-75mm, far exceeding conventional 10mm CTV-PTV margins, which were exceeded in 96% of fractions.
Conclusion: Deep learning auto-segmentation achieved clinically acceptable GI organ accuracy with centroid localization within 4-11mm of expert contours. Inter-fraction positional uncertainty was substantial, exceeding standard margins in nearly all patients regardless of time interval. These findings support routine daily online adaptive replanning in MR-guided abdominal radiotherapy, as conventional margins are insufficient for observed organ displacement.
Segmentation Accuracy, Inter-Fraction Displacement, and Calculated Margins
DSC - Dice Similarity Coefficient,HD95 -95th-percentile Hausdorff Distance,Centroid Err - 3D distance between centers of auto-segmented organ and expert-contour,3D Disp-3D displacement,AP -Anterior-Posterior,LR -Left-Right, CC -Cranio-Caudal ,=7d Disp- Displacement measured between scans =7 days apart, >10mm -Percentage of patients/fractions exceeding 10mm displacement threshold,vH M - Van Herk margin| Metric | Stomach | Small Bowel | Large Bowel | AP (mm) | LR (mm) | CC (mm) | =7d Disp | >10mm | vH M |
| DSC | 0.91 | 0.78 | 0.88 | - | - | - | - | - | - |
| HD95 (mm) | 11.4 | 25.4 | 24.3 | - | - | - | - | - | - |
| Centroid Err | 4.4±3.7 | 10.7±8.9 | 11.4±13.8 | - | - | - | - | - | - |
| 3D Disp (mm) | 37.4±27.0 | 36.6±21.1 | 41.0±26.9 | - | - | - | - | - | - |
| Directional | - | - | - | 14.0 | 20.8 | 25.9 | - | - | - |
| =7d Disp | 28.1±23.3 | 28.6±20.4 | 33.7±26.2 | - | - | - | - | 92% | - |
| S (mm) | 22.1 | 19.2 | 25.1 | - | - | - | - | - | - |
| s (mm) | 19.0 | 12.7 | 17.7 | - | - | - | - | - | - |
| vH M | 68.5 | 56.9 | 75.1 | 36.2 | 54.6 | 81.0 | 73-84 | 98%pts | >>10 |