Longitudinal assessment of interfractional geometrical variations of gastrointestinal organs in patients undergoing abdominal radiotherapy.

Abstract

e16457 Background: Significant anatomical variations of organs-at-risk (OARs), particularly the stomach and duodenum, pose major challenges in abdominal radiotherapy. This study aimed to quantitatively characterize the interfractional geometric stability of these organs during a standard treatment course to provide a scientific basis for adaptive planning and margin optimization. Methods: We retrospectively analyzed 10 patients with abdominal cancer who received radiotherapy. For each patient, diagnostic-quality kV-level fan-beam CT (kV-FBCT) scans were performed daily before each treatment fraction using an integrated linac system (uRT-linac 506). A total of 110 serial scans—including the initial planning CT (pCT) and 10 repeat scans per patient sampled during the treatment course—were retrieved. The stomach and duodenum were manually re-contoured on all scans by senior radiation oncologists. Geometric variations were evaluated using: 1) Volumetric change (ΔV%); 2) Surface-based agreement (MDA and HD); 3) Spatial overlap assessment (DSC). Results: A total of 110 daily kV-FBCT scans were evaluated. The stomach exhibited profound geometric instability, with a mean DSC of 0.62 ± 0.11 and extreme volumetric expansion up to 299.81% (mean ΔV%: 45.08% ± 60.66%). The mean MDA and HD for the stomach were 6.18 ± 3.17 mm and 26.55 ± 12.41 mm, respectively. The duodenum showed even poorer spatial overlap (mean DSC: 0.52 ± 0.12). These geometric variations remained unpredictable and significant throughout the treatment course, with no trend of anatomical stabilization observed. Conclusions: During abdominal radiotherapy, the stomach and duodenum undergo substantial and unpredictable interfractional shifts. The low DSC values and large Hausdorff distances observed via daily kV-FBCT suggest that static planning margins are insufficient. Our quantitative data underscore the necessity of online anatomical assessment and adaptive strategies to ensure safe treatment delivery near gastrointestinal structures.