From contouring variability to dosimetric impact: a comprehensive assessment of online adaptive MR-guided SBRT for pancreatic cancer.
Poeta S, Gulyban A, Paquier Z, Michel M, Bodson E, den Begin RV (+3 more)
Abstract
PurposeTo quantify inter-physician contouring variability and evaluate its dosimetric impact in pancreatic ductal adenocarcinoma (PDAC) patients treated with daily online adaptive MR-guided stereotactic body radiotherapy (MRgSBRT), to assess safety and clinical robustness of online contouring.MethodsFifty treatment sessions from ten PDAC patients treated with 50 Gy in five fractions on a 1.5 T MR-Linac were retrospectively analyzed. During each treatment session ("online"), targets and organs-at-risk (OARs) within 3 cm of the tumor were delineated by seven physicians working on the MR-Linac. All structures were re-contoured offline by a single pancreatic-expert radiation oncologist ("reference"). Geometric agreement was assessed using Dice Similarity Coefficient (DSC), Mean Distance to Agreement (MDA), and 95 % Hausdorff Distance (HD95). Dosimetric analyses were performed on delivered treatment plans using both online and reference contours, with evaluation based on fraction-wise worst-case dose metrics.ResultsHigh geometric concordance was observed for stomach and duodenum, whereas smaller or anatomically complex structures (GTV, major vessels, bile duct) showed greater variability. Online GTVs were larger than reference contours. Despite these differences, target coverage (V50Gy and V35Gy) was maintained with online contours. While occasional per-fraction gastrointestinal (GI) dose constraint exceedances were identified using reference contours, fraction-wise worst-case analysis showed GI doses remained within clinical tolerance for nearly all patients.ConclusionDaily online adaptive MRgSBRT for PDAC provides robust target coverage and clinically acceptable OAR sparing, despite contouring variability. Multi-physician online contouring appears safe in clinical practice, although further improvements may be achieved through standardized contouring guidelines and harmonization strategies.