ESTRO 2025 - Abstract Book

S1830

Clinical – Upper GI

ESTRO 2025

3454

Poster Discussion Improved PTV coverage and OAR sparing with online adaptive MR-guided radiotherapy in pancreatic cancer Paul Rogowski 1 , Hidehiro Hojo 2,1 , Mohamed A Schuman 1,3 , Frederick Fuchs 1 , Aurélie Gaasch 1 , Franziska Walter 1 , Chukwuka Eze 1 , Sebastian N Marschner 1,4 , Helmut Weingandt 1 , Vanessa da Silva Mendes 1 , Jan Hofmaier 1 , Kathrin Straub 1 , Maximilian Niyazi 5,1,6 , Claus Belka 1,3,4 , Stefanie Corradini 1 1 Department of Radiation Oncology, University Hospital LMU, Munich, Germany. 2 Department of Radiation Oncology and Particle Therapy, National Cancer Center Hospital East, Kashiwa, Japan. 3 Bavarian Cancer Research Center, BZKF, Munich, Germany. 4 German Cancer Consortium (DKTK), Partner Site Munich, Munich, Germany. 5 Department of Radiation Oncology, University Hospital Tübingen, Tübingen, Germany. 6 German Cancer Consortium (DKTK), Partner Site Tübingen, Tübingen, Germany Purpose/Objective: Ablative doses in stereotactic radiotherapy for pancreatic ductal adenocarcinoma (PDAC) are often limited by the proximity of radiosensitive organs at risk (OAR), which also exhibit significant daily positional variability. This study aims to investigate the benefit of online-adaptive MR-guided radiotherapy (oMRgRT) on target coverage and OAR dose sparing in patients with PDAC. Material/Methods: Consecutive patients with PDAC treated with oMRgRT at our department since 2020 were retrospectively analyzed. Adaptive planning was evaluated by comparing non-adapted (PLAN PREDICT ) and adapted (PLAN REOPTIMIZED ) treatment plans. For each treatment fraction, planning target volume (PTV) coverage and OAR constraints were analysed. Plan adaptation was categorized as "not necessary" when PLAN PREDICT already fulfilled all treatment plan objectives, as "useful" when the PLAN REOPTIMIZED fulfilled all requirements or significantly improved dose coverage with the prescribed dose or high-dose OAR sparing by ≥ 10%, and as "futile" when PLAN REOPTIMIZED failed to fully meet or substantially improve upon these constraints. Results: A total of 58 patients were treated over five fractions, comprising 290 adaptive planning fractions. The prescribed doses were 40 Gy for 44/58 patients (76%) and 33 Gy for 14/58 patients (24. The PLAN PREDICT met all planning goals in only 3% of fractions, while PLAN REOPTIMIZED increased the percentage to 72%. Full plan re-optimization significantly improved PTV coverage (p<0.001) and significantly reduced the volume receiving 33 Gy (V 33Gy ) of stomach (median 0.28 cc vs. 0.01 cc), duodenum (median 1.03 cc vs. 0.07cc) and bowel (median 1.81 cc vs. 0.06cc). Adaptation was considered useful in 277 fractions (95.6%), not necessary in 5 fractions (1.7%) and futile in 8 fractions (2.8%).