ESTRO 2025 - Abstract Book

S1857

Clinical – Upper GI

ESTRO 2025

Conclusion: The Decision Tree model developed based on PTV D50%, IGRT method, and tumor volume demonstrated strong predictive performance for LC following SBRT for liver metastases and robust validation through bootstrap resampling. The model showed high recall for local control failure and can help guide clinical decision-making.

Keywords: SBRT, predictive modeling

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Digital Poster 5-Fraction Stereotactic MRI-Guided Adaptive Radiotherapy Targeting the Primary Tumor for metastatic Pancreatic Cancer Mohamed A Schumann 1,2 , Aurelie Gaasch 1 , Sandra Boyaci 1 , Frederik Fuchs 1 , Franziska Walter 1 , Sebastian N Marschner 1,3 , Chukwuka Eze 1 , Christoph Benedikt Westphalen 4 , Georg Beyer 5 , Julian Holch 4 , Bernhard W Renz 6 , Maximilian Niyazi 7,8,1 , Claus Belka 1,2,3 , Stefanie Corradini 1 , Paul Rogowski 1 1 Department of Radiation Oncology, LMU University Hospital, Munich, Germany. 2 Bavarian Cancer Center, Partner Site Munich, Munich, Germany. 3 German Cancer Consortium (DKTK), Partner Site Munich, Munich, Germany. 4 Department of Medicine III, LMU University Hospital, Munich, Germany. 5 Department of Medicine II, LMU University Hospital, Munich, Germany. 6 Department of General, Visceral and Transplantation Surgery, LMU University Hospital, Muncih, Germany. 7 Department of Radiation Oncology, University Hospital Tübingen, Tübingen, Germany. 8 German Cancer Consortium (DKTK), Partner Site Tübingen, Tübingen, Germany Purpose/Objective: Metastatic pancreatic ductal adenocarcinoma (mPDAC) carries a poor prognosis and significant morbidity in cases of local tumor progression. This single-center prospective observational study aims to evaluate the safety and efficacy of five-fraction stereotactic MR-guided adaptive radiotherapy (MRgRT) for primary tumor control in patients with mPDAC. Material/Methods: Consecutive patients with newly diagnosed or metachronous mPDAC treated with MRgRT for the primary tumor at LMU University Hospital were prospectively included. All treatment indications were validated by multidisciplinary tumor boards. Primary outcomes were overall survival (OS) and local control (LC), analysed using Kaplan-Meier methodology. Subgroup analyses evaluated outcomes according to metastatic burden (oligometastatic [≤5 metastases] vs. polymetastatic), number of affected organs (single vs. multiple organs or peritoneal carcinomatosis), and post-chemotherapy response of primary tumor and metastases (primary stable vs. progressive and extrapancreatic disease stable vs. progressive distant metastases, respectively) (table 2). Adverse events were graded prospectively using CTCAE v5. Results: Twenty-three mPDAC patients treated with five-fraction MRgRT using a 0.35T MRIdian system (ViewRay Inc.) between January 2020 and December 2023 were included (table 1). Dose prescription was either a uniform dose of 33 Gy or 33 Gy to an extended target volume with a simultaneous integrated boost of 40 Gy to the macroscopic tumor, prescribed to the 80% isodose line. Palliative chemotherapy preceding MRgRT was given to 91% of patients (n=21). Median follow-up was 13 months. Median OS following MRgRT was 12 months, with a one-year OS rate of 48%. Median OS from diagnosis of metastatic disease was 20 months. The one-year LC rate was 92%, with only two cases of locoregional recurrence. Oligometastatic patients had a better median OS of 13 months compared to 6 months for polymetastatic patients, though not statistically significant (p=0.4). Patients with single-organ metastases had a significantly higher median OS (14 months) than patients with multi-organ involvement (5 months, P=0.0004). Patients with stable extrapancreatic