ESTRO 2025 - Abstract Book

S3179

Physics - Inter-fraction motion management and offline adaptive radiotherapy

ESTRO 2025

3471

Digital Poster Dosimetric impact and feasibility of daily adaption on ultra- hypofractionated MR-guided SABR for pancreatic cancer: EMERALD Phase 1 Trial Ben George 1,2 , Elena Moreno-Olmedo 1,3 , Suliana Teoh 3 , Robert Owens 3,1 , Lynda Swan 4 , Tim Maughan 2,5 , James Good 1,6 , Somnath Mukherjee 2,1 1 Oncology, GenesisCare UK, Oxford, United Kingdom. 2 Department of Oncology, University of Oxford, Oxford, United Kingdom. 3 Department of Oncology, Oxford University Hospitals NHS Foundation Trust, Oxford, United Kingdom. 4 Oncology Clinical Trials Office, University of Oxford, Oxford, United Kingdom. 5 Institute of Systems, Molecular & Integrative Biology, University of Liverpool, Liverpool, United Kingdom. 6 Department of Oncology, University Hospitals Birmingham, Birmingham, United Kingdom Purpose/Objective: The EMERALD trial investigated the dosimetric impact and feasibility of stereotactic MR-guided adaptive radiotherapy (SMART) for pancreatic cancer, using three ultra-hypofractionated schedules: five-, three-, and single fraction regimens. For each study arm, technical treatment parameters were documented, and dosimetric comparisons were made between baseline, predicted, and reoptimised plans. Dosimetric metrics included: changes in the planning target volume (PTV) coverage (PTV V(100%) and PTVHigh V(95%)); prescription dose spillage; and, doses to gastrointestinal organs at risk (GI OARs). These comparisons evaluate the precision and benefits of adaptive RT. The trial measured total in-room time for each session to demonstrate the feasibility of SMART in managing pancreatic cancer, which presents unique challenges in adaptive radiotherapy. Material/Methods: This study is a single-centre, phase 1, non-randomised, three-arm safety trial investigating SMART in patients with localised or locally recurrent pancreatic cancer. Three cohorts were investigated, each receiving a different SMART regimen: 50 Gy in 5 fractions, 39 Gy in 3 fractions, and 25 Gy in a single fraction. For each arm, dose-volume histogram (DVH) parameters were obtained from the treatment planning system for baseline, predicted, and reoptimised plans for the fractions available. Additionally, technical treatment parameters, such as beam-on time, number of segments, monitor units, and number of beams, were collected from treatment plan documentation. Results: Between August 2022 and October 2023, the EMERALD trial delivered SMART to twenty patients with pancreatic cancer. A total of 49 treatment fractions were delivered, all successfully utilising daily adaptation. Technical parameters for the treatment plans were extracted from documentation files (Table 1). Dosimetric data were available for 37 fractions (Table 2), allowing analysis of treatment plan quality and safety. After daily adaptation, all re-optimized plans adhered to mandatory dose constraints for organs-at-risk (OARs), while maintaining PTVHigh V(95%) coverage across all dose levels. Additionally, gastrointestinal (GI) OAR doses were reduced in 77% of adapted fractions, with an average of 1.90 cc (range -0.08 – 16.48) reduction in GI OAR volume receiving the tolerance isodose across all plans, demonstrating improved sparing of healthy tissue with adaptive planning. Conclusion: The EMERALD trial demonstrated that SMART effectively maintained optimal dosimetry for ultra-hypofractionated treatments, while successfully reducing GI OAR doses and achieving high plan quality. Additionally, the total in-room times required for each session were found to be manageable, supporting the feasibility of delivering this adaptive treatment in a clinical setting without excessive time burden.

Keywords: pancreatic cancer, SABR, adaptive radiotherapy