ESTRO 2025 - Abstract Book

S3175

Physics - Inter-fraction motion management and offline adaptive radiotherapy

ESTRO 2025

3432

Digital Poster A Pilot Study on Passive Adaptive SABR Treatment for Pancreatic Cancer

Sriram Padmanaban 1 , Maxwell Robinson 1 , Andrew Buckle 1 , Sarah Ruane 2 , Robert Owens 3 , Somnath Mukherjee 3 1 Radiotherapy Physics, Oxford University Hospitals NHS Foundation Trust, Oxford, United Kingdom. 2 Radiotherapy, Oxford University Hospitals NHS Foundation Trust, Oxford, United Kingdom. 3 Clinical Oncology, Oxford University Hospitals NHS Foundation Trust, Oxford, United Kingdom Purpose/Objective: The proximity of pancreas to GI tract, motion management, and GTV/GI tract visualisation on pre-treatment cone beam CT (CBCT) pose significant challenges in pancreatic SABR treatments. Subsequently, some form of mitigation strategy to avoid delivery of excessive GI tract dose is mandated within the clinical practice with daily online adaptation being considered the gold standard. In the absence of online adaptive radiotherapy platform at local centre, a passive adaptive process was piloted to allow safe SABR delivery and minimise replans or treatment cancellations. Material/Methods: Two plans were created for each patient: SABR plan (40Gy/5#) and backup conventional ICRU plan (35Gy/5#). SABR plan, with a peaked dose distribution meeting the UK 2022 SABR consensus OAR constraints, was delivered preferentially. Backup plan had PTV D 99% > 33.25Gy and D max ≤ 36.75Gy. To help CBCT matching, an isodose that touched the GI tract on planning CT was copied to an IGRT structure. During treatment days, if GI tract overlapped with the IGRT structure on CBCT, the patient was taken off the bed and repeat CBCT was attempted after a short gap. If unsuccessful, the backup ICRU plan was delivered. The total delivered dose and homogeneity index (HI) (D 0.1cc /D99%) to GTV was estimated after the course was completed. Results: The pilot study included 16 patients (Fig 1). Full SABR plan was delivered to 8 patients. 7 patients had a mix of SABR and backup ICRU plans. In these 7 patients, taking patients off the bed and repeating CBCT after a gap allowed delivery of SABR plan in 9/18 fractions. One patient had a highly complex overlap of GI tract with PTV and received backup ICRU plan on all 5#s. The mean (range) delivered D 99% and D 0.1cc dose to GTV were 38.5Gy (33.9 to 46.4Gy) and 46.9 (36.2 to 51.1Gy) respectively. The delivered vs. intended HI was 1.22±0.13 vs. 1.27±0.11. There were no attempts to replan within the cohort.