ESTRO 2025 - Abstract Book

S1809

Clinical – Upper GI

ESTRO 2025

2795

Digital Poster Impact of reduced PRV margins on OAR doses for pancreas SABR treatments using the Elekta Unity MR Linac Hubert Stankiewicz 1 , Rachael Hall 1 , Andrew Brocklehurst 2 , Linnéa Freear 1 , Philip Whitehurst 1 , Ganesh Radhakrishna 2 , Robert Chuter 1,3 1 Christie Medical Physics and Engineering, The Christie NHS Foundation Trust, Manchester, United Kingdom. 2 Clinical Oncology, The Christie NHS Foundation Trust, Manchester, United Kingdom. 3 Faculty of Biology, Medicine and Health, The University of Manchester, Manchester, United Kingdom Purpose/Objective: SABR treatments for pancreatic cancer patients deliver high fractional doses in immediate proximity to dose sensitive gastrointestinal (GI) OARs. MRgART exploits improved soft tissue contrast to accurately delineate OARs and facilitate online plan adaptation. GI OARs display significant intra-fraction motion and therefore PRVs are used to mitigate risk of exceeding the GI OARs tolerances. This study investigates the impact of reduced PRV margins on OAR doses during MRgART pancreas SABR treatments. Material/Methods: This study analysed 10 patients (50 fractions) treated for pancreatic cancer with 40Gy/5# SABR on MR-Linac. Offline reference IMRT plans were generated using Elekta Monaco v.5.51.11 TPS and maintained 5mm PRV doses within OAR tolerance (D 0.5cc ≤35Gy for duodenum, stomach, and small bowel, ≤38Gy for large bowel) [1]. Reference plans were adapted online using adapt-to-shape (ATS) workflow. After each fraction, OARs were re contoured on post-treatment MRI (MRI post ) to estimate the D 0.5cc delivered to GI OARs. To investigate the impact of reducing PRV margins, offline reference plans were retrospectively re-optimised using 3mm PRV (10 patients) and no PRV (5 patients), before being re-adapted to daily images via ATS. Finally, ATS-adapted plans were re-calculated on appropriate MRI post to evaluate predicted OAR doses. The fractional and cumulative OAR D 0.5cc doses in 5mm PRV, 3mm PRV and no PRV re-calculated plans were compared to OAR tolerances. The fractional dose was scaled to the prescribed 5 fractions. Results: Reducing PRV margins increased the instances of scaled fractional OAR D 0.5cc and cumulative OAR D 0.5cc exceeded OAR tolerance ( Fig.1 ). Notably, duodenum D 0.5cc exceeded the scaled tolerance in 4 (8% of analysed), 10 (20%) and 16 (64%) fractions with 5mm PRV, 3mm PRV and no PRV, respectively. Cumulative OAR D 0.5cc remained within tolerance for all patients with 5mm PRV. Estimated OAR D 0.5cc would have exceeded tolerance for 3(30%) patients for 3mm PRV plans and for 4(80%) patients for no PRV plans ( Fig.1 ). Median and range values of cumulative OAR D 0.5cc are detailed in Table.1 . PTV coverage (V 40Gy [%]) improved reducing PRV margins in offline reference plans, median values: 62.7%, 71.1%, 81.3% for 5mm PRV, 3mm PRV, no PRV, respectively. Conclusion: This study estimates the dosimetric impact of reducing PRV margins on delivered OAR doses. The clinically implemented technique (5mm) ensured OAR D 0.5cc remained within tolerance. Delivering MRgART without PRVs results in exceeding tolerance doses. Although 3mm PRV are unlikely to account for OAR intra-fraction motion in all cases, it provides improved balance between OAR dose control and target coverage.