ESTRO 2025 - Abstract Book

S4397

RTT - Treatment planning, OAR and target definitions

ESTRO 2025

Conclusion: The technique presented offered a successful path to treating palliative patients on the Halcyon system. PTV coverage statistics of generated plans were comparable to IMRT/VMAT plans of radical intent. Planning times were similar between automated ring-based IMRT planning and conventional TrueBeam VS planning techniques. In many cases multiple script runs were required but as the preconfigured site-specific templates are improved this issue is expected to lessen.

Keywords: Palliative,Halcyon,IMRT

4088

Digital Poster An approach to internal target volume margins for kidney SBRT on the MR-Linac Jennifer Dang 1 , Rachel M Glicksman 1,2 , Marc V Barcelona 2,3,4 , Peter Chung 1,2 , Enrique Gutierrez 1 , Joelle Helou 5 , Aran Kim 1 , Vickie Kong 1,2 , Andrew McPartlin 1,2 , Srinivas Raman 6 , Jeff D Winter 1,2 1 Radiation Medicine Program, Princess Margaret Cancer Centre, Toronto, Canada. 2 Department of Radiation Oncology, University of Toronto, Toronto, Canada. 3 Department of Radiotherapy, Jose R. Reyes Memorial Medical Center, Manila, Philippines. 4 Department of Radiation Oncology, Manila Doctors Hospital, Manila, Philippines. 5 Verspeeten Family Cancer Centre, London Health Sciences Centre, London, Canada. 6 BC Cancer - Vancouver, BC Cancer, Vancouver, Canada Purpose/Objective: Kidney SBRT on the MR-Linac leverages excellent MR soft tissue contrast and CINE MR motion tracking capabilities. One challenge is that the extent of motion captured with 3D MR imaging planning sequences is not well defined. Quantifying motion is crucial for determining patient-specific margins to ensure adequate target coverage during treatment. This study describes an ITV margin determination approach using multi-modality imaging and evaluates potential for daily ITV margin adaptation to account for inter-fraction target motion variability. Material/Methods: Motion data for eight patients enrolled on clinical trials who underwent kidney SBRT with abdominal compression on the MR-Linac from April 2021 to May 2024 were reviewed. The CT-generated ITVs (ITV-CT) were generated from the union of the gross tumour volume (GTV) contoured on 4DCT inhale and exhale phases and compared to GTVs contoured on an average 3D T2-weighted sequence (MRplanning) acquired on the MR-Linac. Treatment ITVs were required on the MRplanning when the GTV-MRplanning differed more than 1mm in length or 5% in volume compared to the 4DCT-generated ITV. Treatment superior-inferior ITV margins on the MRplanning were determined by superior-inferior motion measured on CINE imaging acquired during MR-Linac simulation. Any left-right/anterior posterior motion noted on 4DCT was incorporated into the 3D MR ITV margin. Online GTV motion was collected using 2D CINE imaging to determine the online-generated ITV margin adequacy. Descriptive statistics were utilized to compare target volumes and motion. Results: ITVs were generated for 7 of 8 patients with measurable GTV motion. Small GTV volume differences existed between GTV-MRplanning and average GTV-CT (mean=4.5%, sd=8.1%). GTV-MRplanning were volumetrically on average 20.6% (sd=13.5%) less than the ITV generated based on the 4DCT datasets. However, ITV-MRplanning were on average 22.6% (sd=24.5%) larger than ITV-CT. Differences in daily target motion from reference ranged from – 3.8mm to 5.1mm (Figure 1). ITV margins were greatest in the inferior-direction, with an average of 6.1mm (sd=1.2mm) across the seven patients that exhibited motion. ITVs were not modified during treatment for any patients.