ESTRO 2025 - Abstract Book

S3293

Physics - Intra-fraction motion management and real-time adaptive radiotherapy

ESTRO 2025

3056

Digital Poster Accuracy analysis of the 2D-Cine imaging based rigid target tracking for prostate cancer patients at the 1.5 T MR-Linac system Marcel Nachbar 1 , Monica Lo Russo 2 , Moritz Schneider 1 , David Mönnich 1 , Simon Böke 2 , Cihan Gani 2 , Sarah Kübler 2 , Jessica Boldt 2 , Maximilian Niyazi 2 , Daniela Thorwarth 1 1 Section for Biomedical Physics. Department of Radiation Oncology, University Hospital Tübingen, Tübingen, Germany. 2 Department of Radiation Oncology, University Hospital Tübingen, Tübingen, Germany Purpose/Objective: The 1.5 T MR-Linac (Unity, Elekta) now allows tumor tracking based on sagittal and coronal 2D cine MRIs with an online motion monitoring (OMM) system, facilitating intrafraction plan adaptation based on organ movement. This requires a rigid translational registration of a target structure across sequential images. However, such a structure propagation along 2D-images lacks the information of a 3D-MRI and does not account for target rotation. Therefore, this study aimed to assess the accuracy of the OMM tracking system for prostate cancer patients by comparing rigid OMM-based tracking and pre- and post-treatment 3D-MRI-based prostate registration. Material/Methods: For a total of 18 online adaptive treatment sessions from six prostate cancer patients, high-resolution 3D-MRIs were acquired pre- and post-treatment. For each session a board-certified physician with extensive MR-Linac experience performed a rigid registration, including rotations, between the respective images. For comparison the sequential 2D cine MRIs, acquired during each treatment session were analyzed using a workflow emulator software (Elekta), with the clinical target volume (prostate and 1 cm proximal seminal vesicles) as the tracked structure. The OMM time-position datapoints were processed in MATLAB (V2024a) and extrapolated to the expected timepoint of the post-treatment MRI using a linear regression. The differences between the OMM predicted target offsets and the 3D-MRI registration offset were assessed. An exemplary case is illustrated in figure 1.