ESTRO 2023 - Abstract Book

S615

Monday 15 May 2023

ESTRO 2023

Conclusion CT-STAR resolved strict OAR constraints while maintaining adequate tumor coverage when compared to non-adapted SBRT plans for patients with UC thoracic tumors. A phase I prospective protocol is currently under development to evaluate the safety of this treatment planning paradigm for patients with UC early-stage NSCLC.

PD-0750 Systematic investigation of setup positioning for MRI-guided RT of the upper extremity M. Cobanaj 1,2 , S. Schneider 1,2 , E. Troost 1,2,3 , A. Hoffmann 1,2,3

1 OncoRay-National Center for Radiation Research in Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany; 2 Institute of Radiooncology-OncoRay, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany; 3 Department of Radiotherapy and Radiation Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden, Dresden, Germany Purpose or Objective Cone-beam CT studies have shown that extremity soft-tissue sarcoma (ESTS) volumes may change substantially during radiotherapy (RT), leading to plan adaptations. MRI-guided RT is expected to improve the targeting precision in ESTS patients, as it provides superb soft-tissue contrast. Closed-bore high-field (HF) MRI scanners are most adopted in clinical settings. However, the versatility of open low-field (LF) MRI scanners makes them a worthwhile candidate to be investigated for MRI-guided proton therapy (PT). The present study aims to evaluate the setup reproducibility for the upper extremity positioning in both LF and HF MRI scanners. Materials and Methods Five healthy volunteers (BMI 20.5±2.2) laid both supine and prone with neutral and pronation forearm positions in the HF and LF MRI scanners. Participants laid with their arm stretched above the head on the HF scanner and abducted 90° on the LF scanner. Additionally, for two of them, supine positioning with the arm next to body in the HF MRI was possible. An in- house developed arm holder with a 180° hand rotation range allowed reproducible positioning on the MRI couches. In a systematic study, setup positioning comparison within and between HF scans and LF scans was performed. Position differences, soft-tissue deviations, and Dice similarity coefficients (DSC) were derived after image registration. Setup differences were expressed as two couch shifts and rotation values ( Δ x, Δ y, Δα ). Soft-tissue deviations were defined as skin-to-bones distances at angles of 30° ( Δ dsb) (Figure 1).