ESTRO 2023 - Abstract Book

S650

Monday 15 May 2023

ESTRO 2023

Conclusion On the central axis slice, an automated tumour signal (PCA) is well correlated with observed tumour COM changes. When used to reconstruct the tumour volume, stitching artefacts are less for PCA than those observed for other signals. OC-0780 Delivered dose reconstruction for MR-guided SBRT of pancreatic tumors with fast 3D cine MRI G. Grimbergen 1 , G. Pötgens 2 , H. Eijkelenkamp 1 , B. Raaymakers 1 , M. Intven 1 , G. Meijer 1 1 University Medical Center Utrecht, Department of Radiation Oncology, Utrecht, The Netherlands; 2 Eindhoven University of Technology, Department of Biomedical Engineering, Eindhoven, The Netherlands Purpose or Objective Intrafraction motion can be a concern for safe and efficient SBRT of pancreatic tumors. In MR-guided SBRT, the tumor motion during beam-on is typically monitored with (interleaved) 2D cine MRI. However, the tumor surroundings are not captured in these images, and tumor motion might be distorted by through-plane movement. In this study, the feasibility of using highly accelerated 3D cine MRI to reconstruct the delivered dose during MR-guided SBRT was assessed. Materials and Methods A 3D cine MRI sequence was developed for fast, time-resolved 4D imaging. The sequence features a low spatial resolution (5x5x6 mm ³ ), allowing for rapid volumetric imaging at 430 ms. The 3D cines were acquired during the entire beam-on time of 23 fractions, delivered in five different patients that underwent online adaptive MR-guided SBRT for pancreatic tumors in a 5x8 Gy regimen on a 1.5 T MR-Linac. For every cine dynamic, a 3D deformation vector field (DVF) was extracted using deformable image registration. The linac log files were used to calculate the partial dose plans that had been delivered in the time interval between consecutive cine acquisitions. Each partial dose plan was warped with the corresponding DVF using energy-mass transfer, and afterwards summed to obtain a total delivered dose (see fig. 1). Tumor motion was determined by warping the GTV contour with the same DVF. Key DVH parameters of the GTV and neighboring OARs (duodenum, small bowel and stomach) were calculated in the delivered dose and compared to the planned dose.