ESTRO 2021 Abstract Book

S695

ESTRO 2021

Conclusion Overall, kidney motion velocities during PIBH and PEBH, and in sessions 1 and 2 were not significantly different, but showed large inter-subject variations. Because of the kidney motion velocity of ~5mm/min during PBH, methods to suppress residual and respiratory motion have to be explored. PD-0861 Intrafraction pancreatic tumor motion patterns during ungated MR-guided SBRT with abdominal corset G. Grimbergen 1 , H. Eijkelenkamp 1 , H. Heerkens 1 , B. Raaymakers 1 , M. Intven 1 , G. Meijer 1 1 University Medical Center Utrecht, Department of Radiation Oncology, Utrecht, The Netherlands Purpose or Objective Stereotactic Body Radiotherapy (SBRT) has been shown to be a promising therapy for non-metastatic unresectable pancreatic tumors, which account for 40% of all pancreatic carcinomas. However, intrafraction motion, caused by respiratory motion and organ drift, is the main concern for efficient dose delivery in ungated upper abdominal RT. The aim of this study is to characterize respiration and drift motion patterns of pancreatic tumors, across a clinical cohort of 13 patients that underwent MRgRT while wearing an abdominal corset. Materials and Methods We included 13 patients that underwent online adaptive MR-guided SBRT for malignancies in the pancreatic region (5x8Gy) on the MR-Linac. An abdominal corset was fitted, which has been shown to substantially reduce respiratory motion [1]. Interleaved coronal and sagittal Cine MRs of the tumor region were made at 2 Hz during the entire beam-on time of each fraction. We used deformable image registration to obtain GTV motion profiles, with position over time in the cranio-caudal (CC C , CC s : in both the coronal and sagittal plane), anterior-posterior (AP), and left-right (LR) direction. To isolate the motion caused by respiration and baseline drift, the motion profiles were respectively high-pass and low-pass filtered. From these decoupled motion profiles, the respiratory amplitude and maximum and minimum baseline drifts were extracted.