ESTRO 36 Abstract Book

S469 ESTRO 36 _______________________________________________________________________________________________

this study was to develop and validate a 4D-MRI guided mid-position (midP) correction strategy on an MR-Linac. Material and Methods Experiments were performed on an MR-Linac (ATL1, Elekta AB, Sweden), using the CIRS MRI-LINAC Dynamic Phantom (CIRS Inc., USA). The moving cylinder was filled with anisotropic MRI contrasts and a Perspex spherical target. Motion was performed in CC direction using a Lujan 4 motion pattern with a 20mm amplitude and 4s period. First, a T2-weighted MRI scan was acquired in midP. The cylinder and target were segmented and the target was expanded with a non-uniform margin (LR, AP:10mm; CC:20mm). A density overwrite of 1 was assigned to the structures and a treatment plan consisting of a single anterior beam shaped around the PTV was created in Monaco (Version 5.19.01 Research). Then, baseline shifts in CC direction of 5, 10, 15 and 20mm were applied to the phantom motion. For every shift, a retrospective self- sorted 4D-MRI was acquired (axial single-shot TSE, 2x2x5mm 3 , TE/TR=60/400ms, 30dyn) and each phase was registered to the midP reference image to calculate the time average displacement. The plan was adapted accordingly, performing a virtual couch shift (simple dose shift) using aperture morphing in Monaco. All plans were delivered while electronic portal imaging device (EPID) cine images were acquired. The time average displacement of the target was calculated from the EPID images and geometric accuracy of the workflow was quantified as the distance of the average position of the target to the field edges in the EPID images. Results In Figure1, MRI and EPID images of the midP and a shifted inhale and exhale position are shown. Table1 shows the time average displacement of the target in the 4D-MRI and the EPID images with respect to the reference as well as the distance of the average target position to the field edges. The geometric accuracy of the 4D-MRI guided workflow was 0.3±0.4mm in CC, which includes the 4D-MRI registration accuracy.

Kaas, Natasja Janssen, Ben Floot and Marco van den Berg (NKI). PO-0862 Correlation of Liver and Pancreas Tumor motion with Normal Anatomical Stru ctures R. Kaderka 1 , A. Paravati 1 , R. Sar kar 1 , J. Tran 1 , K. Fero 1 , N. Panjwani 1 , D. Simpson 1 , J. Murphy 1 , T. Atwood 1 1 University of California San Diego, Department of Radiation Medicine and Applied Sciences, San Diego, USA Purpose or Objective Target motion caused by respiration remains the central challenge to delivering SBRT in the abdomen. For targets in the pancreas and liver, SBRT oftentimes necessitates placement of metal fiducials to determine tumor position with fluoroscopy, due to difficulty in visualizing tumors on non-contrast imaging. Metal fiducials have limitations in that they represent an invasive procedure which can introduce treatment delays. Furthermore, fiducials can migrate from their intended position, and the metal can introduce imaging artifacts which make tumor delineation a challenge. We hypothesized that upper abdominal tumor motion would correlate with the motion of nearby organs and could thereby serve as a fiducial-less proxy for tumor motion. Material and Methods Fifteen patients (12 with pancreas and 3 with liver tumors) underwent a 4-dimensional (4D) CT simulation prior to treatment with SBRT. 4D CT images were divided into 10 phases and normal tissues were contoured on a single 4D- CT phase and propagated to the other phase s using deformable image registration. As a means of quality control for image registration and contour propagation the liver was manually contoured on all phases for 5 patients by physicians and compared to the automated contour propagation using a Dice coefficient. Motion was defined from the center-of-mass of each structure, and a patient- specific linear tumor position prediction model based on liver position was developed. Results We found a strong overlap of manually entered contours and the automatically segmented contours with a mean Dice-coefficient of 0.95 (standard deviation 0.01). The linear models accurately predicted tumor motion with a mean absolute error of 0.5 mm and no error greater than 3.0 mm. Mean absolute and maximum errors by direction and tumor type are listed in the table below.

Anterior- posterior direction

Superior- inferior direction

Left-right direction

Pancreas tumors mean absolute error (mm) Pancreas tumors maximum error (mm) Liver tumors mean absolute error (mm)

0.3

0.4

0.5

1.0

1.7

2.6

0.3

0.4

0.8

Liver tumors maximum error (mm)

1.4

2.7

3.0

Conclusion This study demonstrates that normal organ motion could serve as a fiducial-less proxy for tumor motion with SBRT in the upper abdomen when on-site real-time 4D volumetric imaging becomes available during treatment. Deformable image registration has been demonstrated to be a reliable and fast tool for segmentation of normal organs. Moving this motion management approach into clinic requires additional research to optimize 4D image quality and understand inter-fraction reproducibility. PO-0863 Suggestion of optimal planning target volume margins for stereotactic body radiotherapy of the spine

Conclusion 4D-MRI guidance on an MR-Linac was shown to be feasible and had sub-millimeter accuracy. Such a correction strategy has great potential for moving targets that are difficult to visualize on alternative image guidance modalities. Acknowledgements: This research was partly sponsored by Elekta AB, Stockholm, Sweden. The authors would like to thank CIRS Inc., Robert Spaninks (Elekta) and Jochem