ESTRO 2020 Abstract Book

S292 ESTRO 2020

PH-0530 End-to-end measured geometric dose delivery accuracy and precision in MR linac adapted radiotherapy U. Bernchou 1,2 , R.L. Christiansen 1,2 , D. Tilly 3,4 , A. Bertelsen 1 , H.L. Riis 1 , H.R. Jensen 1 , F. Mahmood 1,2 , C. Brink 1,2 1 Odense University Hospital, Laboratory of Radiation physcis, Odense, Denmark ; 2 University of Southern Denmark, Department of Clinical Research, Odense, Denmark ; 3 Uppsala University, Medical Radiation Physics- Department of Immunology- Genetics and Pathology, Uppsala, Sweden ; 4 Akademiska Sjukhuset, Department of Medical Physics, Uppsala, Sweden Purpose or Objective The purpose of the current study was to develop an MR compatible end-to-end (E2E) phantom system and thereby measure the geometric dose delivery accuracy and precision of adaptive radiotherapy (ART) at a high-field MR linac (MRL). Material and Methods The E2E phantom was created using a 3D printer and made of MR visible silicone rubber. Five channels allow an awl to produce marks in a radiochromic film sandwiched in the centre of the phantom (see figure 1A). The phantom supports both vertical and horizontal film alignment. An eleven beam reference IMRT plan based on a T2 weighted MR planning scan (0.3x0.3x1mm 3 voxel size) of the phantom was created in the treatment planning system (TPS) dedicated to the MRL. At the MRL, ART dose delivery was performed by placing the phantom in one of nine treatment positions: Centrally or shifted 20 mm horizontally in the right-left (RL) and/or caudal-cranial (CC) direction. Each session was repeated three times with the film in both coronal and sagittal orientation, resulting in 54 ART irradiations in total. At each session, the phantom was imaged using a T2 weighted MR session scan (0.8x0.8x2 mm 3 voxel size) (see figure 1B). Automatic rigid registration of the planning and session image was performed in the dedicated TPS. Based on this registration, an adapted plan was created by shifting the isocentre position in the phantom. After dose delivery, the film was scanned in two different flatbed scanners, grey levels converted into dose, and marks in the film were manually pinpointed. The spatial dose delivery error (positional difference of planned and delivered dose) was measured in the frame of reference where marks in the film coincide with delineations of the channels in the DICOM images (see figure 1B) using rigid registration of dose images. The dependency of the dose delivery error on the isocentre shift was investigated using Spearman’s correlation coefficient, R.

Results The mean difference between dose delivery errors measured using the two different flatbed scanners was 0.1 mm or below with a standard deviation (SD) of 0.1 mm in both CC, LR, and posterior-anterior (PA) direction. The mean ART dose delivery error of the MRL was 0.3 mm (SD 0.7 mm), 0.1 mm (SD 0.2 mm), and 0.5 mm (SD 0.2 mm) in the CC, LR and PA direction, respectively. The dose delivery error depended on the isocentre shift in the CC direction (R=0.52, p<0.001) and PA direction (R=0.63, p<0.001), but not the LR direction (R=0.03, p=0.87) as seen in figure 2. Dose delivery errors above 1.0 mm were only found in the CC direction. Visual inspection of the planning and session images leading to these errors revealed an image misalignment of the automatic registration of the dedicated TPS.