ESTRO 36 Abstract Book

S82 ESTRO 36 2017 _______________________________________________________________________________________________

of a transversal 3D T1-weighted TSE MR image (optimized MR sequence for bone metastases). A deformable registration was performed (ADMIRE, v1.13.5, Elekta AB, Sweden) of the obtained MR-images with the reference CT for automatic contour propagation and CT deformation. Furthermore a 3-field IMRT technique treatment plan was generated automatically using a research version of Monaco (v5.19.01. Elekta AB, Sweden) with a prescribed dose of 8 Gy. Additional to the MRI, independent position verification was performed using two orthogonal MV beams and the plan was delivered to the phantom. The offline and online procedures were tested three times, each time for a different lumbar vertebra. The duration of the individual procedures within the online workflow was measured.

About 80% of absolute shift errors in all MR-P xy s were not significantly different from MR-ref in all directions. No specific trend between PAT-f and shift error was observed, implying fusion accuracy could not be predicted by PAT-f value. The errors of T2W were significantly smaller than T1W in almost all of the shifts (p<0.05). Junior observers performed significantly better than seniors when fusing T1W images (p<0.05). For treatment verification purpose, T1W MR-P 42 (88s) and T2W MR-P 22 (173s) were chosen since they were the fastest sequences with comparable image quality to MR-ref. However when considering fusion errors, MR-P 44 for T1W (49s) and T2W (59s) images, with an even shorter scan time, should be opted instead as its verification accuracy had no significant difference in most shift directions from MR-ref (Fig.2) despite significantly degraded image quality.

Results Time measurements of the individual procedures were as follows (table 1); MRI acquisition time of 5:02 minutes, deformable image registration, contour propagation and generation of the deformed CT within a range of 40-44 seconds, development of an automatic treatment plan within a range of 6:20-6:30 minutes and position verification and dose delivery within a range of 2:32-2:42 minutes. The total time of these online procedures ranges from 14:34-15:01 minutes. These time measurements do not include the additional time for patient setup, data transfer and the time needed for a physician to evaluate the propagated contours and treatment plan at the MR- Linac. This can potentially increase the total workflow time.

Conclusion The degradation of image quality by increasing PAT-f does not necessarily affect the fusion accuracy. MR-P 44 with acceptable verification accuracy and shortest scan time is therefore proposed for MRI-guided RT in C and HN regions. OC-0163 Online workflow for the First-in-Man study on bone metastases at the MRI-linear accelerator L.T.C. Meijers 1 , S.J. Hoogcarspel 1 , A.N.T.J. Kotte 1 , C.N.N. Nomden 1 , G.G. Sikkes 1 , I.H. Kiekebosch 1 , E.N. Groot de 1 , G.H. Bol 1 , B. Asselen van 1 , I.M. Jurgenliemk- Schulz 1 , L.G.W. Kerkmeijer 1 , B.W. Raaymakers 1 1 UMC Utrecht, Radiotherapy department, Utrecht, The Netherlands Purpose or Objective The first treatment on the MR-Linac in the UMC Utrecht includes patients with bone metastases, the first-in-man study. A fast workflow is required since all processes will be performed in an online setting. The aim of the present study is to test the feasibility of an online workflow for these patients at the MR-Linac within a 30 minute time limit. Material and Methods A workflow with offline and online procedures was developed for the MR-Linac. To test this workflow, the Alderson phantom was used, a human body shaped phantom with tissue-equivalent material. The phantom yields sufficient CT and MR contrast. The offline workflow included acquisition of a reference CT scan and manual delineation of the spinal cord and one lumbar vertebra (vertebra = CTV, PTV-CTV margin = 5 mm). The online workflow at the MR-Linac, shown in figure 1, included setup of the patient (= Alderson phantom) and acquisition

Conclusion From a technical perspective, the online workflow developed for the first-in-man study on the MR-Linac can be performed well within 30 minutes to treat patients with bone metastases. Current work is focused on automation of the data transfer process. OC-0164 Set-up reproducibility on an MR-Linac A. Betgen 1 , T. Vijlbrief 1 , L. Wiersema 1 , V.W.J. Van Pelt 1 , J.J. Sonke 1 , U.A. Van der Heide 1 1 Netherlands Cancer Institute Antoni van Leeuwenhoek Hospital, Department of Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective MRI integrated Linacs are becoming available for improving the accuracy of radiation therapy. The MR-Linac (1ATL, Elekta AB, Sweden) is an integration of a 7MV linear accelerator and a modified 1.5T Ingenia MRI (Philips Healthcare, NL). Like on a conventional MRI, the table only moves in longitudinal direction while the patient is in the bore. No laser system is available. An indexation at