ESTRO 36 Abstract Book

S84 ESTRO 36 _______________________________________________________________________________________________

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.

the tabletop for the patients’ positioning devices has to provide a reproducible position. The treatment plan will be adapted to the patients’ position, anatomical variation and organ motion of the day. The aim of this study was to characterize the accuracy of patient set-up with a table indexing system in the absence of lasers and skin marks. Material and Methods This investigation was performed on a conventional MRI. MR-scans were acquired at 3 different time points from 8 volunteers. The pelvis was chosen as anatomical region of interest. The tabletop of this MRI is comparable with the tabletop of the MR-Linac. A head support and a knee support, indexed on the table, were used for stability and reproducibility. The first MR-scan was defined as reference scan and the following two MR-scans were registered to this reference on bony anatomy. The setup variability was analyzed in terms of group mean (M), systematic (Σ) and random errors (σ) for both translations and rotations. The results were compared to retrospective set-up data of 79 patients treated for rectum carcinoma (5x5 Gy), aligned with lasers and skin marks and measured with CT-scan and Cone Beam-CT position verification. Because the group of volunteers is relatively small, the comparison to the rectal cancer patient group is on a descriptive basis only. Results When comparing retrospective set-up data of rectal cancer patients to the group of volunteers in this investigation, for translations, the group mean for the patient group seem to show a better set-up reproducibility in the LR and CC direction as compared to the volunteers. This resulted in group means closer to zero with corresponding smaller Σ errors and σ errors. In the AP direction, the mean and standard errors did not seem to show apparent differences. For rotations the results for both groups were comparable. The results are presented in table 1.

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 For volunteers, without the use of laser alignment, translations seem to be larger in LR and CC direction. Rotations were comparable for both groups. However, for daily practice, the impact of this increased uncertainty is likely small relative to uncertainties of internal organ motion that can be in the cm range. In daily on-line corrections, the combination needs to be considered in positioning pelvic cancer patients without skin marks, on an MR-Linac. OC-0165 TPUS vs CBCT: comparison of daily inter- modality derived setup shifts for prostate radiotherapy. E.P.P. Pang 1,2 , K. Knight 2 , M. Baird 2 , J.M.Q. Loh 1 , E.T.Y. Chen 1 , G.K. Low 1 , C.C.C. Yap 1 , A.H.S. Boo 1 , J.K.L. Tuan 1,3 1 National Cancer Centre Singapore, Division of Radiation Oncology, Singapore, Singapore 2 Monash University, Faculty of Medicine- Nursing and Health Sciences Medical Imaging & Radiation Sciences, Melbourne, Australia 3 Duke-NUS, Graduate Medical School, Singapore, Singapore

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