ESTRO 36 Abstract Book

S500 ESTRO 36 _______________________________________________________________________________________________

were reconstructed to show the uptake of intravenous contrast agent. The sequences were tested on healthy volunteers and one patient using a 3T MR system (Ingenia; Philips Healthcare, The Netherlands) and reviewed by two MR-experts and one radiologist. Pineapple juice was given orally to distend the stomach and suppress signal from the stomach filling. Gadolinium was used as intravenous contrast agent for the patient only. Results Visual inspection showed that for TD and staging, T2w exhale respiratory navigator triggered, rather than a respiratory sensor, provides excellent contrast with limited motion artifacts. For TP, mDixon with a large FoV, a high signal to noise ratio (SNR) and HR in one BH is feasible. For motion modeling, 4D T2w MRI resulted in a good slice ordering, high SNR and HR. For MM, TSE Cine- MRI gave a good SNR and HR without artifacts. For staging and treatment response monitoring, FB DWI with an increased number of averages gave the best result, only limited motion and susceptibility artifacts were visible. FB 4D THRIVE DCE resulted in a good temporal resolution and limited motion artifacts. (Figure 1)

volume (in one contour: vagina, cervix and uterus), left and right femur. The manual contours of the first day were used as input for the automatic/semi-automatic contouring strategies. Automatic contouring software (ADMIRE research v1.13.5 Elekta AB, Stockholm, Sweden) was used for MR based deformable registration and contour propagation. For the automatic contouring strategy the daily propagated contours were based on an intra-patient atlas consisting of the manual contours of the first day and propagated contours of other previous days.The semi-automatic contouring strategy included additional manual adjustments made by a technologist after each daily automatic contour propagation serving as input for the following days. All automatic and semi- automatic contours were compared with the manual contours of the corresponding day by calculating dice coefficients, mean and Hausdorff distances. Timing measurements were done for both strategies. Results Higher median dice coefficients with smaller ranges were found for the semi-automatic strategy compared to the automatic strategy (figure 1). However, large variations after manual adjustments were still found for the GTV. Outliers found in the mean and Hausdorff distances of the automatic strategy were not seen in the semi-automatic strategy (figure 2). The contours were automatically propagated for day 2, 3, 4 and 5 in respectively 18, 38, 54 seconds and 1:13 minutes on average. The propagated contours of the semi- automatic strategy were manual adjusted with an average time of 14:49 minutes (in comparison with approximately 45 minutes for full manual contouring). Manual adjustments of the cranial and caudal slices of the contours were most time consuming.

Conclusion We developed a comprehensive imaging protocol for the entire RT guidance treatment chain. The complex motion artifacts were reduced by applying either navigator triggering or BH techniques. The new gastric cancer protocol looks therefore very promising and will be used for MR-based delineation for RT. [1] van de Lindt T, et al. ESTRO 35 2016 Abstract-book:PV- 0325; 171-172 PO-0905 (Semi-)Automatic contouring strategies for rectal boost treatment on the MR-Linac C.N. Nomden 1 , M.P.W. Intven 1 , A.N.T.J. Kotte 1 , I.H. Kiekebosch 1 , S. Mook 1 , I.M. Jürgenliemk-Schulz 1 , G.G. Sikkes 1 , L.T.C. Meijers 1 , E.N. De Groot 1 , G.H. Bol 1 , B. Van Asselen 1 , L.G.W. Kerkmeijer 1 , B.W. Raaymakers 1 1 UMC Utrecht, Radiation Oncology, Utrecht, The Netherlands Purpose or Objective The MR-Linac enables online treatment adaptations in response to changes in anatomy. This stresses the need for fast contouring strategies for target and OARs. Unfortunately, manual delineation in an online workflow is time consuming and therefore suboptimal. The purpose of this study was to investigate whether automatic and semi-automatic contouring strategies result in clinical acceptable contours for an online workflow on the MR- Linac. Material and Methods Fifteen patients with early staged rectal cancer were scanned at an 1.5T MRI for five consecutive days. The scan consisted of a T2 weighted MRI; voxelsize 0.63x0.63mm, slice thickness 4 mm and a total number of 30 slices. For each scan the following contours were delineated by an experienced radiation oncologist (manual contours): GTV, mesorectum, bladder, rectum, sphincter, gynecological