Part I | Chapter 2 44 ten patients on the same T2-weighted MR image, showing a median DSC of 0.94 (IQR 0.93-0.95). These results were acquired with a different method, using one MR set per patient, and completely new delineations by physicians in an offline setting, thus making it difficult to compare with our results. Another study with similar methods to Pathmanathan et al.15 reported a slightly lower median DSC of 0.88. Adaptation times were generally longer and showed more variation in earlier patients compared to patients that were treated at a later date, with a mean (SD) adaptation time of 14.4 (± 3.8) min in patient 1-10 and 9.5 (± 1.4) min in patient 21-30 (Figure S1, Supplementary A). This might partly be explained by a learning effect. There was no clear difference in the number of RTTs that were involved per patient in those with more and those with less variation (results not displayed). Our contour adaptation times are in line with Bertelsen et al.12, reporting a median adaptation time of 11.5 min (range 1-24 min) in their patients treated on an MR-Linac. While contour adaptation times are generally comparable to those observed in physicians, this still raises a concern regarding anatomical changes that might occur during contour adaptation. This problem arises from the applied ATS workflow and the software used for warping the contours to the daily MR image. In the current workflow, a PV scan is obtained at the end of re-contouring. In case of major anatomical differences, an ATP step can be applied, or it can be decided upon to redo the contours (ATS step) on the PV scan, e.g. in case of a large gas pocket in the rectum. Presently, we work towards a Figure 5 – Dose-volume histograms (DVHs) for the CTV for four exemplary ‘outlier’ cases as judged by Observer 3. Dose distributions are based on the online (RTTs-contour based) dose plan as delivered to the patient. DVHs are given for the RTTs CTV contour (solid blue line), the adapted CTV contour by Observer 1 (dashed yellow line), and the adapted CTV contour by Observer 2 (dotted orange line). Legend: RTTs = Radiation therapists. Obs = Observer.
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