Daily online contour adaptation by radiation therapists 39 direct supervision, with exception of each patient’s first fraction. In this patient cohort, contour adaptations were performed by a group of eight RTTs. Contour adaptation timings During each fraction, all steps of the online workflow were timed. The time taken for the contour adaptation by the RTTs was measured as the time interval between the start of manual adaptation and the start of treatment plan calculation. During this time, both the CTV as well as OAR contours within a 2.0 cm ring around the CTV contour were adapted. Evaluation of contours All 150 CTV contours were independently judged by two radiation oncologists (Observer 1 and 2) and adapted if deemed necessary. Both observers were blinded for each other’s adapted contours. Fraction 1 was chosen as a reference for interfraction analysis, since for each patient a radiation oncologist was present for approval of the contours during the first fraction and due to the fact that no ground truth was available for each fraction. Evaluation of the contours consisted of three parts. Firstly, CTV volumes were obtained using Volumetool®6 and relative volume differences between the contours from fraction 1 and the contours from fraction 2 to 5 were calculated, separately for the RTTs and for Observer 1 and 2. Secondly, interobserver Dice’s similarity coefficient (DSC) per fraction was calculated between the RTTs and the observers as well as between both observers for fraction 2 to 5, using MATLAB (version R2019a). Finally, a third, ‘senior’ observer judged the RTT contours on clinical acceptability, taking into account the extent of the adaptations (if any) that were performed by Observer 1 and 2 and expected interobserver variability in the base and apex region.9,10 All analyses were performed using R statistical software (version 3.6.2). Figure 1 – MR-Linac workflow for prostate cancer treatment. Legend: CT = Computed Tomography. MR =Magnetic Resonance. ATS = Adapt-to-Shape. RTTs = Radiation therapists. ATP = Adapt-to-Position. 2
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