Part I | Chapter 3 58 manual editing of propagated contours – which inherently is a subjective visual judgement by the operator – is the limiting factor. Keeping that in mind, we decided to work from this perspective. Furthermore, the 3 min cut-off for manual editing was arbitrarily chosen, as this cut-off will depend on multiple aspects that have yet to be investigated for the implementation of a VF workflow. This includes primarily the amount of intrafraction motion that is expected in the time from the end of image acquisition to actual start of beam-on, during which DIR and contour editing are performed. The timings of such a workflow will ultimately affect the final dose distribution and therefore influence the potential benefits. Additionally, clinical goals such as applying 1 mm CTV-PTV margins will guide the process to determine what is needed from a technical point-of-view. Ideally, the time dedicated to visual inspection and manual contour editing is a few seconds, implying that the contours are always spot-on. Until we can fully rely on accurate auto-contouring solutions, operator intervention will remain essential. Finally, the cut-off was set as a benchmark in the light of current manual adaptation times.15,24 Besides exploring clinical usability of propagated contours, the clinical feasibility of employing adaptive workflows for MRI-guided prostate cancer radiotherapy should be tested. Our current work has focused on the image registration and contour propagation in a standalone pipeline. Future work should include an assessment of technical feasibility when incorporated in a (pre-)clinical VF workflow and certification of workflow software for intended use. Conclusion Concluding, the employed DIR algorithm performed well for intrafraction propagation of bladder and prostate CTV contours. Generally, rectum contours were acceptable, but sometimes needed more manual editing to fit the anatomy. Nevertheless, adaptation times were below 3 min for most cases. This work paves the way for exploring adaptive workflows using intrafraction DIR, contour propagation, and re-planning.
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