General introduction and thesis outline 19 Online clinical workflow During the actual treatment delivery (online phase), the workflow consists of several steps (Figure 4). For prostate cancer SBRT treatment, a so-called Adapt-to-Shape (ATS) workflow is used, in which a daily treatment plan is created for the anatomy of the day, using a new MRI scan with updated contours.70 Before treatment can be started, the patient is positioned on the treatment couch. After patient positioning, an initial daily MRI scan is acquired to assess the daily anatomy (step 2). Using deformable image registration (DIR), this daily MRI scan is registered to the pre-treatment planning MRI and the target and OAR contours are warped from the pre-treatment image to the daily MRI (step 3). An operator – either a radiation therapist or a physician – checks the contours andmanually adapts them wherever needed (step 4). Using the daily MRI and the updated contours, a new daily treatment plan is created (step 5). Just before the end of dose calculation, a second daily MRI scan is acquired – a so-called position verification (PV) scan – to verify the position of the target (step 6). In case any major shifts have occurred between the initial daily MRI and the PV MRI scan, a virtual couch shift (VCS) or Adapt-to-Position (ATP) can be applied to re-align the position.70,71 After approval of the treatment plan, treatment delivery is started with simultaneous acquisition of 3D cine-MR images to track the prostate motion (step 7).62 The duration of one treatment fraction is approximately 45 min (including patient positioning), which is well tolerated by patients. The current, commercially available Unity workflows allow for interfraction adaptation of the treatment plan according to the anatomy of the day. Ultimately, these machines will enable realtime adaptation during treatment delivery, to counteract intrafraction motion and deformations. However, at the moment of writing, real-time intrafraction adaptive treatments are not yet clinically available on 1.5 T MR-Linac systems. Figure 3 – Transversal MR image with the Gross Tumour Volume (GTV), Clinical Target Volume (CTV), and Planning Target Volume (PTV) contours for primary prostate cancer SBRT. 1
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