Daily online contour adaptation by radiation therapists 37 Introduction The clinical introduction of magnetic resonance imaging (MRI)-guided linear accelerator (MR-Linac) systems has led to major changes in the workflows for treatment of various types of cancer.1,2 MRLinac systems integrate an MR imaging scanner with a linear accelerator, enabling ‘online’ – when the patient is on the treatment table – imaging, contour adaptation, and treatment (re)planning.3 To make optimal use of the capabilities of these systems, online treatment steps are added to the workflow. One example is deformable image registration (DIR) of pre-treatment and daily MR images. After registration, the pre-treatment contours are propagated to the daily MR scan, followed by manual adaptation to fit the anatomy of the day. This so-called ‘Adapt-to-Shape’ (ATS) workflow is aimed at delivering the highest precision treatment to the patient, thereby potentially reducing toxicity and improving oncological outcomes.4,5 Consequently, these new online tasks increase the workload of those involved in the radiotherapy treatment of patients. With increasing numbers of patients being treated on MR-Linac systems worldwide, new approaches for the online workflow are being implemented. This also encompasses the transition of tasks from radiation oncologists to radiation therapists (RTTs). One of the most timeconsuming steps in the ATS workflow for MR-Linac treatment is the manual adaptation of the propagated contours on the daily online MR scan by radiation oncologists. Since October 2019, RTTs in our centre are trained and certified to perform the online contour adaptation and to approve the contours for prostate cancer treatment. Besides this, they also perform all other steps of the workflow, such as image registration, treatment planning, and approval of the treatment plan. With the delegation of the contour adaptation task from radiation oncologists to RTTs, both need to be confident with the associated responsibilities. The purpose of this study was to evaluate the clinical prostate contours as adapted and approved by RTTs in the online MR-Linac setting. Materials and methods Patient characteristics For this study 30 consecutive patients, who were treated for low- or intermediate-risk prostate cancer (NCCN criteria) at the Radiotherapy Department of the University Medical Centre Utrecht between January and March 2020, were included. All patients were part of an institutional review board approved registration and imaging study. Patients were treated with five fractions of 7.25 Gy over the course of 2.5 weeks on a 1.5 T Unity MR-Linac (Elekta AB, Stockholm, Sweden). Offline workflow Patients underwent a computed tomography (CT) and/or MR simulation scan prior to the first fraction. From January 30th, 2020 onwards, an MR-only workflow was implemented, discarding the need for a CT simulation scan (17 out of the 30 patients). Pre-treatment delineations were performed 2
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