Part I | Chapter 2 36 Abstract Background and purpose Magnetic resonance imaging (MRI)-guided linear accelerator (MR-Linac) systems have changed radiotherapy workflows. The addition of daily online contour adaptation allows for higher precision treatment, but also increases the workload of those involved. We train radiation therapists (RTTs) to perform daily online contour adaptation for MR-Linac treatment of prostate cancer patients. The purpose of this study was to evaluate these prostate contours by performing an interfraction and interobserver analysis. Materials and methods Clinical Target Volume (CTV) contours, generated online by RTTs from 30 low- and intermediate-risk prostate cancer patients treated with 5 x 7.25 Gy, were used. Two physicians (observers) judged the RTT contours and performed adaptations when necessary. Interfraction relative volume differences between the first and the subsequent fractions were calculated for the RTTs, Observer 1, and Observer 2. Additionally, interobserver Dice’s similarity coefficient (DSC) for fraction 2-5 was calculated with the RTT- and physician-adapted contours. Clinical acceptability of the RTT contours was judged by a third observer. Results Mean (standard deviation) online contour adaptation time was 12.6 (± 3.8) min and overall median (interquartile range [IQR]) relative volume difference was 9.3% (4.4–13.0). Adaptations by the observers were mostly performed at the apex and base of the prostate. Median (IQR) interobserver DSC between RTTs and Observer 1, RTTs and Observer 2, and Observer 1 and 2 was 0.99 (0.98–1.00), 1.00 (0.98–1.00), and 1.00 (0.99–1.00), respectively. Contours were acceptable for clinical use in 113 (94.2%) fractions. Dose-volume histogram (DVH) analysis showed significant CTV underdosage for one of the seven identified outliers. Conclusion Daily online contour adaptation by RTTs is clinically feasible for MR-Linac treatment of prostate cancer.
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