Introduction: Cancer incidence is expected to increase in Europe by 18% in eighteen years. To account for the increasing patient numbers, the workload per patient needs to be reduced. One step towards future-proof radiotherapy is automated MR-only radiotherapy as it could eliminate the need for (i) a planning CT and (ii) for manual organ at risk (OAR) delineations. The aim of this study was to evaluate the feasibility of an automated MR-only workflow for head-and-neck radiotherapy. Method: Automated MR-only radiotherapy consisted of a Zero-Echo-Time-based synthetic CT for dose calculations and automated T2w-based OAR delineations. Automated MR-Only RT was compared to the clinical workflow consisting of CT-based dose calculation and CT-based OAR delineations. Both approaches were benchmarked to a gold standard consisting of the planning CT for dose calculations and manual delineations on the T2w MR scan. Dice similarity coefficients (DSC), 95% Hausdorff distances and absolute DVH metrics were compared between the clinical and MR-only workflow using a linear mixed-effect model. A p-value < 0.05 was deemed significant. Results: Seventeen head-and-neck cancer patients were included. The automated MR-only delineations were more accurate compared to the clinical CT delineations (DSC of 0.79 vs. 0.67; 95% Hausdorff distance 4.0 vs 5.8 mm (p-values < 0.001)). The average dose calculation errors of the automated MR only RT were smaller than the clinical workflow (+0.34 Gy vs. −1.39 (p-value < 0.01)). Discussion: The automated MR-only head-and-neck radiotherapy workflow was more accurate than the standard CT based clinical workflow, demonstrating the feasibility of automated MR-only RT to decrease the workload for head-and-neck RT treatment preparation.

Background and Purpose: MRI-based synthetic CTs (synCTs) show promise to replace planning CT scans in various anatomical regions. However, the head-and-neck region remains challenging because of patient-specific air, bone and soft tissues interfaces and oropharynx cavities. Zero-Echo-Time (ZTE) MRI can be fast and silent, accurately discriminate bone and air, and could potentially lead to high dose calculation accuracy, but is relatively unexplored for the head-and-neck region. Here, we prospectively evaluated the dosimetric accuracy of a novel, fast ZTE sequence for synCT generation. Materials and Methods: The method was developed based on 127 patients and validated in an independent test (n = 17). synCTs were generated using a multi-task 2D U-net from ZTE MRIs (scanning time: 2:33 min (normal scan) or 56 s (accelerated scan)). Clinical treatment plans were recalculated on the synCT. The Hounsfield Units (HU) and dose-volume-histogram metrics were compared between the synCT and CT. Subsequently, synthetic treatment plans were generated to systematically assess dosimetry accuracy in different anatomical regions using dose-volume-histogram metrics. Results: The mean absolute error between the synCT and CT was 94 ± 11 HU inside the patient contour. For the clinical plans, 98.8% of PTV metrics deviated less than 2% between synCT and CT and all OAR metrics deviated less than 1 Gy. The synthetic plans showed larger dose differences depending on the location of the PTV. Conclusions: Excellent dose agreement was found based on clinical plans between the CT and a ZTE-MR-based synCT in the head-and-neck region. Synthetic plans are an important addition to clinical plans to evaluate the dosimetric accuracy of synCT scans.