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.