Clinical experience with online adaptive radiotherapy (oART) for head-and-neck (H&N) cancer is still limited, and an improved understanding of the process, risks, and benefits is needed to move forward [Citation1]. Added complexity is introduced to the treatment workflow, including deformable image registration (DIR) processes, automated contouring and online re-optimization, and quality assessment (QA) methods [Citation2]. Rapid online dose calculation is required and is in the presented clinical workflow based on the DIR of the planning CT to the anatomy of daily CBCTs. Dosimetric uncertainty as a result of suboptimal DIR should be quantified as part of safety assessment, and challenges are expected to be diagnosis/site-specific [Citation3]. In the current work, our institution’s first clinical experience with oART for H&N cancer is presented. The focus is on the dosimetric effect of uncertainty in the DIR/synthetic CT generation, as well as the dosimetric difference between the daily adapted plans and the original plan.