Treatment adaptation to biological and anatomical changes occurring during treatment can increase the chance of cure at minimized radiation-induced toxicity in head and neck cancer patients. This trial investigates the feasibility of using repetitive per-treatment [18F]FDG-PET acquired during treatment in adaptive [18F]FDG-PET-voxel intensity-based intensity-modulated radiotherapy (IMRT) or volumetric-modulated arc therapy (VMAT) for head and neck cancer.