The investigators hypothesize that treatment adaptation to biological and anatomical changes, occurring during treatment, can increase the chance of cure at minimized or equal radiation-induced toxicity in head and neck cancer patients. This trial compares standard intensity-modulated radiotherapy (IMRT), using only pre-treatment planning 18F-FDG-PET/CT scans to adaptive 18F-FDG-PET-voxel intensity based IMRT or volumetric-modulated arc therapy (VMAT) using repetitive per-treatment planning 18F-FDG-PET/CT scans for head and neck cancer.