Purpose: To assess possible consequences for radiotherapy (RT) planning, e.g., reduction of treatment volume by a decreased tumour volume in Fluor-18-fluoro-deoxy-glucose-Positron emission tomography (FDG-PET) based on a close-meshed evaluation of FDG uptake in primary head and neck cancer (HNC) during RT. Materials and method: PET data were analysed using a source-to-background based algorithm. The following parameters were obtained: max. standardised uptake value (SUVmax), PET-based gross tumour volume (GTV-PET) and metabolic volume (MV). Results: While the median SUVmax decreased (initial: 15.2, 1st/2nd week: 10.2, 3rd/4th week: 6.5, 5th/6th week: 6.4), the median values of GTV-PET (9.3 cm ³, 12.4 cm ³, 14.0 cm ³, 17.9 cm ³) and MV (92.2 cm ³, 61.7 cm ³, 60.0 cm ³, 71.3 cm ³) seemed to increase during radiotherapy. The intra-individual development of SUVmax could be divided into two groups: group A having continuously decreasing values of SUVmax (n=10 patients), and group B having a temporary increase of SUVmax (n=13). Conclusions: Data suggest that a reduction of treatment volume is not possible by an adaptive re-planning based on FDG-PET, e.g., at 50 Gy. This may be caused by a consecutive therapy associated inflammation. This limitation is probably related to the use of a source-to-background based algorithm.