This is a single‐centre observational study. Patients with proven locally advanced NSCLC will be treated with concurrent chemoradiotherapy according to the standard clinical protocol of the NKI‐AVL. During treatment, the biological behaviour of the tumour will be monitored with serial quantitative FDG (fluorodeoxyglucose) PET/CT scans. From these images, the time during treatment where a 50% reduction in FDG uptake relative to day 1 is reached (T50) will be derived. The T50 of progression‐free surviving patients will be compared with that of relapsed or deceased patients, in order to find a value that predicts early treatment failure. An additional FDG PET/CT scan at the same day as regular follow up CT scan will be made, two months after treatment, to have a baseline after treatment for follow up.