Lung cancer is a suitable model for studying the quality criteria of radiotherapy, as irradiation of the intra-thoracic organs is complex. The recent availability of conformational techniques allows the analysis of these difficulties, as treated volumes are adapted to the target volume in a three-dimensional setting. Radiotherapy has then become a major therapeutic option of the treatment of non-small cell lung cancer. Radiation protection and technical quality criteria for radiotherapy have been integrated for a long time into the global control programs for civil use of ionizing radiation. Clinical quality criteria were defined more recently both from radio-biological data available in the literature and by the analysis of historical radiation accidents and risk management. Good clinical practice in radiotherapy is part of the global quality requirements for cancer treatment, and remains the key to maintaining quality in an increasingly technological area of medicine.