positron emission tomography with (18f)2-fluoro-2-deoxy-d-glucose (pet-fdg) is an imaging metabolic technique. pet-fdg is more precise than ct for the evaluation of mediastinal affectation of patients with non-small cell lung cancer, offering a high negative predictive value. it can detect occult metastases in 11 % of the patients, although the etiology of the extra-thoracic isolated receptions needs confirmation. theoretically, pet-fdg can influence the planning volume of radiotherapy, mainly of patients with atelectasis. the quantification of the metabolic activity by means of pet-fdg is influenced by the size of the lesion, the glucose levels, and the time elapsed since the isotope injection. more clinical assays are needed to standardize pet execution procedures, its use as a prognostic factor and the evaluation of treatment response.