Background: three-dimensional conformal radiation therapy (3DCRT) planning remains the standard option in the management of locally advanced NSSCL, a technique that makes the radiation oncologist face the challenge of target volume delineation based on CT scan alone, which will eventually affect target volume coverage, i.e. gross tumor volume (GTV) and planning target volume (PTV), as well as dose to the surrounding normal tissues at risk. Purpose: To prospectively study the impact of fusing 18F-fluoro-deoxy-2-glucose hybrid positron emission tomographic (FDG-PET) images with CT images on the planning target volume (PTV) delineation, target coverage, and critical organ dose in radiation therapy planning of non–small-cell lung carcinoma. Methods and Materials: Twenty patients with Stages I–III NSCLC were referred to our radiotherapy department in the period between Jan 1st 2015 and Aug 30, 2016, planned for treatment via radiotherapy alone or with concurrent chemo-radiation. Each patient underwent a planning CT with immobilization devices. FDG-PET scan was ordered for every patient and done in the department of nuclear medicine very soon after or before the day of CT simulation. Both the CT and PET/CT image data sets were fused and used in the radiation treatment planning workstation for contouring. Each FDG-PET study was reviewed with the interpreting nuclear radiologist before tumor volumes were contoured. A three-dimensional conformal radiation therapy (3DCRT) plan was calculated based on contours done on the CT scan only. A second plan based on the fused PET/CT images was generated. The PTV was defined by a 20 mm margin around the GTV. The two 3DCRT plans for each patient were compared with respect to the GTV, PTV, mean lung dose, volume of normal lung receiving >20 Gy (V20), and mean esophageal dose. Results: The FDG-PET findings altered the AJCC TNM stage in 6 of 20 (30%) patients; 2 patients were diagnosed with metastatic disease based on FDG-PET and received palliative radiation therapy. Of the 18 patients who were planned with 3DCRT, PET clearly altered the radiation therapy volume in 10 (66%), for example, PET helped to distinguish tumor from atelectasis in all 4 patients with atelectasis. Unsuspected nodal disease was detected by PET in 2 patients, and 1 patient had a separate tumor focus detected within a different lobe of the lung. Increases in the target volumes led to increases in the dose to organs at risk (mean lung dose, V20, and mean esophageal dose). Decreases in the target volumes in the patients with atelectasis led to decreases in these normal-tissue toxicity parameters. Conclusions: Radiation targeting with fused FDG-PET and CT images resulted in modifications in radiation therapy planning in over 50% of patients by comparison with CT targeting. The future plan of having a PET-CT simulator in our department will make it possible to have the planning CT and PET-CT done on the same day and in the same position, eliminating all the difficulties faced during the fusion process.