Abstract: Objective: To investigate the potential benefit of using 18F- deoxyglucose γ - camera PET- CT (FDGhPET- CT) to delineate the gross tumor volume (GTV) of non- small cell lung cancer (NSCLC) complicated with atelectasis in patients who are to be treated with three- dimensional confor-mal radiotherapy (3DCRT) in comparison to patients undergoing conventional inspection. Methods:Seventeen patients with histopathologically and cytologically confirmed NSCLC complicated with varying degrees of atelectasis were studied. All patients were scanned with both thoracic CT and 18F- deoxyglucose γ - camera PET- CT. The GTV was delineated based on both CT image and hPET- CT image (CT- GTV, hPET- CT- GTV) and the GTV from each imaging technique (designated CT- GTV and hPET- CT- GTV) was compared before and after treatment. Results: The paired CT- GTV and hPET-CT- GTV values for each patient were different from each other. Fifteen of 17 patients' GTV was reduced an average of 19.3% (26 cm3) with a median CT- GTV of 135 cm3 (95- 190 cm3) and a median hPET- CT- GTV of 109 cm3 (65- 155 cm3) (P=0.004); 2 of 17 patients' GTV increased by 20% (28.5cm3). The lower values obtained for hPET- CT- GTV were due to the ability of hPET- CT to differentiate cancer- induced atelectasis from gross tumor, reducing the target volume and sparing more of the sur-rounding normal tissue. This factor alone displays the superiority of 3DCRT using hPET- CT. Conclusions: The incorporation of hPET- CT data with gross tumor delineation improves the accuracy of 3D-CRT for non- small cell lung cancer patients with atelectasis complications.