Abstract: Objective: To investigate the value of spectral CT in differential central lung cancer from obstructive atelectasis for achieving the accurate delineation and radiotherapy localiation. Methods: The CT radiotherapy localiation images of three phase contrast enchancement by spectral CT of 65 patients confirmed by pathology with central lung cancer and obstructive atelectasis with were collected and analyzed. The scores and detection rates of tumor-lung interface between the polychromatic image (PI), the best monochromatic image (BMI), the effective atomic number (Eff-Z) and the best monochromatic image combined with iodine concentration map (BMI-ICM) were compared. Results: Statistical differences in the scores of tumor-lung interface were observed between PI, BMI, Eff-Z and BMI-ICM; the pairwise comparison showed that the subjective score of BMI-ICM was the highest, and PI was the lowest. The pulmonaty artery phase detection rates of tumor-lung interface of three phase contrast enchancement were the highest. There was statistical difference of iodine concentration (IC) and CT score between lung cancer and obstructive atelectasis. The difference of IC was higher than the CT score. Conclusion: Spectral CT is helpful to distinguish central lung cancer delineation from atelectasis. It provides a new method to delineate the radiotherapy localiation for radiotherapy plan of optimization.