Abstract: Objective: To compare the errors of geometric center of the conformal radiotherapy planning in patients with lung cancer, before and after the proofreading of the three-dimensional (3D) directions by CT simulation localizer, us-ing the image fusion technology, and to clearly identify the direction of the errors and verify the range of the errors, so as to improve the precision of radiotherapy. Methods:Philips 16-row big pore diameter spiral CT simulator exclusively for radio -therapy planning was used for a scout view in 30lung cancer patients with thermo-plastic (body mask) phantom immobiliza -tion. In each of the patients, a set of images was transmitted to a three-dimensional radiation treatment planning system (3-D TPS, Eclipse7.0), then the radio-therapeutic plan was devised with an ascertainment of the geometric center. The cen-ter was proofread by the CT simulator to get a corrected center which was marked on the body. A rescan was performed ac -cording to the site of corrected center, and the new image was also transmitted to the 3-D TPS, with the correct center. The image fusion technology was used to check and analyze the errors of the 2 centers in 3D directions. Results: The average accuracy and precision in the X direction were 2.20mm and 1.20mm, respectively, with the mean error of2.20± 1.20mm, while the accuracy and precision were2.36mm and 1.36mm, respectively, in the Y direction, with the mean error of2.30±1.95mm. These values were 3.12mm and 1.78mm, respectively, in the Z direction, with the mean error of 3.50± 3.48mm. The overall accuracy and precision were respectively 4.99mm and 1.34mm in the directions. There was statistical signifi-cance in comparison of the differences in the mean errors of the 3 directions, using the rank sum test for an inter-group comparison ( χ2=6.773 , P=0.034 ). Conclusion:In the patients with lung cancer, the error of radiation treatment planning cen-ter during simulation is in the acceptable limit. The main source of the errors in the 3D directions is in the cephalic and pedalian directions.