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We utilized CT image fusion technique for replanning during the course of three-dimensional conformal radiotherapy (3D-CRT), and determined its dosimetric effects on both target volumes and normal tissues. Ten patients with non-small-cell lung cancer (NSCLC) were studied. The primary treatment plan was generated from the pre-treatment planning CT scan. Two treatment plans were established in the replanning of the 3D-CRT following the mid-treatment CT scan. Treatment plan A was generated from the fused images with the reference of the primary treatment plan; treatment plan B independently from the second CT scan images by the physicists who were blinded to the primary treatment plan. For the planning target volume, there were no significant differences between treatment plan A and B in D99 and D95 of the whole treatment. However, the doses to normal structures in the whole treatment, mean lung dose (MLD) of the lung, Dmax of the spinal cord and esophagus in the treatment plan A were significantly lower than those in the treatment plan B. CT image fusion is needed in replanning during the course of 3D-CRT for NSCLC, which may ensure safe doses to normal tissues.