Abstract: Objective:To evaluate the applicability of different thresholds for setup correction during radiotherapy of pul -monary tumors. Methods: Thirty patients with pulmonary tumors were treated with radiotherapy with a thermoplastic body mask. They received a CBCT scan at each fraction after initial setup and after re-positioning. The CBCT was registered to the planning CT using XVI software and setup errors on lateral (X), cranial-caudal (Y) and anterior-posterior (Z) axes were analyzed. Radiation could only start when the setup errors in all axes were ≤2 mm, otherwise the patient would be repositioned until the action limit was met. The initial setup errors and post-correction of residual errors were analyzed off-line. Four assumed correction threshold levels (1 mm, 2 mm, 3 mm and5 mm) were designated for evaluation of clinically applicable online correction. The post-correction residual errors and setup margins (Msetup) were compared among the four assumed threshold levels.Results:A total of 860 CBCT scans were performed on30patients, of which 584 were performed after initial setup and276 after re-positioning. The pre-correction interfractional setup errors were largest on the Y axis. The percentages of pre-correction errors on the Y axis which were less than 1 mm, 2 mm, 3 mm and 5 mm in size, were 15%,26%,48.7% and 63.7%, respectively, while those for the post-correction residual errors were 78.4%,95.2%,98.3% and 99.6%, respectively. Before correction, the maximum systematic and random errors were4.2 mm and5.0 mm, respectively, with setup margins ranging6.9-13.8 mm. After correction, the residual errors with threshold levels of 1 mm, 2 mm, 3 mm and5 mm were≤1.0 mm, ≤1.0 mm, ≤1.2 mm and≤2.2 mm, respectively, and the setup margins were ≤2.2 mm, ≤2.2 mm, ≤3.1 mm and ≤4.4 mm, respectively. Conclusion:Measurement and correction of interfractional setup errors before each fraction using CBCT can help to improve the setup precision for pulmonary radiotherapy immobilized with a thermoplastic frame. The correction threshold levels with CBCT should be set at 2 mm and 3 mm for pulmonary tumor radiotherapy.