Background and purpose: Correct target positioning is an important factor affecting the precision of stereotactic body radiotherapy (SBRT) in patients with lung tumors. This study investigated the setup errors in patients with malignant lung tumors receiving SBRT with cone-beam CT (CBCT) and analyzed the factors influencing setup errors. Methods: Twenty-nine patients with solitary malignant lung tumors were enrolled in the study. Each patient underwent SBRT with CBCT before each treatment. Setup errors in CBCT were obtained according to the matched and planned CT images in anterior-posterior (AP)

superior-inferior (SI) and left-right (LR) directions. The expanding margins of clinical target volume (CTV) to planning target volume (PTV) according to the analyzed setup errors were then calculated. And the influencing factors of setup errors were analyzed. Results: A total of 155 CBCT images from 29 patients were obtained during the treatment. The setup errors were (-1.68±3.62)

(-1.34±3.90) and (0.36±2.15) mm in the AP

SI and LR directions

respectively. The absolute setup errors were (3.16±2.42)

(3.29±2.48) and (1.74±1.30) mm in the AP

SI and LR directions

respectively. The suggested expanding margins of CTV to PTV were 9.6

10.0 and 5.3 mm in the AP

SI and LR directions according to the setup errors. The setup errors in the AP direction of peripheral lesions and in the SI direction of inferior

right and metastatic lesions were relatively larger (P=0.007

0.008

0.000 and 0.000).. Conclusion: In patients with malignant lung tumors receiving SBRT

the setup errors were more obvious in the SI and AP directions. Tumor motion management techniques including CBCT

breathholding technique are required to reduce the setup error in patients with lung tumors receiving SBRT.