Abstract: 【Abstract】Objective To analyze the setup errors in radiotherapy on esophageal carcinoma by CBCT. Methods Patients received a total of 444 CBCT scans after the initial setup，re-positioning and radiation delivery. The CBCT images were registered to the planning CT images，and the setup errors on axe axes X，Y and Z were analyzed. Results a total of 11 patients received 444 CBCT. The setup errors were larger in the left-fight and superior-inferior directions comparing to the anterior-posterior direction. Compared with the initial setup errors，the re-positioning setup errors decreased significantly in all six axes(P＜0.05)The setup errors after radiation delivery were significantly increase comparing to the re-positioning setup errors(P＜0.05). Without real-time correction of setup errors，the estimated margins required for CTV-PTV was 8.49 mm in X-axis，9.09 mm in Y-axis and 5.67 mm in Z-axis. While with the real-time correction，the margins were only required 1.80 mm，2.47 mm and 2.21 mm in X，Y and Z axis，respectively. Conclusions The spatial distributions of setup errors were smallest in the anterior-posterior direction，moderate in the left-right direction and largest in the superior-inferior direction. The variations of intra-fractional setup error are obvious in esophageal tumors and should be taken into account during treatment planning. Real-time online correction of setup errors by CBCT can reduce the inter-fractional error，improve the precise of radiotherapy in patients with esophageal carcinoma，and reduce the CTV-PTV margin significantly.

Key words: 【Key words】 Esophageal neoplasm, Cone-beam computed tomography (CBCT)                 【Key words】 Esophageal neoplasm, Cone-beam computed tomography (CBCT), Setup errors, CTV-PTV margin