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目的：应用机载千伏级锥形束CT（KV-CBCT）监测食管上段癌调强放疗患者疗程中摆位误差的变化和影响因素。方法每周按治疗计划标志点对16例调强放疗食管癌患者摆位，利用机载KV-CBCT扫描获取摆位误差，计算在相同治疗阶段所有患者三维线性方向和水平面旋转方向摆位误差平均值、标准差及95%可信区间（CI）；分析随疗程进行摆位误差的变化情况；探讨体重指数（BMI）和体重变化对摆位误差的影响。结果随疗程进行，摆位中心点向腹侧偏移明显。BMI≥25 kg/m2患者在垂直方向及水平面旋转方向首次摆位误差绝对值较BMI＜25 kg/m2患者明显增大；BMI≥25 kg/m2患者在垂直方向及水平面旋转方向的系统误差较BMI＜25 kg/m2患者明显增大；患者体重下降会引起摆位中心点往腹侧偏移和水平面逆时针旋转。结论在食管上段癌调强放疗中：(1)BMI≥25 kg/m2患者有必要进行自适应放疗；(2)放疗疗程中体重减少超过7.10%时，需要及时调整放疗计划。“,”Objective Using KV-cone beam CT (KV-CBCT) on IGRT to study the setup errors for upper esophageal carcinoma treated by fractionated intensity-modulated radiation therapy (IMRT). Methods Sixteen patients diagnosed with early upper esophageal carcinoma by pathology were selected. They were treated by fractionated IMRT. During the treatment courses, patients were posed by the original positions every week in order to get fractional setup errors with on-board KV-CBCT. The mean values, standard deviations, 95%confidence intervals (CI) of three dimensional and the horizontal rotation errors were calculated. The effects of body mass index (BMI) and weight loss on setup errors was analyzed. Results The displacements increased with the treatment progress along the ventral-dorsal direction. Absolute values of the first setup errors, the system errors of the ventral-dorsal direction and the horizontal rotation in the patients with BMI≥25 kg/m2 are more apparent than those with BMI<25 kg/m2. Weight loss can obviously result in the center of actual setup position shifting to the ventral direction and the horizontal rotation counterclockwise. Conclusion 1.The esophageal carcinoma patients with BMI≥25 kg/m2 should be treated by adaptive radiation therapy;2.The treatment planning should be adjusted when the weight lost over than 7.10%during the treatment course.