Objective:To investigate whether adjuvant skin-marker positioning can decrease the set-up errors in overweight patients with thoracic and abdominal tumors.Methods:A total of 60 overweight patients with thoracic and abdominal tumors treated with radiotherapy in the First Affiliated of Fujian Medical University between January 2018 and December 2018 were randomly divided into two groups. In group A, conventional skin-marker positioning was adopted. In group B, conventional skin-marker positioning combined with adjuvant skin-marker position was employed. All patients were immobilized with thermoplastic positioning body membrane with head-body plate fixation. The set-up errors in the right-left, head-foot and dorsoventral directions were obtained from cone-beam CT (CBCT) scan system before radiation delivery. The set-up errors were statistically compared between two groups by using t-test. Results:In group A, the set-up errors in the right-left, head-foot and dorsoventral directions were (4.47±2.91) mm, (5.43±2.61) mm and (3.87±2.40) mm, significantly higher compared with (2.97±1.68) mm, (3.21±1.62) mm and (2.59±1.57) mm, respectively (all P<0.001). Conclusion:Adjuvant skin-marker positioning method can reduce the set-up errors and enhance the positioning repeatability in overweight patients with thoracic and abdominal tumors receiving radiotherapy.