Objective To assess the feasibility of utilizing the ExacTracDynamic surface monitoring system （ETD） for setup and body surface monitoring in patients with thoracic tumors undergoing intensity?modulated radiotherapy （IMRT）. Methods Patients receiving IMRT for thoracic tumors were included in this study. The enrolled patients were alternatively assigned to either conventional cross curve positioning （control group） or surface monitoring system?assisted positioning （experimental group）. ETD X?ray images were utilized for calibration purposes prior to radiotherapy， enabling the determination of setup errors. A region of interest （ROI） was delineated on the body surface above the sternum， and real?time body surface monitoring was performed based on this ROI during radiotherapy. Post?radiotherapy X?ray images were obtained to verify patient position. Data regarding left?right （X）， head?foot （Y）， abdomen?back （Z）， pitch， roll， and yaw directions were recorded and analyzed. Results A total of 60 patients were enrolled， with 754 fractions of radiotherapy in the control group and 718 fractions in the experimental group. The setup errors in the X and Z directions were significantly smaller in the experimental group compared to the control group （P < 0.05）. Moreover， there was a significant reduction in the number of setup errors ≤ 0.50 cm for X， Y， and Z directions， as well as ≤ 1.00° for Roll angle in the experimental group compared to the control group （P < 0.05）. Additionally， differences were observed between surface monitoring and X?ray image verification regarding position deviation along Y and Z directions （P < 0.05）， although these deviations remained within submillimeter levels on average. Conclusion Surface monitoring system?assisted positioning can enhance radiotherapy setup accuracy among thoracic tumor patients， particularly along X and Z directions. Furthermore， when setting ROI above sternum on body surface area， surface monitoring provides better reflection of target area's position deviation.