Objective:To analyze the influence of CT couch tilt on the accuracy of isocenter in radiotherapy.Methods:Based on the schematic diagrams of patient positioning on CT couch and accelerator couch

a geometric illustration of isocenter trajectory was generated to obtain the equation for calculating the deviation of isocenter position between the planning system and the radiotherapy accelerator.The positioning image data of the nasopharynx and the pelvis were used to calculate the deviations of isocenter positions at different tilt angles.Results:Isocenter position deviation increased with the tilt angle of CT couch.With fixed tilt angle of CT couch

the deviation varied in the positioning of different body parts.The BC distance reached 1.85 mm and 3.14 mm

respectively

in the nasopharynx and the pelvis at ∠FEN of 1°.Conclusion:The tilt angle of CT couch directly influences the accuracy of isocenter in radiotherapy

which highlights the importance of quality control of CT couch regarding the accuracy of tilt angle adjustment.