Abstract: Background and purpose: For off-center patients, split arcs are used for radiotherapy plan to increase the tumor control rate, as the traditional linear accelerator (linac) gantry cannot span 180°. This method increases the idling time and the risk of collision between the gantry and the patient. The purpose of this study was to compare and analyze the potential advantages in volumetric-modulated arc therapy (VMAT) plans between the United Imaging Healthcare (UIH) linac 540° gantry and the traditional 360° gantry. Methods: In this study, 100 arc plans were randomly generated according to different arc lengths and arbitrary starting points, and they were divided into two groups. Group A included arcs that could span 180°, and group B plan consisted of two arcs on both sides of 180° with total arc length equal to group A. By using the trajectory simulation software, the double S velocity model was used to describe the rotational trajectory, and the execution efficiency simulation was carried out considering the reset time. In addition, the efficiency advantages of group A and B were also analyzed based on 10 actual clinical cases. Results: Comparing the execution time of group A and B, the difference mainly came from the idling time of gantry without considering the reset time. For clinical routine, the idling time was proportional to the proportion of the double arc plan. The maximum saving time was the full double arc plan, saving about 32 minutes. The maximum execution time of full arc plan could be reduced by about 85 minutes in clinical situation. Conclusion: For VMAT plan in which tumors are deviated from the center of patient, the UIH machine can save the idling time of the gantry and improve efficiency while meet the clinical dose distribution. In addition, it can effectively avoid the risk of collision between the gantry and the patient.

Key words: UIH 540°, Gantry angle, Angle optimization