Introduction:We have developing a collision-detection simulator between treatment units and a patient for dynamic wave arc(DWA)irradiation achieving dose delivery to a target by continuous ring and gantry rotation in Vero4DRT system.The purpose of this study was to perform verification of the simulator by measuring distance between the gantry and the couch at several gantry and ring angles; and couch positions in the Vero4DRT system and comparing them with the corresponding distance computed by the simulator.Methods:Using an optical tracking system(Polaris,NDI),the gantry-couch distance was measured at several gantry,ring angles; and vertical,longitudinal couch positions,respectively.On the other hand,the corresponding distance was computed under the same conditions by the simulator.Then,non-coplanar coordinates of vertices on the outline of the couch were measured at the initial position and the position shifted in the vertical direction,respectively.Subsequently,the corresponding coordinates were computed by the simulator.Next,a 4×4 matrix that gives coordinate system transform from the simulator to the treatment room was calculated.Then,coordinates in the simulator were transformed to those in the treatment room.Next,the vector from the rotation center of the gantry to the center of the couch was measured while the corresponding vector was computed by the simulator.The simulator was adjusted using the displacement vector between them.After that,the gantry-couch distance at the same positions was recomputed.Results:Each coordinate was measured with an accuracy of 0.01 mm.The difference between measured and simulated distance was 16.8 mm on average(maximum:32.0 mm); however,it was reduced to 7.6 mm(maximum:17.0 mm),respectively.Conclusion:We have observed that our collision-detection simulator had an accuracy of < 20 mm for the gantry-couch distance after geometric correction although further improvement was required.