Displacement direction Discrepancies between CBCT 6-DOF match and known displacements including deviations from the radiation isocenter (RMS, SD) Translation (mm) AP 0.86, 0.49 SI 1.01, 0.30 LR 0.95, 0.33 Rotation (degree) Pitch 0.38, 0.37 Roll 0.19, 0.10 Yaw 0.22, 0.19

Table 1: Translational and rotational discrepancies between CBCT 6D image registration and known displacements in a head phantom study. Abbreviations: LR = left-right; SI = superior-inferior; AP = anterior-posterior; Pitch = rotation around LR direction; Roll = rotation around SI direction; Yaw = rotation around AP direction.