Compatibility assessment of Varian and Elekta robotic couch‐assisted six‐dimensional patient positioning correction systems with external independent imaging modalities

Abstract Aim This study aimed to evaluate the compatibility of robotic couch‐assisted six‐dimensional (6D) patient positioning systems with gantry‐mounted and in‐room independent imaging modalities in two different accelerators. Materials and Methods Imaging and 6D patient setup workflow were evaluated for both machines. The imaging workflow consists of onboard (OBI) cone beam computed tomography (CBCT) imaging and in‐room ExacTrac imaging to evaluate setup errors. Setup images were acquired using CBCT, and positional corrections were performed using a 6D robotic couch. Further compatibility of CBCT and ExacTrac was evaluated for both coplanar and noncoplanar beam geometries. Result Varian 6D couch, PerfectPitch, comprises three translational axes (x, y, z): the table column's rotational axis (yaw) and the two additional axes of rotation (pitch and roll) that are affected by the couch's bellows. In this approach, after accounting for 6D shifts, the couch remains physically rotated about its rotational axis (yaw:zROT ≠ 0°). This residual error in yaw rotation was counted in subsequent noncoplanar beam geometries. If the initial yaw couch error was −5.8°, after applying this error a beam planned at 30° couch rotation will appear as 24.2° (= 30°–5.8°) for both CBCT and ExacTrac images. In Elekta's HexaPOD six degrees of freedom (DoF) couch, the column rotational axis is not counted as one of the six DoF. Instead, HexaPOD's bellows can effectively execute all three rotations, and the 3D couch movements that correct setup errors are limited only to the translational axes. A noncoplanar beam planned at a couch rotation of 30° appeared at 30° after patient positional correction. Conclusion Varian 6D robotic couch solutions are only partially compatible with independent in‐room imaging systems.