Abstract Purpose: To assess the accuracy of the Varian PerfectPitch six-degree-of-freedom (6DOF) robotic couch by using a Varian SRS QA phantom. Methods: The stereotactic radiosurgery (SRS) phantom has five tungsten carbide BBs each with 7·5 mm in diameter arranged with the known geometry. Optical surface images and cone beam CT (CBCT) images of the phantom were taken at different pitch, roll and rotation angles. The pitch, roll, and rotation angles were varied from −3 to 3 degrees by inputs from the linac console. A total of 39 Vision RT images with different rotation angle combinations were collected, and the Vision RT software was used to determine the rotation angles and translational shifts from those images. Eight CBCT images at most allowed rotational angles were analysed by in-house software. The software took the coordinates of the voxel of the maximum CT number inside a 7·5-mm sphere surrounding one BB to be the measured position of this BB. Expected BB positions at different rotation angles were determined by multiplying measured BB positions at zero pitch and roll values by a rotation matrix. Applying the rotation matrix to 5 BB positions yielded 15 equations. A linear least square method was used for regression analysis to approximate the solutions of those equations. Results: Of the eight calculations from CBCT images, the maximum rotation angle differences (degree) were 0·10 for pitch, 0·15 for roll and 0·09 for yaw. The maximum translation differences were 0·3 mm in the left–right direction, 0·5 mm in the anterior–posterior direction and 0·4 mm in the superior–inferior direction. Conclusions: The uncertainties of the 6-DOF couch were examined with the methods of optical surface imaging and CBCT imaging of the SRS QA phantom. The rotational errors were less than 0·2 degree, and the isocentre shifts were less than 0·8 mm.

Rotation setup errors for SRS treatments are typically less than two degrees which usually causes less dosimetry discrepancy for a typical SRS treatment as the target size is likely less than 4 cm. However this might not be true for the single isocenter and multiple metastatic targets' technique where, the distances between the isocenter of the treatment and those tumor centers can be very large. A 6DOF couch can be used to correct this setup error by using optical surface and/or Conebeam CT (CBCT) images. The accuracy and consistence between optical surface and CBCT images were studied. The CBCT system and optical surface images of an SRS phantom were taken at different pitch, roll, and rotation angles varied from -3 to 3 degrees by intentionally changing values on the service console. A total 39 combinations of pitch, roll, and rotations using the 6DOF couch were generated. The optical surface images were taken and Vision RT software was used to determine the rotation angles and isocenter shifts. Eight CBCT images corresponding to +- 3 degree changes in pitch, roll, and rotation were taken. The center positions of the 5 tungsten carbide BBs contained in the SRS QA phantom were determined using in-house software. A least square fit was used to determine translation/rotation parameters. Among the 39 calculations from optical images, the maximum rotation angle differences were 0.2, 0.1, and -0.2 degrees for pitch, roll, and couch rotation, respectively. The maximum isocenter shifts were -0.80 mm, 0.70 mm, and -0.10 mm in lateral, longitudinal, and vertical axes, respectively. The maximum residual rotation angles from analysis of CBCT images were 0.20 degree for pitch, roll, and rotation. By using 6DOF couch, the rotation errors can be reduced dramatically compared to initial rotation errors. To overcome this rotation errors without 6DOF couch, one has to introduce extra margin for target which is far away from the treatment iso-center. Six DOF can avoid extra CTV-PTV margin. The residual rotational errors studied for the couch were less than 0.2 degree. This accuracy for the 6DOF system is sufficient for SRS applications even for single isocenter multiple tumor technique and extra CTV-PTV margin might be not necessary for some cases.