Verification and validation (VnV) of control software for an isocentric rotation chair positioner used to treat patients with head/neck cancers in a fixed carbon-ion beamline

To utilize the full capability of the isocentric rotating chair positioner for the image-guided radiotherapy treatment (IGRT) in a carbon-ion fixed beamline, a verification and validation (VnV) procedure was deployed prior to commissioning the positioner for its clinical application. Based on the structural characteristics of the motion mechanisms and the treatment workflow, we developed the mathematical manipulations to construct an operational-matrix implemented for executing 6 Degree-of-Freedom (6-DoF) displacements required by IGRT system in the IEC fixed (I-fix) coordinate. We initially found that a residual misaligned Z-axis of the rotating-platform (Z(Irpf)) introduced deviations in the order of a few mm for rotations over 360 degrees. We established the sets of categorized VnV-variable to evaluate few residual (but minor) misalignments of the various mechanisms that can cause deviations in the measured displacements. We developed a concept of correlation using each set of VnV-variable to construct a correction-matrix applied on the operational matrix. Each correction uses the Hexapod to model and compensate the deviations induced by a specific misaligned mechanism. We applied the correction-matrix using VnV-variable values of 0.05 degrees and 2.79 mm for the misalignment of Z(Irpf). These values were determined during the VnV procedure. By applying correction-matrix, we achieved an accuracy of +/- 1.0 mm in-translation and +/- 1.0 degrees in-rotation of 6-DoF displacements having pitch/roll within 2 degrees for a clinical trial treating 17 patients using treatment fields without pitch/roll. To utilize this positioner for treatment fields with +/- 20 degrees pitch/roll, we need to construct and implement an additional correction-matrix in the control software to achieve the required clinical accuracy.