The reproducibility of a HeadFix relocatable fixation system: Analysis using the stereotactic coordinates of bilateral incus and the top of the crista galli obtained from a serial CT scan

We analysed the repositioning accuracy of bite-plate fixations from serial QA-CT (quality-assurance CT) taken during the course of stereotactic radiotherapy. A total of 72 series of CT examinations from 15 consecutive patients, who underwent stereotactic radiotherapy for various intracranial tumours, were examined. Three or four CT scans were obtained for the purpose of QA for the right and left incus, as well as the crista galli. The stereotactic coordinates of the centres of the incus and the top of the crista galli were semi-automatically obtained for each QA-CT scan. Positional displacements for these anatomical reference points and the centre of the points were obtained. The mean displacements for these points in the 3D directions ranged from -0.10 to 0.08 mm (standard deviations: 0.44-0.94). The absolute positional displacement ranged from 0.93 to 1.09 mm (standard deviations: 0.52-0.88 mm). The rotations of the head were 0.49 0.36°. Our 3D measurement technique using anatomical landmarks revealed excellent stability of the mouthpiece fixation system in terms of translational and rotational displacements. This technique can also be used as a QA method for other fixation devices.