A program was developed in house to rotate reference computed tomography images. The angles measured by the system were compared with the known rotations. The accuracy of ET/NB was evaluated with a head phantom with seven lead beads inserted, mounted on a treatment couch equipped with a robotic tilt module, and was measured with a digital water level and portal films. Multiple hidden target tests (HTT) were performed to measure the overall accuracy of the different positioning techniques for radiosurgery (i.e., frameless and frame-based with relocatable mask or invasive ring, respectively).
The ET/NB system can detect rotational setup errors with an average accuracy of 0.09¡ã (standard deviation [SD] 0.06¡ã), 0.02¡ã (SD 0.07¡ã), and 0.06¡ã (SD 0.14¡ã) for longitudinal, lateral, and vertical rotations, respectively. The average positioning accuracy was 0.06¡ã (SD 0.04¡ã), 0.08¡ã (SD 0.06¡ã), and 0.08¡ã (SD 0.07¡ã) for longitudinal, lateral and vertical rotations, respectively. The results of the HTT showed an overall three-dimensional accuracy of 0.76 mm (SD 0.46 mm) for the frameless technique, 0.87 mm (SD 0.44 mm) for the relocatable mask, and 1.19 mm (SD 0.45 mm) for the frame-based technique.
The study showed high detection accuracy and a subdegree positioning accuracy. On the basis of phantom studies, the frameless technique showed comparable accuracy to the frame-based approach.