Accuracy of positional correction for the floor-mounted kV X-ray IGRT system in angled couch positions.

Stereotactic irradiation (STI) requires high geometric accuracy. We evaluated the positional correction accuracy after treatment couch rotation for non-coplanar STI with a frameless mask. A steel ball was embedded as a virtual target in a head phantom with a human cranial bone structure, and the head phantom was placed in the isocenter of the treatment-planning system with the image-guide system. The Winston-Lutz test at treatment couch angles of ±90°, ±45°, and 0° was performed, and the amount of displacement from the center position at the treatment couch angle of 0° was calculated. After treatment couch rotation through each treatment couch angle, the amount of center displacement was compared between cases with and without a positional correction by the image-guide system, and then the accuracy of the positional correction after treatment couch rotation was examined. The maximum amount of three-dimensional displacement without and with positional correction after treatment couch rotation was 0.52 mm at a treatment couch angle of -90° and 0.49 mm at a treatment couch angle of -45°. These results indicate that the image-guide system provides accuracy within about 0.50 mm regardless of the positional correction even after rotation of the treatment couch.