Room: Karl Dean Ballroom A1
Purpose: In Stereotactic Radiosurgery (SRS) couch walkout can introduce spatial dose errors when non-coplanar beams are applied. We describe a device and practical procedure for measuring vertical as well as horizontal couch walkout and present results for a Varian Edge accelerator.
Methods: The device, the Universal Alignment Ball (UAB), consists of a 25.4 mm diameter acrylic plastic sphere with a concentric 6.35 mm diameter tungsten carbide ball. The UAB is positioned at CBCT isocenter by registering the surface of the plastic sphere in three dimensions to computer-generated contours, circumventing image artifacts that a conventional metal BB placed at isocenter would cause. A dial gauge having 0.01 mm resolution is attached to the collimator. It slides along the surface of the plastic ball, measuring couch deviations from CBCT isocenter along the three major axes as the couch is rotated. Horizontal walkout can be equally measured by taking MV portal images of the UAB and matching the surface of the tungsten ball to a computer-generated circular contour.
Results: Setup of the UAB at CBCT isocenter was accomplished with better than 0.1 mm accuracy. Couch walkout based on MV portal images agreed with dial gauge measurements when averaged over the 180Â° couch rotation within 0.03 Â± 0.07 mm along the x-direction and within 0.00 Â± 0.10 mm along the y-direction. Average couch walkout vector length was 0.17 Â± 0.09 mm measured with the dial gauge and 0.23 Â± 13 mm according to MV portal image measurements. Dial gauge-measured vertical couch walkout from CBCT isocenter was â‰¤ 0.03 mm at any couch angle.
Conclusion: The UAB is a practical device that provides for couch walkout measurements with better than 0.1 mm accuracy. It can be used in conjunction with a dial gauge or by registering MV images with computer-generated contours.