Room: Exhibit Hall | Forum 6
Purpose: The Leksell Gamma Knife ICON system (LGKS) CBCT has the ability to correct for rotational and translational inter-fractional motion. We tested the rotational repositioning accuracy of the LGKS with a prototype 6 degree of motion platform and an anthropomorphic phantom with gafchromic film.
Methods: Repositioning accuracy was tested by acquiring an initial CBCT defining the stereotactic reference of a cube with a central bb positioned at the origin of the LGKS coordinate system. Rotational shifts of 1.00, 5.00, and 10.00 +/- 0.01 degrees were applied to the cube before acquiring a subsequent CBCT. This was registered to the stereotactic reference for angular repositioning accuracy. To test for repositioning accuracy of shot delivery, a plan was created with an anthropomorphic phantom (AP) with four spatially separated shots. The rotational corrections were analyzed by measuring the widening focal spot size. Additionally, translational shifts were applied verifying treatment interruption beyond set 3mm limit and the accuracy of the High Definition Motion Management system and translational repositioning accuracy.
Results: Rotational repositioning accuracy was tested for roll, pitch and yaw with maximum registration differences of 0.09, 0.03, 0.08 degrees. Translational repositioning was verified for shifts up to 1cm with an accuracy of 0.2mm. Registration was performed with matching window levels and a tight region of interest (ROI) on anterior edge of the cube. Treatment delivery of widely variable positioning fractionated treatment of the AP showed a widening of a four shot plan delivery of 2mm of the FWHM.
Conclusion: The rotational repositioning accuracy of the LGKS CBCT system was within 0.1 degrees in all three rotational directions for small, medium and large rotations. Although there is no correction from differences in attenuation which may cause differences in planned and delivered dose, the repositioning accuracy for both rotations and translation is within 0.2 mm.