Quality assurance tests for the Gamma Knife Icon™ image guidance system.

Introduction: The Gamma Knife Icon™ comes with an image guidance system for tracking patient motion and correcting for inter- and intrafractional shifts, mainly used with frameless thermoplastic immobilization. The system consists of a cone-beam CT (CBCT) and a couch-mounted infrared camera (IFMM). We report our quality assurance program for Icon's image guidance system.

Methods: The manufacturer-provided tool is used for daily checks of CBCT positional precision. Catphan phantom is used for monthly checks of CBCT image qualities (uniformity, contrast to noise ratio (CNR), and spatial resolution) for the two acquisition presets (low-dose and high-quality presets). On a semi-annual schedule, we use a frame tool to check the agreement of CBCT-based and Frame-based stereotactic space coordinates by comparing the locations of five attached ball bearings in CT-sim scans (Frame-based coordinates determination) and in Icon's CBCT scans. On an annual basis, the accuracy of IFMM, image registration, and delivery-after-shift are tested using a translational stage. A weighted CT dose index is measured annually with a pencil chamber in CTDI head phantom.

Results: The CBCT precision check: 0.12 ± 0.04 mm (maximum deviations average). CBCT image quality: spatial resolution range: [6,7] lp/cm (low), and [7,8] lp/cm (high); uniformity: 12.82 ± 0.69% (low), and 13.01 ± 0.69% (high); CNR: 1.07 ± 0.08 (low), and 1.69 ± 0.10 (high). Agreement of CBCT-based with Frame-based stereotactic coordinates range: [0.33, 0.66] mm. Accuracy of IFMM: 0.00 ± 0.12 mm (average) with 0.27 mm (max.); image registration: 0.03 ± 0.06 mm (average) with 0.23 mm (max.); and delivery-after-shift: 0.24 ± 0.09 mm (average) with 0.42 mm (max.). CTDI : 2.3 mGy (low), and 5.7 mGy (high).

Conclusions: The manufacturer-required QA checks together with additional user-defined checks are an important combination for a robust quality assurance program ensuring the safe use of Gamma Knife Icon™'s image guidance and motion management features.