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

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.

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.

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_w : 2.3 mGy (low), and 5.7 mGy (high).

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.