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Intra-fraction motion gating during frameless Gamma knife® Icon™ therapy: The relationship between cone beam CT assessed intracranial anatomy displacement and infrared-tracked nose marker displacement
Gavin Wright, Jannie Schasfoort, Natalie Harrold, Paul Hatfield and Peter Bownes

Methods: Gamma Knife Icon™’s high-definition motion management (HDMM) system gates treatment delivery should intra-fraction displacement of a nose marker exceed some user-defined threshold. A method, previously-validated with a phantom, is used to relate intra-fractional displacements of the nose marker to displacements of patient targets. Additionally, novel analysis is performed to ascertain the relationship between nose marker displacement and displacement of a 3D grid of coordinates throughout stereotactic space. This spatial information is used to retrospectively review HDMM threshold levels based upon real target locations.

Results: For 41 targets from 22 patients, the mean(standard deviation) and maximum target-to-nose displacement ratio was 0.54(0.32) and 1.65, respectively. On average, displacements typically exceed those of the nose only for coordinates at the most extreme peripheral corner of the investigated 3D grid of points. Allowing target displacement of up to a maximum of 0.8mm, retrospective review indicated that at the locations of the 41 targets a median(range) HDMM threshold of 1.4(1.0-1.9)mm could have been adopted, compared to our standard threshold of 1.0mm.

Conclusions: Intracranial targets typically displace by a magnitude around half that of the nose. Novel analysis to determine the spatial variation of target-to-nose displacement ratio suggests, for our 41 targets, HDMM threshold could have been increased from our standard. Cases for which HDMM threshold could be safely increased would minimise treatment gating events and expedite treatment delivery to offer patient comfort benefits.

Keywords: Gamma Knife, Icon, mask, intra-fraction motion, gating, nose tracking, CBCT

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