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Sources and Magnitudes of Uncertainties Using the Gamma Knife Extend System

D Schlesinger

D Schlesinger*, Z Xu, F Taylor, J Sheehan, University of Virginia, Charlottesville, Virginia

SU-C-BRCD-2 Sunday 1:30:00 PM - 2:15:00 PM Room: Ballroom CD

Purpose: The Gamma Knife Extend System uses a vacuum-assisted dental fixation mounted on a carbon-fiber frame to achieve non-invasive immobilization appropriate for multi-fraction Gamma Knife procedures. At simulation and before each fraction, patient position is determined through the use of high-precision digital mechanical measurement probes and a reference positioning template (RCT). The objective of this study is to quantify uncertainty in the Extend System from patient setup, intrafraction immobilization, and observer uncertainty in taking positioning measurements.

Methods: Positioning measurements for 10 patients/ 36 fractions were used to estimate setup and immobilization uncertainty. Setup uncertainty was quantified by recording the residual difference between pre-fraction measurements after repositioning and reference measurements taken at the time of CT imaging. Intrafraction positioning uncertainty was estimated by the difference between post-fraction and pre-fraction position measurements. Observer uncertainty was estimated by three observers taking three measurements on each of the 44 measurement holes in the RCT using a phantom as a subject.

The distributions of the systematic (S) and random (s) components of the setup and immobilization measurement differences were calculated. The distribution of the random component of the observer measurement variation was determined.

Results: The mean radial setup difference from the reference measurements was 0.65mm, SD=0.24mm, S = 0.17mm, s = 0.16mm. The mean intrafractional positional difference was 0.46mm, SD=0.30mm, S=0.19mm, s= 0.22mm. The distribution of the random component of observer uncertainty was 0.03mm.

Conclusions: The Extend System appears to have uncertainties similar in magnitude to other noninvasive fixation systems. Care should be taken to acquire measurements that can implicitly account for rotations of the patient's head. Further work is required to determine the sensitivity of the vacuum interlock to detect patient motion and to determine the systematic inter/intra-observer uncertainty.

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