The Gamma Knife Extend System makes possible multi-fraction Gamma Knife treatments. The Extend System consists of a vacuum-monitored immobilization frame and a positioning measurement system to determine the location of the patient’s head within the frame at the time of simulation imaging and before each treatment fraction. The measurement system consists of a Repositioning Check Tool (RCT) that attaches to the Extend frame, and associated digital measuring gauges.

     The purpose of this study is to evaluate the pre-fraction repositioning and intra-fraction immobilization performance of the Extend System for the first ten patients (n=36 fractions) treated at the University of Virginia.

     

     For each patient, the RCT was used to acquire a set of reference measurements of the patient’s position at the time of CT simulation. Before each fraction repositioning measurements were acquired, and the patient position was adjusted until the residual radial difference from the reference position measurements were less than 1mm (if possible). After treatment, position measurements were acquired and the difference from the pre-fraction position was calculated as a measure of immobilization capability. Analysis of patient setup and immobilization performance included calculation of group mean, standard deviation (SD), and distribution of systematic (components affecting all fractions) and random (per-fraction) uncertainty components.

     Across all patients and fractions, the achieved mean radial setup difference from the reference measurements was 0.65mm, SD=0.24mm. The distribution of systematic uncertainty was 0.17mm. The distribution of random uncertainty was 0.16mm. The RMS differences for each plate were right=0.35mm; left=0.41mm; superior=0.28mm; anterior=0.20mm. The mean intrafractional positional difference across all treatments was 0.46mm, SD=0.30mm. The distribution of systematic uncertainty was 0.19mm. The distribution of random uncertainty was 0.22mm. One treatment fraction was excluded from the analysis because the vacuum monitoring interlock detected patient motion, requiring mid-fraction repositioning.

     Data was collected prospectively and reviewed retrospectively.

     With appropriate patient selection, the Extend system can be used to reposition patients with sub-millimeter precision, and appears to have adequate immobilization capability.

     However, 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.