Evaluation of stability of stereotactic space defined by cone-beam CT for the Leksell Gamma Knife Icon

Abstract

The Gamma Knife Icon comes with an integrated cone-beam CT (CBCT) for image-guided stereotactic treatment deliveries. The CBCT can be used for defining the Leksell stereotactic space using imaging without the need for the traditional invasive frame system, and this allows also for frameless thermoplastic mask stereotactic treatments (single or fractionated) with the Gamma Knife unit. In this study, we used an in-house built marker tool to evaluate the stability of the CBCT-based stereotactic space and its agreement with the standard frame-based stereotactic space. We imaged the tool with a CT indicator box using our CT-simulator at the beginning, middle, and end of the study period (6 weeks) for determining the frame-based stereotactic space. The tool was also scanned with the Icon's CBCT on a daily basis throughout the study period, and the CBCT images were used for determining the CBCT-based stereotactic space. The coordinates of each marker were determined in each CT and CBCT scan using the Leksell GammaPlan treatment planning software. The magnitudes of vector difference between the means of each marker in frame-based and CBCT-based stereotactic space ranged from 0.21 to 0.33 mm, indicating good agreement of CBCT-based and frame-based stereotactic space definition. Scanning 4-month later showed good prolonged stability of the CBCT-based stereotactic space definition.

Keywords: CBCT; Gamma knife; Leksell space; QA tool; icon; stereotactic radiosurgery.

Figure 1

(a) The manufacturer's CBCT tool used for calibrating the CBCT position, and (b) the QA tool Plus used in daily CBCT precision tests.

Figure 2

Different views of the in‐house marker tool: an assembled G stereotactic frame with five ball‐bearing markers (pointed with red arrows) attached to a taut string tied to two posts of the G‐frame.

Figure 3

(a) CT scanning of the in‐house marker tool with the CT indicator box attached to; and (b) the same tool mounted on the Gamma Knife Icon couch adapter in preparation for CBCT.

Figure 4

A snapshot of the Leksell GammaPlan treatment planning software showing a zoomed‐in and maximally contrasted CBCT image of one of the markers (marker (a) is shown here). The coordinates reported for each marker were the average of three readings of the geometrical center of markers as determined visually by a single observer.

Figure 5

Results of daily CBCT precision QA test. The graph shows the variation of maximum deviation in image volume between the daily QA tool Plus and the expected baseline taken during calibration of the tool.

Figure 6

Daily variations of CBCT‐based stereotactic coordinates from the mean frame‐based in the lateral, vertical, and longitudinal directions and the magnitude of vector difference (r) of each marker over a period of 6 weeks.