Spatial shifts in frame-based Gamma Knife radiosurgery: A case for cone beam CT imaging as quality assurance using the Gamma Knife® Icon™

Abstract

Background: Cone beam CT (CBCT) imaging has been integrated into the most recent version of the Leksell Gamma Knife for the primary purpose to facilitate fractionated therapy.

Case description: This case study presents three patients where the CBCT system of the Gamma Knife Icon discovered potentially clinically significant frame shifts. In each case, patients were imaged with volumetric MR prior to stereotactic frame placement. Immediately following frame placement, diagnostic stereotactic CT imaging was acquired with a stereotactic indicator box attached to the frame. Following treatment planning and immediately before radiosurgery, a CBCT was acquired using the on-board imaging functionality of the Gamma Knife Icon, which provides a registration of the patient's anatomy to stereotactic space independent of that provided by the stereotactic frame/fiducials. Co-registration of the CT and CBCT provides an estimate of the difference between these two estimates of stereotactic coordinates. The vector magnitudes of the differences measured at the center of stereotactic space were 0.93mm, 2.64mm and 2.18 mm for Case 1, Case 2 and Case 3 respectively.

Conclusions: Use of the CBCT functionality of the Gamma Knife Icon to verify the consistency of frame placement can prevent clinically significant targeting errors due to frame slippage or frame adapter mounting errors, and allows any required adjustments to be made without interrupting the overall treatment workflow.

Keywords: Gamma Knife; cone beam CT; frame shift; quality assurance; radiosurgery; stereotactic frame.

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Figure 1

Treatment planning imaging for a right vestibular schwannoma. (A) The post frame placement diagnostic CT demonstrates positional shift during image acquisition, most clearly indicated by bending of the straight vertical bars of the head frame as shown by the white arrows. (B) The fusion of the Icon CBCT and diagnostic pre-procedural CT in the axial plane demonstrates misalignment, most evident in the area indicated by the white arrow. (C) The treatment plan defined by the diagnostic CT placed over the treatment planning MR co-registered to the CBCT-based stereotactic coordinates demonstrates significant portions of GTV that would have been undertreated and areas of normal parenchyma that would have received full prescription dose.

Figure 2

Treatment planning for a left cerebellar metastasis. (A) Axial view of one of the frame pins on the stereotactic fan beam CT. (B) Coronal view of one of the frame pins on the stereotactic fan beam CT. (C) Axial view of the same pin on the pre-treatment CBCT. (D) Coronal view of the same pin on the pre-treatment CBCT. Comparison of the stereotactic fan beam CT with the pretreatment CBCT shows that the pin was not securely anchored with an accompanying frame shift.

Figure 3

Treatment planning imaging for multiple brain metastases. (A) Axial view of one of the frame pins on the stereotactic fan beam CT. (B) Coronal view of one of the frame pins on the stereotactic fan beam CT. (C) Axial view of the same pin on the pre-treatment CBCT. (D) Coronal view of the same pin on the pre-treatment CBCT. Comparison of the stereotactic fan beam CT with the pretreatment CBCT shows that the pin was not securely anchored with an accompanying frame shift.

Figure 4

Treatment planning imaging for multiple brain metastases. The uncorrected plan of one of multiple brain metastasis (blue line) based off the stereotactic fan beam CT registered to the planning MRI overlaid with the corrected volume (yellow line) generated from the pretreatment CBCT registered to the planning MRI. Using the stereotactic CT for planning would have undertreated the metastasis and applied full dose to normal brain parenchyma.