Background: Distortion of the Leksell stereotactic frame can occur during surgical and radiosurgical frame-based procedures. The targeting accuracies of surgical procedures such as DBS surgery or frame-based biopsy, or Gamma Knife radiosurgical procedures, are related to the stereotactic frame that is commonly referred to a non-deformable referential system. Objective: To evaluate the clinical impact of frame distortion on accuracy of targeting in various stereotactic procedures. Methods: We studied the influence of increasing levels of distortion of the Leksell frame both on surgical procedures using the stereotactic arch, and on radiosurgical procedures with the Gamma Knife. For surgical procedures we applied the Target Simulator of Elekta to a frame submitted to different levels of frame distortion, and we measured the modifications of accuracy of targeting. For radiosurgical procedures the Leksell frame was applied on an anthropomorphic phantom and modifications of target coordinates were measured by an imaging method using the stereotactic CBCT of the Gamma Knife ICON. Results: For surgical procedures, we found a linear relation between the amount of the frame distortion and the extent of the deviation from the stereotactic target using the stereotactic arch (R 2 = 0.99709). The level of bending of the frame is also linearly related to the level of inaccuracy of stereotactic targeting based on acquisition of MRI and CTscan with the Elekta imaging boxes (R 2 = 0.96825). The inaccuracy of targeting related to frame bending can be avoided by a systematic control at the end of frame placement and by the use of the CBCT of the Gamma Knife Icon. Conclusion: Distortion of the frame is a significant source of clinical inaccuracy of targeting for stereotactic procedures. Frame bending must be checked after each frame placement and corrected if needed to avoid targeting inaccuracy during stereotactic surgery and radio-surgery procedures.
We have observed, as other, in our clinical daily practice that the pressure exerted on the fixation screws to secure the frame on the patient’s head can create a significant distortion of the Leksell head frame [
The aim of our work was to study the clinical impact of targeting inaccuracy produced by distortions of the Leksell frame when applied in various stereotactic conditions.
The present study was approved by the local Institutional Review Board of the Ethical Committee of our institution (ref. P2015/207). Informed consent of all patients included in this study was obtained.
Our analysis has been focused on two different stereotactic applications of the Leksell G frame.
The first experiment would study the effect of frame distortion on targeting in 60 surgical procedures performed with the Leksell stereotactic arch applied on the frame. For this purpose, we have used the Target Simulator of Elekta® (Elekta Instruments, Sweden). We applied the Target Simulator on a stereotactic frame not strained by any significant pressure. We fixed the stereotactic arch on the frame and we used a Sedan biopsy needle of 2.5 mm diameter to reach the target (
The second experimental study focused the influence of distortion of the Leksell
frame on the radiation targeting of the Leksell Gamma Knife Icon® (Elekta Instruments, Sweden). We placed the Leksell frame on the head of an anthropomorphic phantom and we applied different conditions of pressure in order to create increasing distortions of the stereotactic frame. A series of 30 measurements were performed. We evaluated the targeting inaccuracy induced by distortion of the frame with the following method: we compared the stereotactic coordinates of a target located in the head issued from 2 different stereotactic CTscan. The first one was the conventional stereotactic 1-mm 3D CTscan acquired after having placed the CT Indicator box of Elekta® (Elekta Instruments, Sweden) on the frame, as used in a clinical routine. The location of the fiducial markers sets on the tomographic images was used by the Stereotactic Reference Definition program of Leksell GammaPlan 11.0 to define the stereotactic coordinates of tomographic images. The indicator box is docked on the frame; so, bending of the frame will unavoidably influence the stereotactic coordinates issued from the box. The second stereotactic CTscan was the stereotactic Cone Beam CT (CBCT) of the Gamma Knife Icon [
We placed the Target Simulator on the stereotactic frame strained by different pressures to create 6 values of frame bending: 0.5 mm, 1.0 mm, 1.5 mm, 2.0 mm, 2.5 mm and 3.0 mm. We measured the distance of the tip of the stereotactic cannula from the target. Les values recorded were respectively 0.3 mm, 0.8 mm, 1.1 mm, 1.5 mm, 1.8 mm and 2.2 mm (
We performed measurements of the variation between the stereotactic coordinates of the CT-related targeting and CBCT-related targeting, when the frame was not distorted and with 5 levels of frame distortions at 0.5 mm, 0.85 mm, 1.0 mm, 1.5 mm and 2.0 mm (
DISTORTION | X-axis | Y-axis | Z-axis | Vectorial |
---|---|---|---|---|
0 | −0.63 | −0.31 | −0.10 | 0.71 |
0.5 | −1.21 | −0.18 | 0.03 | 1.22 |
0.85 | −1.24 | −0.26 | 0.10 | 1.27 |
1.0 | −1.33 | −0.09 | 0.13 | 1.34 |
1.5 | −1.63 | −0.03 | 0.24 | 1.65 |
2.0 | 2.08 | −0.02 | 0.41 | 2.12 |
Analysis of the gafchromic films of the irradiated target obtained after that the correction proposed by the Gamma Knife ICON system has not or has been applied, confirmed the accuracy of the correction applied. The
The stereotactic frame is widely used for surgical or radiosurgical procedures when a high accuracy of targeting is expected. Our practice as well as some experimental published studies [
The results of our experience using the Target Simulator of Elekta as experimental model of frame distortion have shown that distortion of the Leksell frame has a direct and significant impact on the accuracy of targeting for stereotactic procedures using the stereotactic arch. The fixations of the stereotactic arch applied on both sides of the frame are in their turn distorted by frame bending, which will as a consequence influence the accuracy of targeting of the stereotactic instruments. We found a linear relation between the amount of frame distortion and the extent of deviation from the target.
We have performed some experiments to evaluate the accuracy of the stereotactic coordinates issued from CT and MR acquisitions when the Leksell stereotactic frame is subjected to some distortion constraint. We have demonstrated that frame distortion will affect significantly the accuracy of the CT- and MR-based stereotactic coordinates. The MR and CT indicator boxes are applied on the frame and when the frame is bent, the imaging series will provide inaccurate stereotactic coordinates. There is a linear relation between the importance of frame distortion and the level of deviation of the stereotactic coordinates issued from imaging acquisition.
For surgical procedures using the Leksell stereotactic arch, the two sources of inaccuracy in targeting will occur: the stereotactic error from imaging acquisition is added to the stereotactic error from fixation of the stereotactic arch to the bended frame. For Gamma Knife radiosurgical procedures, only the error from imaging acquisition will occur.
Preventive measures can be taken to avoid distortion of the Leksell frame during fixation on the patient’s head. The first one is to follow the instructions for use provided by Elekta [
For radiosurgical procedures, the used the Gamma Knife Icon® to adjust stereotactic coordinates when the frame and frame-based imaging acquisitions have some sources of errors, as frame distortion. The stereotactic CBCT that can be acquired before irradiation gives 3D CT images that can be used to obtain stereotactic coordinates of the target independently of the frame [
Distortion of the Leksell frame is a source of inaccuracy of frame-based coordinates of stereotactic targets for surgical or radiosurgical procedures. The amount of deviation from the target is related to the level of frame bending. Image acquisition with the CT- or MR-box and fixation of the stereotactic arch on a distorted frame represent the two sources of inaccuracy in stereotactic targeting. The use of an undeformable box at the end of application of the frame and use of the CBCT of the Gamma Knife Icon can avoid the consequences of frame distortion as a source of targeting inaccuracy in clinical use.
The authors report no conflict of interest concerning the materials or methods used in this study or the findings specified in this paper.
No grant or any source of financial or material support was received for this study.
No any portion of this study has been presented or published previously.
Renier, C. and Massager, N. (2018) Clinical Impact of Targeting Inaccuracy Due to Distortion of the Leksell Stereotactic Frame. Open Journal of Modern Neurosurgery, 8, 315-322. https://doi.org/10.4236/ojmn.2018.83026