Targeting inaccuracy caused by mechanical distortion of the Leksell stereotactic frame during fixation

Abstract Background The stereotactic frame represents the mainstay of accuracy for targeting in stereotactic procedures. Any distortion of the frame may induce a significant source of error for the stereotactic coordinates. Objective To analyze the sources of distortion of the Leksell frame G induced by fixation to the patient's head and to evaluate the clinical impact of frame distortion on the accuracy of targeting in stereotactic procedures. Methods We analyzed the torques exerted on the fixation screws after frame placement in a series of patients treated stereotactically by an experienced team. We studied the risk for frame bending in an experimental model of stereotactic frame fixation, with increasing torque of fixation screws in a homogeneous and heterogeneous distribution of torques between the four screws. We assessed the impact of expanding dimensions of bending of the Leksell frame both on surgeries utilizing the stereotactic frame, and on radiosurgical procedures with the Gamma Knife. Results Frames were fixed clinically at a range of torques of 0.147–0.522 Nm (mean = 0.348 Nm). The torques did not vary significantly with time. Heterogeneity between the two opposite pairs of screws is often limited, but can reach 96.3%. Distortion of the frame may occur even at minimal levels of torque. Heterogeneity between the two opposite pairs of screws will significantly raise the amount of frame distortion. We found a direct correlation between measures of the frame distortion and extend of the deviation from the stereotactic target in clinical models of stereotactic procedures. Conclusion Stereotactic frames were subjected to distortion due to the torque used for frame fixation. The risk of distortion increased with the torque used and the heterogeneity between the torques of the fixation screws. Distortion of the frame was a significant source of inaccuracy of targeting for stereotactic procedures in clinical practice.


| INTRODUCTION
Efficient stereotactic surgery and Gamma Knife radiosurgery procedures require a high spatial accuracy. This accuracy is achieved by the use of a stereotactic frame, which is firmly fixed to the patient's head. The stereotactic frame with a localizer fixed to it allows the registration of the patient's anatomy for the three-dimensional coordinate system defined by the frame for accurate placement of surgical instruments or for targeting radiosurgical irradiation. To obtain high accuracy during this stereotactic procedure, the stability of all elements of the system is crucial. Among other things, the mechanical stability of the stereotactic frame is a critical issue since it represents the central element for the definition of stereotactic coordinates and the support for targeting. 1 Treuer et al. 2 published the results of a study on the influence of stereotactic frame distortions on targeting accuracy where they showed that in a series of patients undergoing a stereotactic procedure with a stereotactic frame, a mean frame bending of 0.74 mm and maximal bending of 1.30 mm occurred. They advocated that a too firm attachment of a stereotactic head frame to the patient's skull may cause distortion of the frame in clinical practice. They further suggested that the alloy Al-Cu-Mg used as components of the conventional frames allows such distortion and that the use of frames in ceramics could resolve the problem of frame bending. 2 It remains to be known if the distortion of the stereotactic frame will affect the accuracy of stereotactic targeting. Current stereotactic procedures use CT and/or MR imaging acquired after placement of a CT-or MR-indicator box on the frame. What error frame bending will induce on the stereotactic coordinates obtained with this imaging is difficult to estimate. Treuer et al. 2 assessed that with a CT-or MR-localizer with lateral plates fixed with a plate at the top of the localizer, frame distortion will have no significant clinical impact on the stereotactic accuracy of the system. They also asserted that frame bending will have no significant clinical effect in radiosurgery, although noted that frame distortions may lead to some minimal errors in stereotactic localization and consequently could limit the target point accuracy in radiosurgery, which contradicts the earlier assertion.
The aim of our study was to analyze in which circumstances the Leksell stereotactic frame G (Elekta Instruments AB, Stockholm, Sweden) could be mechanically deformed during a stereotactic procedure. The present work focused on the Leksell frame distortions that could be induced by the torques exerted during fixation of the frame on the patient's head (Fig. 1). We analyzed Leksell frames mounted with conventional posts and conventional screws, as suggested in the Instructions for Use. 3 Also, the genuine clinical effect of distortion of the stereotactic frame stays obscure: Does frame bending truly incite a problem in stereotactic focusing for surgeries, for example, DBS surgical procedure and frame-based biopsy, and for Gamma Knife radiosurgical treatment? Different referential systems are fixed on the stereotactic frame for these techniques, and the fixation of different stereotactic instruments (stereotactic arc for surgical procedures, frame adapter for Gamma Knife) to the frame could likely impact in various ways the outcomes of frame distortion on the accuracy of targeting. Our study consists of three different parts. In the first part, we analyzed the clinical expertise of routine procedures of frame fixation by an experienced team and defined the normal range of torques exerted on the Leksell frame by fixation to the patient's head.

| MATERIALS AND METHODS
In the second part, we study the risk of frame distortions created by the use of different levels of torque exerted on the frame with the four fixation screws. In the third part of this study, we studied the clinical impact of frame bending on two different stereotactic applications of the Leksell frame G.

2.A | Clinical experience of frame fixation
Our team had a 20-year experience with more than 4000 applications of stereotactic head frame in a routine clinical setting of stereotactic and functional neurosurgery and Gamma Knife radiosurgery. For this clinical study, we used a high-precision digital torque screwdriver (E.S404, Facom S.A.S. ® , Morangis, France), selfcalibrated, which allows measurements of torque with an accuracy of 3%. Using this screwdriver, we measured the torque exerted individually by the four screws at the end of the frame fixation procedure in 75 patients. We made the same measurements at the end of the stereotactic procedure, before frame removal. The measurements were expressed in Newton-meter (Nm); conversion to inchpound is: 0.1 Nm (Newton-meter) = 0.885 inch-pound; 4 inchpounds = 0.452 Nm. The torques T1, T2, T3, and T4 (recorded at end of frame fixation) and T5, T6, T7, and T8 (recorded at end of the stereotactic procedure, before frame removal) were measured as the torque of the left anterior, right anterior, right posterior, and left posterior screw, respectively.

2.B.1 | Homogeneous torque exerted
In the first part of this experimental study, we checked the influence of an increase in levels of torques homogeneously exerted on the four screws on frame bending. For this experiment, we fixed the frame with four screws tightened to the same torque, according to a protocol for incrementing to different torque levels from 0 to 0.8 Nm in steps of 0.1 Nm. For each level of torque, we measured frame bending for each of the seven Leksell frames G analyzed.

2.B.2 | Heterogeneous torque exerted
In the second part of this study, we analyzed frame distortion caused by frame fixation when torques were exerted at different levels between the 2 sets of 2 diagonally opposite screws, as shown in Fig. 3. T1 and T3 were set at a similar level of torque, T2 and T4 were also set at a similar level of torque, and different levels of torques were set between T1 and T2. Although we found from our clinical experience that only one screw set at a significantly different torque than the three others could alter coplanar properties of the frame, distortion of the frame seems much more important when the two couples of diagonally opposite screws are set at different levels of torque. From this experience, we followed a protocol of increasing torque levels for both couples, as listed in Table 3, columns 1 and 2. Frame bending was measured with the same procedure as already described above.

2.C. | Clinical impact of frame distortion on surgery and radiosurgery targeting accuracy
The clinical consequences of frame bending have been studied on two different stereotactic applications of the Leksell Frame G.  We co-registered both data sets and obtained two sets of stereotactic coordinates of the target. The difference in stereotactic coordinates will represent the inaccuracy of targeting induced by frame distortion in real clinical conditions. The ICON procedure using CBCT as reference will give corrections of coordinates in the three directions and vectorial correction that must be applied to correct the inaccuracy of targeting. 4,5 In order to confirm the accuracy of the correction applied, we used gafchromic films to irradiate the target without and with the correction proposed by the Gamma Knife ICON.

3.A | Clinical experience of frame fixation
The standard procedure of frame fixation recommended by Elekta 3 and described by others 6 was used. The results of these measurements are presented in Table 1. The mean torque exerted on the four screws by frame fixation was 0.348 Nm (3.08 inch-pound) and SD was 0.8 Nm. The minimum torque used for secured fixation was 0.147 Nm, and the maximum torque was 0.522 Nm.
To evaluate the applied torque as a function of time, we measured four torques at T1 to T4 at the end of the frame fixation procedure, and again at the end of the stereotactic treatment several hours later, just before frame removal. We calculated the mean value of the four torques measured at each time point. The median ratio between the two values was 0.99 (SD 0.1), and ranged from 0.73 to 1.19.
We also studied the difference between the torques that were

3.B | Frame distortion model
We applied our experimental protocols of frame distortion with homogeneous torques and heterogeneous torques to 7 different Leksell frames G. All these frames were certified to be within industry standards for accuracy during the 6-month maintenance provided by Elekta Instruments ® , including new frames as well as frames with several years of use.

3.B.1 | Homogeneous torque exerted
The results of the homogeneous torque exerted are presented in Table 2. Data were expressed as mean values for the measurements carried out on the seven frames for each torque level. Figure 5(a) shows the linear regression between the torque exerted on the frame and the mean frame distortion measured in the seven frames.
The mean and maximum frame bending increased to 0.50 and

3.B.2 | Heterogeneous torque exerted
The results of the heterogeneous torque exerted are shown in Table 3. We found that the heterogeneity in the torques applied was associated with a marked increase in frame distortion. The dif-

3.C.2 | Gamma Knife procedures
The measurements performed when the frame was not distorted showed that the difference in stereotactic coordinates was a translation of −0.31 mm in X-axis, −0.55 in Y-axis, and −0.18 in Z-axis.
When increasing distortions were applied to the frame, the differences in stereotactic coordinates were increasing, as shown in Table 4. The vectorial deviation was 0.71 mm when the frame was not bent (physical inaccuracy of the frame) and increased to a maximum of 2.12 mm with increasing frame distortion. The trend line T A B L E 1 Analysis of torques exerted for frame fixation in a series of 75 patients.

Number of patients 75
Torque (T) after frame fixation  Elekta does not stipulate detailed recommendations for the torque exerted for an adequate frame fixation. In the "Instructions for Use" 3 provided by Elekta, it is recommended that "do not overtighten" and "avoid excessive force when tightening the fixation screws, otherwise wrong target can be treated". The explanation for this inaccuracy is that it will be due to bent fixation posts or damaged fixation screws, not to frame bending. This warning has been further addressed by Elekta in a recent Product Bulletin 7 but the risk of frame distortion is not addressed. On the basis of our study, we suggested that the target inaccuracy induced by overtightening the fixation screws may be due to frame bending. less than theirs. We did not routinely use torque wrenches for frame fixation instead we applied the stereotactic frame with the wrenches provided by Elekta ® (Elekta Instruments AB, Stockholm, Sweden).
Safaee et al. did not provide any reference for the assertion that frames could be distorted by overtightening. 6 Our experiments confirmed that torques greater than 4 inch-pounds increased the risk of frame distortion, but torques around 4 inch-pounds could also give rise to significant frame distortion. We then used a box that is not We performed some experiments to evaluate the accuracy of the stereotactic coordinates issued from CT and MR acquisitions when the Leksell stereotactic frame was subjected to some distortion constraint.
We demonstrated that frame distortion will affect significantly the accuracy of the CT-and MR-based stereotactic coordinates. The MR and CT indicator boxes were attached to 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.
Graph showing the relation between frame distortion (Xaxis, in mm) and corrections of the stereotactic coordinates provided by the Cone Beam CT -related ICON procedure (Y-axis, in mm).
For surgical procedures using the Leksell stereotactic arch, the two sources of inaccuracy in targeting will occur: the stereotactic error from imaging acquisition and the stereotactic error from the fixation of the stereotactic arch to the bent frame while for Gamma Knife radiosurgical procedures, only the error from imaging acquisition will occur.
We have shown from our experiment with gafchromic films that the correction proposed by the Gamma Knife ICON system allows correcting efficiently the inaccuracy of targeting induced by frame bending. The stereotactic CBCT of the Gamma Knife ICON apply an accurate correction algorithm. Stereotactic radiosurgery with all devices would probably in the future include systematically a stereotactic 3D imaging acquisition before or even during irradiation to check for inaccuracy of targeting, even when a conventional rigid stereotactic frame is used.

| CONCLUSION S
The Leksell stereotactic frame G may be subject to distortion induced by the torque exerted by frame attachment to the patient's head.
This bending is significantly related to the level of torque applied, and especially to differences of torques applied by the fixation screws.
F I G . 8. Gafchromic films after irradiation of the target when the stereotactic coordinates from the CT indicator box were used with a frame not distorted (left), when the stereotactic coordinates from the CT indicator box were used with a frame distorted by 3.0 mm (median), and when the stereotactic coordinates from the Cone Beam CT of the ICON procedure were used with a frame distorted by 3.0 mm (right). The target was reached with a high accuracy when the frame was not bent (left). With a frame distorted by 3.0 mm, the target was significantly shifted when stereotactic coordinates from the CT indicator box were used (median) and was reached with a high accuracy when the stereotactic coordinates from the Cone Beam CT of the ICON procedure were used (right).
F I G . 9. Dose profile analyses of the films using stereotactic coordinates from the CT indicator box (left) and using stereotactic coordinates from the Cone Beam CT of the ICON procedure (right). The stereotactic coordinates of the target were shifted by 2.635 mm on the dose profile of the films using stereotactic coordinates from the CT indicator box. X-axis: measurement of the deviation of the stereotactic Xcoordinate (in mm) from the center of the radiation target. Y-axis: radiation dose (in Gy).