Introduction

Frame-based stereotactic procedures are the gold standard because of their superior stereotactic accuracy for both biopsies and placement of depth electrodes [1]. Non-frame-based systems are routinely used when lower precision is acceptable.

Frame-based stereotactic procedures are a multiple-step procedure in standard practice. Previously our practice was to apply the stereotactic head ring in the morning at the ward. The stereotactic CT scan was performed at the Department of Radiology (DR) before noon, and the biopsy was performed after that. In case of intubated patients the head ring was placed in the OR, then the patient was transferred to the DR, scanned and transferred back to the OR. We here present a single-step method for a stereotactic procedure utilizing intraoperative CT (iCT). We describe the practicality and results from our first 8-month usage of this technique.

Method

The patient records including OR registration were retrospectively surveyed and served as quality control; thus, no patient informed consent was needed according to the local ethics committee. Additional informed consent was obtained from all individual participants for whom identifying information was included in this article.

For the procedure a combined fixation and low profile frame holder was designed for the operating table (Figs. 1 and 2).

Fig. 1
figure 1

This low profile frame holder is fixed to the operating table. The patient's head is fixed to the frame and the CT scan performed. With the frame attached to the table the vertical position can be obtained compensating for the fixed position of the gantry. The patient position will not be changed until after the biopsy

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Fig. 2
figure 2

Low profile frame fitted during biopsy demonstrating multiple locations for access possibilities. In cases of infratentorial biopsies the table is elevated to maximal height, allowing the surgeon to work sitting

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The procedure for awake patients is as follows:

  • In the OR the HRAIM (Radionics® Radiosurgery Head Ring Assembly) base frame is fitted to the patient’s head. The patient’s head is positioned centrally in the scanner gantry (Toshiba & iCT Brainsuite Brainlab AG, Germany), and intravenous contrast is simultaneously administrated, if necessary. The staff leaves the operating room (OR), and the iCT-scan is performed within 3 min. The iCT moves on rails, and the operating table position is fixed. The images are automatically transferred to iPlannet (Brainlab AG, Germany) and fused with a preoperative MRI including a biopsy trajectory plan if formulated beforehand. On planning the biopsy entry position was carefully chosen to be the top of a gyrus, avoiding cortical vessels as seen on Gd-MRi.

  • The time spent on image transfer, fusion and final confirmation of the plan is generally less than 10 min. The biopsy procedure is performed while the patient stays on the same operating table, with the same head fixation. Since the iCT moves away on rails, the operating procedure (as well as table) is free of physical interference from the scanner (Fig. 2). This allows the patient to maintain a supine position in all cases of biopsy, thus avoiding a brain shift using a 2.7 mm twist drill with no escape of fluid. For the DBS patient the position was changed after scanning to a semi-sitting position so the trajectories were near vertical and the shift of the brain (with a possible brain shift using a 10-mm burr hole) along the course.

  • If intubation is needed, the above steps are followed after intubation.

Results

During the first 8 months (December 2012–August 2013) this procedure was performed 65 times. Our system has allowed us to perform biopsies from all intracranial locations (Table 1). The duration of the procedure from the patient's arrival in until the departure from the OR for awake patients was on average 81 min [median 76, 33 to the 202 min; for intubated patients (children) on average 89 min (median 89, 78–100 min)]. For the placement of depth electrodes for evaluation of epilepsy or deep brain stimulation (DBS), including IPG (implantable pulse generator) placement, the duration was on average 281 min (median 309, range 243 to 377), including an iCT scan to document the electrode positions. Of the 40 patients for whom a biopsy confirmed a glioma (malignant or low grade), 15 were taken in the frontal lobe, 10 the parietal, 4 the temporal, 1 the occipital lobe and 10 the central core. In the posterior fossa/pons four biopsies were performed, two in the pons respectively confirming a demyelating disease and facilitating aspiration of a cyst in a pons glioma. The other two cases in the posterior fossa were tumor diagnostic in the cerebellar peduncle.

Table 1 Tumor locations
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Eight pediatric cases were performed, all using general anesthesia (Table 1). Before implementation of iCT, pediatric cases were cumbersome because of the transportation of intubated and sedated patients between our OR and Department of Radiology two floors below. This was resource intensive and carried a potential risk for the patient (Table 2).

Table 2 Time calculated from the patients' arrival in the OR until the procedure was completed, including placing an IPG (implantable pulse generator) in DBS; time in min
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Figure 3 shows the different types of procedures performed and the pathologies diagnosed with the biopsies. The most common indication was diagnostic biopsies of tumors where the diagnosis was confirmed 48 times in adults. In four of five pediatric cases where the stereotactic biopsies demonstrated that the tumor was benign, surgical resection of the tumor was performed at a later date. A definite histological diagnosis could be obtained in all cases of biopsy. The position of the depth electrodes for intracerebral EEG was documented on CT performed immediately after placement and integrated into the preoperative plan. The accuracy of the position of the tip was within a few millimeters.

Fig. 3
figure 3

The number of patients with different indications for a frame-based stereotactic procedure. *Nn Non-neoplastic, DBS deep brain stimulation

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Besides the enhanced safety, workflow and comfort for the patient, this new setup allows for a mediate control CT scan. This is a great help if hemorrhage is suspected or verification of a biopsy location or electrode placements is needed. In all patients where the procedure was used for placing depth electrodes for epilepsy (2 patients) and for aspiration of a cyst (1 patient), a second CT scan was performed. In none of the cases did it lead to a renewed stereotactic procedure although this would be possible had for example a cyst aspiration been insufficient. Verification of placement of electrodes in epilepsy surgery patients is an integral part of the intracranial EEG investigation.

Discussion

This retrospective study describes a single-stage procedure for stereotactic biopsies using iCT. The many advantages include reduced time for the patient with the head ring mounted, less discomfort with no need to move from one department to another wearing the frame, elimination of the transfer of intubated patients, easy access to the CT for follow-up of cyst aspirations, electrode placement, possible hemorrhage and other complications. With this technique the procedural time may be as short as 33 min, and all locations in the brain are accessible, like when using conventional frame-based stereotaxy.