Arlan Mintz, MD
Assistant Professor of Neurological Surgery
Director, Adult Surgical Oncology

Karl Lozanne, MD
PGY-6 Resident

Pittsburgh, June 1, 2008 -- Traditional methods to obtain a tissue biopsy from within the brain require the rigid fixation of a stereotactic frame, followed by imaging with a CT or MRI and then the target location can be calculated.

Frameless stereotactic techniques have provided a useful alternative to rigid head frame fixation. These techniques make use of a pointer-based referencing system that utilizes anatomical landmarks or self-adhesive fiducial markers on the skin. A computer image guidance system stores the pre-operative imaging that is co-registered to the fiducials by a registration probe and camera. To maintain accurate localization throughout the procedure they require invasive rigid head fixation in the form of a Mayfield head holder. Rigid head fixation can be painful and the head movement limitation intolerable to some awake patients. If the patient moves, they risk lacerating the scalp. Furthermore, there is risk of cranial fracture, epidural hematoma, and cerebrospinal fluid (CSF) leak following the application of head pins, especially in young children. From the surgeon’s perspective, rigid head fixation can also be problematic since they are bulky and limit intraoperative flexibility as well as free movement of surgical instruments. If the patient’s head moves relative to the reference arc, the accuracy of the system is reduced, potentially compromising successful execution of the procedure.

Figure 1. (left) Autoregistration mask adhered to face; (right), baseplate with swiveling cannula attached.

We have developed a technique for obtaining a biopsy of intra-axial tumors in awake patients using a frameless, fiducial-less technique without the use of head fixation (pinless). Prior to the procedure a contrast enhanced MRI or CT scan of the brain using thin 1.5 mm slices obtained. The image set is imported into the Stryker iNtellect Cranial Navigation computer work station via computer network. The patient is placed supine on the operating room table with the head comfortably resting on a horseshoe headrest. The patient receives intravenous neuroleptic sedation and an autoregistration mask with multiple light emitting diodes (LEDs) is adhered to the patient’s face covering the bridge and both sides of the nose, the left and right periorbita and malar eminence (Figure 1, above). The area of the face covered by the mask contains sufficient unique geometric features allowing for accurate fiducial-less registration of the entire skull. An image guidance probe with LED technology is registered to the Stryker neuronavigation system using a handheld tracking device visible by the camera. The accuracy of the registration is then assessed with the registered image guidance probe by correlating several surface anatomical landmarks to the patient’s imported MRI images.

Keeping the mask affixed to the patient throughout the procedure provides dynamic reference LED tracking points for the neuronavigation system. This allows for the maintenance of accurate localization despite the potential for head movement. Furthermore, should the mask be displaced from the patient’s face, this is detected and the software either corrects the change or temporarily stops navigation until appropriate registration tracker geometry is recovered. Once registration accuracy is confirmed, the registered probe is utilized in navigation software to choose the target and entry site. The patient’s hair is then minimally clipped, prepped and then covered with a sterile transparent drape. The transparency allows visualization of the autoregistration mask throughout the procedure by the surgeons as well as line-of-sight for the neuronavigation system camera.

A sterile image guidance biopsy probe is registered and the location of the entry point, planned trajectory, and target is confirmed. An incision measuring approximately 3 cm in diameter is then opened over the planned entry site following local infiltration with lidocaine. A 10mm burr hole, centered at the entry site, is made in the direction of the planned trajectory with a perforator bit. A baseplate is rigidly secured onto the skull over the burr hole with three 8 mm screws. The swiveling cannula is attached to the baseplate and a plastic set screw is engaged but not tightened to allow free swiveling of the cannula (Figure 1, above). The biopsy navigation probe with LEDs is registered and inserted into the swiveling cannula. The trajectory for the biopsy is established by aligning the pointer of the navigation probe to the pre-calculated trajectory. This trajectory is maintained by tightening the plastic set screw on the swiveling cannula. The distance from the tip of the pointer to the target is measured by using a virtual tip advancement feature. The depth from the end of cannula to the target is then calculated by adding the length of the swiveling cannula to the distance from the tip of the pointer to the target. A disposable biopsy needle is then prepared by setting a stopper at the calculated depth of insertion from the midpoint of the opening of the biopsy needle (Figure 2, below).

Figure 2. Biopsy needle with stopper set at calculated depth of insertion.

Initial biopsy specimens are sent to pathology for frozen section evaluation. Additional specimens are obtained for subsequent microscopic evaluation of fixed slice preparations. Following microscopic confirmation of specimens adequate for diagnosis and confirmation of appropriate hemostasis, the biopsy needle, swiveling cannula, and baseplate are removed. The craniotomy is covered with a burr hole cover and the skin incision is closed. A post-operative CT of the head is obtained to evaluate for hemorrhage.

We have used this technique in more than 20 patients. The indications for surgery include previously undiagnosed brain lesions and one brain abscess. The volume of the lesions ranged from 0.9 to 75.4 cm3. The pathologies included gliomas of low and high grade, two lymphomas, an oligodendroglioma, a cerebral abscess and a case of atypical glial cells.

This method of frameless surgical navigation using fiducial-less mask registration without rigid head fixation enhances patient comfort and eliminates the risk of complications from head clamp placement. Patients can obtain their pre-operative MRI prior to admission without concern of displacement of adhesive skin markers. The lack of head fixation precludes the associated risk of skull injury, intracranial injury, or scalp laceration. In addition, we believe this fiducial-less, frameless, pinless biopsy technique allows greater surgical flexibility than frame based systems. A larger area of the skull is accessible without interference of pins or frames and intraoperative planning of additional target sites can be undertaken without need for complex calculations.

Furthermore, should there be a need to convert to an open craniotomy, image guidance would remain available without a frame or skull fiducial limiting the surgical approach. Considering that no additional steps are required, we expect reduced operative time.