Indications
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In general, surgical resection is preferred for accessible, symptomatic lesions with a focal growth pattern, while stereotactic biopsy is typically reserved for tumors with a diffuse growth pattern.
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If obstructive hydrocephalus is present, it is treated with a CSF diversion technique. In pediatric cases, and selective adult cases, an endoscopic third ventriculostomy (ETV) is preferred over ventriculoperitoneal shunt (VPS) placement.
Contraindications
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Diffuse pontine glioma (brain biopsy is also controversial).
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Lesions that are clinical and radiologically stable may be initially managed in a non-operative manner with serial imaging. Surgery is performed in cases of tumor progression/new contrast enhancement on MRI or development of obstructive hydrocephalus (third ventriculostomy and/or decompression).
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Because most midbrain lesions have an indolent course, some surgeons discourage open resections for these lesions.
Preoperative Considerations
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Brainstem tumors account for 10–20% of pediatric and 1.5–2.5% of adult intracranial tumors.
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Brainstem gliomas are the most common pathologic entity.
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Pathologies include gliomas, metastatic tumors, cavernous malformations, hemangioblastomas, demyelinating processes, infectious processes, granulomas, infarction or hematomas.
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Classification of brainstem lesions is based on CT and MRI ( ) ( Figure 5.1 ):
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Type I: diffuse.
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Type II: focal intrinsic.
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Type III: focal exophytic.
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Type IV: cervicomedullary.
Figure 5.1
The various locations of brainstem tumors: focal intrinsic tectal plate; focal intrinsic midbrain; focal intrinsic pontine; dorsal exophytic pontine; diffuse pontine; focal intrinsic medullary; dorsal exophytic medullary.
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Growth pattern, location and the presence of hydrocephalus or hematoma modifies the surgical management of the lesion.
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Preoperative imaging studies:
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MRI: Non-contrast enhancement, focal and well-circumscribed exophytic lesions are low-grade features. Contrast enhancement may be indicative of a higher-grade lesion for both focal or diffuse lesions.
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PET (positron emission tomography): Used in the attempt to differentiate low-grade from high-grade gliomas.
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DTI (diffusion tensor imaging): Can elucidate the relationship of motor and sensory tracts to the brainstem lesion. Current uses are limited.
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Preoperative planning: Careful study of the preoperative imaging is required to select the most suitable approach in order to avoid major neurovascular structures, white matter tracts and brainstem nuclei.
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Location: Brainstem gliomas located in the midbrain and medulla are usually focal and low grade. Gliomas located in the pons usually have an infiltrative and more aggressive nature. Obstructive hydrocephalus is common in midbrain and dorsally exophytic tumors.
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The management of diffuse tumors is controversial. At present, it is still acceptable to initiate standard, non-surgical therapy without pathological confirmation. Brainstem biopsy is not that common because of the risk of permanent damage to eloquent structures. However, with the improvement of surgical techniques and preoperative planning, the management is trending towards performing a biopsy for diagnosis and molecular characterization followed by adjuvant therapies if required.
Surgical Procedure
Image-Guided Stereotactic Biopsy
Approaches
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Suboccipital transcerebellar: lower midbrain, pons, middle cerebellar peduncle and medulla. The cerebellar peduncle is a very good target for tumors located in the pons.
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Transfrontal, transventricular: midline pons and medulla. Requires crossing the lateral ventricle, and the tentorial incisura set the lateral limits of the access.
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Transfrontal, extraventricular: lateral pons and middle cerebellar peduncle. Requires the use of a Leksell stereotactic frame. This route avoids the lateral ventricle and the tentorium does not block the access to more lateral lesions.
Patient Positioning
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Transfrontal: Patients can be positioned supine, in the lateral position or sitting. The head is turned contralaterally and the neck is flexed.
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The suboccipital, transcerebellar approach is preferred for pontine lesions. For the suboccipital transcerebellar approach the Leksell frame is secured as inferiorly as possible. The patient is placed in the prone position with the neck flexed. The sitting position is also used by some surgeons.
Frame Biopsy for Transfrontal Extraventricular Approach
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A Leksell stereotactic frame system is used. An MRI is performed following frame placement.
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Reconstruction images are used to plan the point of entry, avoiding sulci and large cortical vessels and to set the target coordinates and trajectory.
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After that, the frame needs to be assembled, the X and Y coordinates are set and the instrument guide and stopholders are fitted in the arch.
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In the operating room the patient is placed in the sitting position and light intravenous sedation is administered. The skin entry point is shaved, prepped and draped. The coordinates are set and checked.
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For the contralateral, transfrontal, extraventricular approach the entry point is placed approximately 4 cm off midline in the coronal plane.
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After the skin incision, a small craniotomy is made.
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After warning the patient of potential mild discomfort, a spinal needle is used to puncture the dura mater. The biopsy needle is slowly passed down the preplanned trajectory. At the same time the patient is examined for any neurologic changes. Biopsy samples are taken using a side-cutting aspiration needle.
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In both cases if the pathologic reading is nondiagnostic, additional samples can be obtained without a second needle pass from another enhancing region of the lesion.
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Once the biopsy sample is obtained, a small volume of air can be injected to confirm the site of the biopsy on a postoperative MRI.
Frameless Biopsy
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Requires the use of preoperative imaging (or intraoperative MRI) and intraoperative neuronavigation.
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Areas corresponding to contrast enhancement are targeted for biopsy in both superficial and deep portions of the lesion. The target region is usually selected to minimize the trajectory through the brainstem.
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A straight trajectory is planned with the navigation system preoperatively. The trajectory is selected to pass through the largest dimension of the lesion when possible to allow for multiple biopsy sites via one needle pass.
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The distance to the site is measured and marked on a 1.7-mm Nashold biopsy needle
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A 0.5-cm linear incision and a burr hole are made at the entry point. The dura mater is opened using a sharp needle.
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A beveled biopsy needle with a blunt stylet connected to the neuronavigation system is used to penetrate the brainstem. The needle is advanced slowly to the target following the planned trajectory and assisted by the neuronavigation system. A side-cutting biopsy needle is inserted through the passage and several samples are obtained.
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Tissue specimens approximately 8 mm long and 1 mm thick are obtained and sent for frozen sectioning.
Open Biopsy
The location and size of the tumor within the brainstem dictates the surgeon’s operative approach for resection (see Table 5.1 and Figure 5.2 ).
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