Introduction
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Metastatic lesions account for over 50% of intracranial lesions in adults.
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The most common primary locations of these metastases are lung, breast, skin and large intestine.
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The majority of intracranial metastases occur at the grey–white matter junction in the supratentorial space.
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Multiple metastases (>3 distinct lesions) are not uncommon.
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The goals of treating brain metastases are to relieve mass effect, improve neurologic symptoms, establish a diagnosis and provide local disease control in the brain.
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In general we aim to achieve en bloc resection of intracranial metastases, but this can be challenging, especially with large tumors that require significant retraction.
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Because these tumors typically reside in the gray–white matter junction below the cortical surface, these tumors can be approached and resected either via a trans-sulcal or transcortical route ( Figure 3.1 ).
Figure 3.1
Surgical corridors that can be used to approach supratentorial subcortical lesions. (A) Lesions located in contact with the cortical surface are typically approached with the transcortical route. (B) Subsulcal locations are best approached by a trans-sulcal approach splitting the sulcus leading to the lesion. If the overlying cortex is non-eloquent, the lesion can be approached transcortically to avoid damaging the vessels running within the sulci. The transcallosal and the sylvian fissure-splitting route are good options for deeper-seated lesions.
Modified from Lang, F.F., Chang, E.L., Suki, D., et al., 2004. Metastatic brain tumours. In Winn, H.R. (Ed.), Youmans Neurological Surgery, fifth ed. WB Saunders, Philadelphia.
Indications
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The decision to surgically resect supratentorial, intracranial metastases requires consideration of various factors, including the patient’s age, functional status, control of primary tumor and presence of extracranial spread. These factors are all used to calculate a patient’s recursive partitioning analysis (RPA) class.
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Patients with good RPA class (class I–II), radiation-resistant tumors and lesions causing symptoms or mass effect (including seizures, motor deficit, headaches, hydrocephalus) are typically candidates for surgery. Historically, the number of intracranial metastases was used to determine whether a patient was a surgical candidate, but there remains conflicting evidence regarding resection of symptomatic lesions only.
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Patients with poor prognoses are generally considered for radiation therapy, including whole brain radiation therapy (WBRT) and/or stereotactic radiosurgery (SRS).
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The choice of approach is usually dictated by preoperative neuroimaging and tumor location. In the trans-sulcal approach the sulcus to enter is planned preoperatively, taking into account the anatomy of the vessels running within it and the areas of eloquence.
Indications of the Trans-Sulcal Approach
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Intra-axial lesions in the subcortical space underlying an evident sulcus.
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This approach is preferred for tumors underlying eloquent cortex, including somatosensory, language and visual cortex.
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For tumors deep in the subcortical space (>1 cm), the trans-sulcal approach may allow easiest access for resection while minimizing disruption of the overlying cortical tissue, which is especially critical in eloquent areas.
Contraindications of the Trans-Sulcal Approach
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Deep intra-axial lesions that require wider exposure to localize and resect the lesion.
Indications of the Transcortical Approach
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Subcortical tumors that do not underlie an obvious sulcus and do not involve eloquent cortical regions.
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This may provide a more direct access for operative resection than the trans-sulcal approach.
Contraindications of the Transcortical Approach
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Tumors involving eloquent regions.
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Tumors that underlie an evident sulcus that may be better resected with a trans-sulcal approach.
Surgical Procedure
Patient Positioning
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Using a combination of anatomic landmarks and intraoperative neuronavigation, the intracranial tumor location is projected onto the scalp.
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In most cases the patient is placed in the supine or in the lateral position. The tumor region is placed at the top of the operative field, as perpendicular to the horizontal plane as possible.
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The head is stabilized using Mayfield skull clamps.
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In the case of multiple intracranial metastases it is often possible to resect all accessible lesions without redraping the patient, using single or multiple craniotomies. Some authors advocate the use of an “in-between” position, so the patient may be maneuvered intraoperatively to place each targeted lesion at the top of the operative field in turn.
Skin Incision
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An incision centered on the tumor is performed. This incision can be linear, curvilinear or trapdoor depending on the location and size.
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The planned incision should be large enough for a generous craniotomy to allow ample visualization of the gyrus and sulcal anatomy, as well as provide flexibility during tumor resection.
Craniotomy
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A large craniotomy allows ample visualization of the relevant sulci and gyri, potential other sulci and the cortical vasculature, namely the veins.
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We generally only require one or two burr holes depending on the location and quality of the dura mater.
Dural Opening
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The dura mater is opened in either a cruciate or C-shaped/trapdoor fashion. A cottonoid can be used in between the arachnoid and the dura mater to protect the cortical surface. Following each subsequent cut the cottonoid can be advanced forwards.
Procedure
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At this point the surgical corridor can be assessed with direct visualization of the sulci and surrounding gyri, intraoperative neuronavigation and/or intraoperative ultrasonography.
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Intraoperative brain stimulation, motor evoked potentials and somatosensory evoked potentials can be used to identify eloquent areas.
Trans-Sulcal Approach
(Tumors in the subcortical space underlying a seen sulcus and/or underlying eloquent cortex — Figure 3.2 .)
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The sulcus or sulci to be entered are identified.
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The arachnoid overlying the sulcus is incised sharply with either an 11-blade or 18-gauge needle, with care taken to avoid injuring the underlying vasculature. The sulcus is typically opened where the subarachnoid space is the largest. In general, the subarachnoid space over the arteries is typically more robust than over the veins.
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The sulcus is opened as widely as possible, using a combination of sharp and blunt dissection. In order to preserve the anatomy, bipolar cautery is not used unless bleeding occurs. Venous bleeding can typically be stopped with gentle pressure with cotton and/or placement of Gelfoam. It is critical to spare the arteries and veins running within the sulci that supply and provide venous drainage of surrounding gyri, respectively.
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We try to avoid the use of retractors to prevent pressure-related iatrogenic injury to the surrounding cortical tissue. Larger and/or deeper tumors may require the need for a fixed retractor system and/or the use of a tubular retractor (Vycor).
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Once the tumor is identified, the tumor can be removed either en bloc or piecemeal. We prefer an en bloc approach (see Figure 3.3 ). With the en bloc approach the tumor is dissected free from the surrounding parenchyma. Typically, the tumor is surrounded by a rim of gliotic parenchyma that separates the lesion from the edematous parenchyma, forming a pseudocapsule. The tumor is resected circumferentially by dissecting the pseudocapsule from the surrounding parenchyma. The borders are identified and held with the use of cottonoids. In order to reduce bleeding during the tumor resection, the small vessels supplying the metastasis are identified and individually coagulated and cut. Once the complete lesion is dissected away from the parenchyma, the tumor can be extracted en bloc.
