eMedicine Specialties > Neurosurgery > Spine

Skull Base Tumors: Workup

Author: Todd C Hankinson, MD, MBA, Resident in Neurological Surgery, Department of Neurological Surgery, Neurological Institute of New York, Columbia University
Coauthor(s): Jeffrey N Bruce, MD, Edgar M Housepian Professor of Neurological Surgery Research, Professor of Neurological Surgery, Director of Brain Tumor Tissue Bank, Director of Bartoli Brain Tumor Laboratory, Department of Neurosurgery, Columbia University College of Physicians and Surgeons; Grant P Sinson, MD, Associate Professor, Department of Neurosurgery, Medical College of Wisconsin; G Timothy Reiter, MD, Assistant Professor, Department of Neurosurgery, Penn State College of Medicine; Director, Department of Spinal Neurosurgery and Neurotrauma, Penn State Hershey Medical Center
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Updated: May 29, 2007

Workup

Laboratory Studies

• Preoperative laboratory studies include a complete blood cell count, electrolyte evaluation, and bleeding and coagulation parameters.
• Masses located in or around the sella turcica merit serum endocrine studies.
• Paragangliomas: Catecholamine secretion can be diagnosed with urine or serum studies.

Imaging Studies

General imaging studies for assessment of skull base tumors include the following:

• CT scan of the brain with thin cuts and sagittal and coronal reconstructions can reveal abnormalities of the bone (eg, erosion, hyperostosis) and calcification in the tumor.
• Brain MRI with and without gadolinium is the best study to evaluate soft tissue masses and structures.
• Cerebral angiography is beneficial if the tumor encroaches on the carotid or other major intracranial arteries or a major venous sinus. It is used to assess whether arteries and venous sinuses are patent. In the case of meningioma and other vascular tumors, angiographic embolization of appropriate feeding vessels can decrease blood loss during operative resection and contribute to the ease of tumor removal.
• MR arteriography and venography may also be used to assess the patency of arterial and venous structures.
• If a metastatic tumor has an unknown primary source, a systemic workup should be performed. Imaging tests include posteroanterior and lateral chest radiography; contrast-enhanced CT scan of the chest, abdomen, and pelvis; and a bone scan.

• Osteoma: Diagnosis is based on the radiographic appearance. The most sensitive test is CT scanning, in which the lesion appears as a circumscribed homogeneous bony density (see Image 1).
• Chondroma: Radiographically, chondromas are best characterized by CT scanning; these tumors appear as lytic lesions with sharp margins and erosion of surrounding bone. Stippled calcification within the lesion helps distinguish it from a metastasis or chordoma.
• Hemangioma: Skull radiographs may demonstrate a circular lucency with trabeculations. CT scanning demonstrates a nonenhancing hypodense lesion. Flow voids may be visualized on MRI.
• Dermoid and epidermoid tumors of the skull: Radiographs demonstrate rounded or ovoid lytic lesions with sharp sclerotic margins. On CT scans, these lesions are nonenhancing and hypodense and involve all 3 bone layers. MRI demonstrates a low-intensity lesion on T1 and a high-intensity lesion on T2.
• Paragangliomas: MRI and magnetic resonance (MR) angiography best characterize paragangliomas because these modalities usually reveal their extensive vascularity, meriting preoperative embolization in many cases (see Images 3 and 4).
• Metastasis to the skull base: MRI scans can reveal even small skull base metastases. Almost all tumors can be visualized, especially if they lie within the cavernous sinus.
• Direct extension of malignant tumors to the skull base: Imaging demonstrates erosion of the skull base and the presence of a soft tissue mass.
• Osteogenic sarcoma and fibrous sarcoma: Fibrous sarcoma is a soft tissue tumor that arises from bone, periosteum, scalp, or dura. It is often accompanied by bony destruction, showing a regular but discrete lytic radiographic picture.
• Vestibular schwannoma: MRI is the best modality for the diagnosis of a vestibular schwannoma. The tumor enhances vigorously, and its origin in the internal auditory canal distinguishes it from a meningioma. CT scanning with contrast demonstrates most tumors larger than 1.5 cm.
• Trigeminal schwannoma: Diagnosis is best established with MRI.
• Meningioma
  • Imaging of skull base meningiomas includes CT scanning, MRI, and sometimes angiography, which can be obtained in conjunction with embolization of feeding arteries. On nonenhanced CT scans, the tumor is isodense or slightly hyperdense compared with the brain. The mass is generally smooth and sometimes lobulated, and calcifications are often appreciated. Contrast enhancement is strong and homogeneous; margins are distinct, and the tumor’s dural base can usually be appreciated (see Image 15). Hyperostosis of the underlying bone is common and can be appreciated radiographically. Hyperostosis of the immediately adjacent bone is seen in 25% of patients (see Images 13 and 14).
  • On MRI, the tumor is isointense (65%) or hypointense (35%) when compared with normal brain on both T1-weighted and T2-weighted imaging. Intense and homogeneous gadolinium enhancement is visible, and the dural tail is often evident. MRI is the best modality to define the relationship between the tumor and surrounding structures.
  • Atypical radiographic features such as cysts, hemorrhage, and central necrosis, which can mimic features of glioma, are visible in about 15% of meningiomas. Malignant meningiomas commonly demonstrate bone destruction, necrosis, irregular enhancement, and extensive edema. MRI demonstrates the rare cases of direct brain invasion by tumor. The differential diagnosis of radiographically atypical meningiomas includes dural metastasis, other primary meningeal tumors (eg, sarcoma), granuloma, or aneurysm. Metastases are commonly associated with abundant surrounding edema and bone destruction; in contrast, hyperostosis and moderate edema suggest meningioma.
  • On angiograms, skull base meningiomas are hypervascular, generally with feeding vessels from the external carotid artery. Encased or compromised arteries and venous sinus involvement can be assessed using MR or CT angiography, which greatly contributes to operative planning. Catheter angiography is reserved for cases in which balloon test occlusion (BTO) or arterial embolization is planned.
• Hemangiopericytoma: Gadolinium-enhanced MRI best characterizes these lesions (see Image 16). CT scanning may help differentiate hemangiopericytoma from a meningioma by demonstrating local osteolysis, rather than the hyperostosis that may be seen with meningioma. Angiography can demonstrate the tumor’s vascularity, and embolization can be a helpful adjunct prior to surgical intervention.
• Tumors of the posterior cranial fossa: MRI usually differentiates a meningioma from a vestibular schwannoma because the latter begins within the internal auditory canal.

Other Tests

• In the appropriate clinical setting, additional tests are used to formally evaluate cranial nerve function. Tests may include a formal visual field evaluation, audiography, or swallowing study.
• Vestibular schwannoma: Hearing function can be evaluated using brainstem auditory evoked potentials, caloric stimulation with electronystagmography, and audiometry.
• Tumors of the posterior cranial fossa: Abnormalities found on electronystagmography, audiography, and brainstem auditory evoked potentials testing are less common than with vestibular schwannomas, but these tests are rarely needed with current imaging techniques.

Diagnostic Procedures

Balloon test occlusion (BTO): Large vessel compromise due to tumor involvement can be documented preoperatively with angiography to allow for adequate presurgical planning. If a large vessel requires ligation during the operative approach or tumor resection, a BTO gives the surgeon an indication of whether the patient might tolerate vessel sacrifice or if a bypass procedure is required.

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