7 Lesions in the cerebellopontine angle (CPA) are invariably neoplastic, representing 5 to 10% of intracranial tumors. They are almost all benign (>95%). Vestibular schwannomas are the most common (80 to 91%), followed by meningiomas and epidermoid cysts. Of malignant etiologies, metastatic disease, particularly melanoma, is most common, and should be considered in the setting of systemic metastases, bilateral disease, thick enhancement outside of the nodular lesion, or a rapidly progressive clinical course. Vestibular schwannomas represent 6 to 7% of intracranial tumors and 95% of intracranial schwannomas. They arise most commonly from age 40 to 70 at the Schwann-glial junction of the eight cranial nerve, which is within the internal auditory canal (IAC), and >95% emerge into the CPA, widening the porus acusticus. On computed tomography (CT), they are isodense to hypodense to brain, widen the porus acusticus, and demonstrate intense, homogeneous enhancement. They are essentially isointense to the pons on T1-weighted magnetic resonance images (T1WI) and T2-weighted magnetic resonance images (T2WI), with intense enhancement. Larger tumors commonly develop cystic degeneration with infrequent internal hemorrhage and rare calcification (Table 7.1). Eight and one-half percent of meningiomas originate within the posterior fossa, most along the petrous dura/CPA, where they represent 10 to 15% of CPA tumors. On CT, they are hemispheric- shaped masses, isodense to slightly hyperdense to brain, with a broad base to the petrous bone, which may demonstrate hyperostosis or invasion. They may calcify, rarely undergo cystic degeneration, and demonstrate intense homogeneous enhancement. On magnetic resonance imaging (MRI), they are isointense to gray matter with homogeneous intense enhancement, with extension along a “dural tail” common (Table 7.1). Epidermoid cysts are congenital lesions of ectodermal origin and intracranially are most commonly found in the CPA cistern. They can also occur in the suprasellar and parasellar regions, the quadrigeminal plate cistern, and the temporal fossa. They often insinuate around neural and vascular structures. They contain keratin and cholesterol due to desquamated epithelium. The radiologist is usually faced with differentiating these lesions from arachnoid cysts (Table 7.2
Cerebellopontine Angle Lesions
Vestibular Schwannomas
Meningiomas
Epidermoid Cysts
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