(Radiographics. 2001;21:419-438.)
© RSNA, 2001
Unusual Lesions of the Cerebellopontine Angle: A Segmental Approach1
Fabrice Bonneville, MD,
Jean-Luc Sarrazin, MD,
Kathlyn Marsot-Dupuch, MD,
Clément Iffenecker, MD,
Yves-Sébastien Cordoliani, MD,
Dominique Doyon, MD and
Jean-François Bonneville, MD
1 From the Department of Neuroradiology, Hôpital J. Minjoz, Boulevard Fleming, 25000 Besançon, France (F.B., J.F.B.); the Department of Radiology, Hôpital d’Instruction des Armées du Val-de-Grâce, Paris, France (J.L.S., Y.S.C.); the Department of Radiology, Hôpital Saint-Antoine, Paris, France (K.M.D.); and the Department of Neuroradiology, Hôpital de Bicêtre, Le Kremlin Bicêtre, France (C.I., D.D.). Presented as a scientific exhibit at the 1999 RSNA scientific assembly. Received April 3, 2000; revision requested May 8 and received July 25; accepted July 27. Address correspondence to F.B. (e-mail: fabbonne@club-internet.fr).
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Abstract
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Tumors of the cerebellopontine angle (CPA) are frequent; acoustic neuromas and meningiomas represent the great majority of such tumors. However, a large variety of unusual lesions can also be encountered in the CPA. The site of origin is the main factor in making a preoperative diagnosis for an unusual lesion of the CPA. In addition, it is essential to analyze attenuation at computed tomography (CT), signal intensity at magnetic resonance (MR) imaging, enhancement, shape and margins, extent, mass effect, and adjacent bone reaction. CPA masses can primarily arise from the cerebellopontine cistern and other CPA structures (arachnoid cyst, nonacoustic schwannoma, aneurysm, melanoma, miscellaneous meningeal lesions) or from embryologic remnants (epidermoid cyst, dermoid cyst, lipoma). Tumors can also invade the CPA by extension from the petrous bone or skull base (cholesterol granuloma, paraganglioma, chondromatous tumors, chordoma, endolymphatic sac tumor, pituitary adenoma, apex petrositis). Finally, CPA lesions can be secondary to an exophytic brainstem or ventricular tumor (glioma, choroid plexus papilloma, lymphoma, hemangioblastoma, ependymoma, medulloblastoma, dysembryoplastic neuroepithelial tumor). A close association between CT and MR imaging findings is very helpful in establishing the preoperative diagnosis for unusual lesions of the CPA.
Index Terms: Brain neoplasms, 154.30 • Brain neoplasms, CT, 154.12112 • Brain neoplasms, diagnosis, 154.30 • Brain neoplasms, MR, 154.121411 Cerebellopontine angle, neoplasms, 154.30
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LEARNING OBJECTIVES FOR TEST 4
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After reading this article and taking the test, the reader will be able to:
- Describe the spectrum of unusual tumors of the CPA.
- Recognize the CT and MR imaging features of these tumors.
- Propose a diagnosis based on suggestive features for almost every tumor.
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Introduction
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Lesions of the cerebellopontine angle (CPA) are frequent and represent 6%–10% of all intracranial tumors (1),(2). Acoustic neuromas, which are also called vestibular schwannomas (3), and meningiomas are the two most frequent lesions and account for approximately 85%–90% of all CPA tumors (1). The other 10%–15% encompass a large variety of lesions that radiologists will encounter more and more frequently because of the remarkable sensitivity and accuracy of magnetic resonance (MR) imaging in evaluation of a CPA syndrome.
In most cases, MR imaging and computed tomography (CT) show typical features of acoustic neuromas or meningiomas and are sufficient to establish the diagnosis. Acoustic neuromas are usually round or oval masses in the cerebellopontine cistern that emerge from the internal auditory canal, widen the porus, and grow posteriorly because of the anterior limit represented by the cisternal segment of the facial nerve (4). They can be heterogeneous due to cystic components. Conversely, meningiomas are usually hemispheric, semilunar masses with a broad petrous base to which they are attached and are usually asymmetric to the internal auditory canal.
After excluding acoustic neuromas and meningiomas, we retrospectively studied a large variety of surgically proved unusual CPA lesions from the collections of four institutions. In this article, we discuss and illustrate the neuroimaging fea-tures of these rare CPA lesions. In addition, a segmental approach to diagnosis of these lesions based on their site of origin is proposed (Fig 1). Although the lesions usually arise from the cer-ebellopontine cistern, they can also invade the CPA from extension of a skull base tumor or even a brainstem tumor. The attenuation at CT, signal intensity at MR imaging, enhancement, shape and margins, extent, mass effect, and reaction of the adjacent bone structures of the skull base are also helpful in making the correct preoperative diagnosis.
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Figure 1a. Table (a) and drawing (axial view) (b) show the segmental approach to diagnosis of unusual lesions of the CPA based on their site of origin. DNET = dysembryoplastic neuroepithelial tumor.
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Figure 1b. Table (a) and drawing (axial view) (b) show the segmental approach to diagnosis of unusual lesions of the CPA based on their site of origin. DNET = dysembryoplastic neuroepithelial tumor.
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Tumors Originating in the CPA
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The CPA is outlined by the meninges of the cerebellopontine cistern; thus, in addition to cerebrospinal fluid (CSF), the CPA contains nerves and arteries and possibly embryologic remnants. Each of these structures can be the site of origin of an unusual CPA lesion.
Masses originating in the CPA are extraaxial and thus widen the homolateral subarachnoid cisterns. They displace or encase neurovascular structures. These lesions can be separated from the brainstem by a thin CSF layer, and there is usually no brainstem edema.
Epidermoid Cyst
Epidermoid cysts or tumors, also known as primary cholesteatomas, are the third most frequent tumor of the CPA (5). They arise from normal epithelial cells included during neural tube closure. Their growth is due to accumulation of keratin and cholesterol produced by desquamation of the squamous epithelium lining the mass. These slow-growing tumors encase and surround nerves and arteries in the cisterns rather than displacing them (6).
On CT scans, epidermoid cysts appear hypoattenuating, almost isoattenuating to CSF, and have characteristic irregular, lobulated margins. As opposed to arachnoid cyst, which is the main differential diagnosis, epidermoid cyst produces no reaction of the adjacent bone structures. Slight marginal calcifications or enhancement after contrast material administration are rarely seen (7). At MR imaging, epidermoid cysts have slightly higher signal intensity than CSF on T1- and T2-weighted images, often with heterogeneous and marbled features (Fig 2). Sometimes, when the signal intensity is very similar to that of CSF, the fluid-attenuated inversion-recovery sequence is more sensitive than conventional sequences in differentiation of epidermoid and arachnoid cysts because it suppresses the signal of CSF (8). With this sequence, epidermoid cysts have high signal intensity, whereas the signal of arachnoid cysts is suppressed. Diffusion-weighted imaging is also well known to allow differentiation of epidermoid and arachnoid cysts. The apparent diffusion coefficient of an epidermoid cyst is significantly lower than that of an arachnoid cyst; therefore, epidermoid cysts have high signal intensity on diffusion-weighted images, whereas arachnoid cysts, like CSF, have very low signal intensity (Table 1) (9). Recently, diffusion-weighted imaging has also been reported to allow confirmation of the presence of residual postoperative tumor (10). Constructive interference in the steady state or three-dimensional fast spin-echo sequences, which produce heavily T2-weighted images and thus excellent contrast between CSF and all other structures, are necessary to precisely assess the extent of the tumor (8).
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Figure 2a. Epidermoid cyst in a 52-year-old woman with epilepsy. (a) Axial T1-weighted MR image shows an epidermoid cyst with characteristic focal marbling in the left CPA (arrow). (b) Axial T2-weighted MR image shows the lobulated margins of the cyst impinging on the pons (arrowhead). (c) Axial heavily T2-weighted (constructive interference in the steady state) MR image shows the extent of the tumor.
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Figure 2b. Epidermoid cyst in a 52-year-old woman with epilepsy. (a) Axial T1-weighted MR image shows an epidermoid cyst with characteristic focal marbling in the left CPA (arrow). (b) Axial T2-weighted MR image shows the lobulated margins of the cyst impinging on the pons (arrowhead). (c) Axial heavily T2-weighted (constructive interference in the steady state) MR image shows the extent of the tumor.
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Figure 2c. Epidermoid cyst in a 52-year-old woman with epilepsy. (a) Axial T1-weighted MR image shows an epidermoid cyst with characteristic focal marbling in the left CPA (arrow). (b) Axial T2-weighted MR image shows the lobulated margins of the cyst impinging on the pons (arrowhead). (c) Axial heavily T2-weighted (constructive interference in the steady state) MR image shows the extent of the tumor.
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Table 1. MR Imaging Characteristics and Suggestive Features of Unusual Lesions Arising from the Cerebellopontine Cistern
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Dermoid Cyst
Like epidermoid cysts, dermoid cysts result from inclusion of ectodermal elements during neural tube closure but may originate a bit earlier (11). Dermoid cysts are midline lesions that rarely invade the CPA laterally and contain elements from all layers of the skin. Thus, fat, hair, sebaceous glands, and sweat glands can be found in addition to squamous epithelium. Typically, dermoid cysts have negative attenuation values on CT scans and high signal intensity on T1-weighted images due to their fatty content, may have a very suggestive fat-fluid level, and contain calcifications (7).
Arachnoid Cyst
Arachnoid cysts are pouchlike intraarachnoid masses of uncertain origin filled with CSF (1). At neuroimaging, their attenuation and signal intensity match those of CSF almost exactly. These masses have smooth and rounded edges, displace neurovascular structures (Fig 3), and erode adjacent bone structures. There is no calcification or enhancement. On T1- and T2-weighted images they can appear similar to an epidermoid cyst. As described, fluid-attenuated inversion-recovery and constructive interference in the steady state sequences as well as diffusion-weighted imaging are helpful in making the distinction (8),(9).
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Figure 3a. Arachnoid cyst in a 27-year-old woman with headaches. (a) Axial T1-weighted MR image shows an arachnoid cyst with signal intensity similar to that of CSF stretching the left seventh and eighth cranial nerve complex (arrow). (b) Axial T2-weighted MR image shows the cyst displacing the vascular structures of the CPA (arrowheads).
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Figure 3b. Arachnoid cyst in a 27-year-old woman with headaches. (a) Axial T1-weighted MR image shows an arachnoid cyst with signal intensity similar to that of CSF stretching the left seventh and eighth cranial nerve complex (arrow). (b) Axial T2-weighted MR image shows the cyst displacing the vascular structures of the CPA (arrowheads).
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Miscellaneous Cysts
Three other rare cystic lesions can be encountered in the CPA: neurocysticercosis, neurenteric cysts, and neuroepithelial cysts. Cysticercosis is a parasitosis due to encystment of Taenia solium larvae in tissues. In the subarachnoid region, cystic lesions can be found in the CPA and usually occur in the racemose form. They appear as lobulated cysts with no mural nodule or enhancement and have signal intensity similar to that of CSF with all sequences (12),(13). They are therefore difficult to diagnose, and cysticercosis should be considered whenever a solitary enlarged cistern is seen in a patient from an area where the disease is endemic. In addition, on CT scans, the lack of adjacent bone erosion can be useful in distinguishing neurocysticercosis from arachnoid cyst, which is the main differential diagnosis.
The origin of neurenteric cysts is not precisely known, but they probably result from dysgenesis of the neurenteric canal during the third week of embryogenesis (14). The cyst walls are lined by a typical epithelium that resembles gastrointestinal or respiratory tract mucosa, and mucinlike secretions fill the cysts. They are generally located in the spine and only a few are intracranial, but those usually occur in the CPA (15). The signal intensity depends on the content of the cyst, but it is usually isointense or slightly hyperintense to CSF on T1- and T2-weighted images (14). No calcification or enhancement is typically seen.
The origin of neuroepithelial cysts is still controversial, but they are probably secondary to simple folding of neuroepithelium out of the primitive ventricular system (16). In fact, these well-defined cysts are lined by a columnar epithelium that resembles ependyma. They can arise anywhere in the central nervous system and are frequent in the choroid plexus and ventricles but uncommon in the CPA (17). Neuroepithelial cysts are mostly asymptomatic but can compress the brainstem and cerebellum in the posterior cranial fossa and produce signs of increased intracranial pressure (18). At neuroimaging, neuroepithelial cysts appear as round or ovoid cystic lesions with possible septa but without calcification or pathologic enhancement; the attenuation and signal intensity are similar to those of CSF (19).
Lipoma
Lipomas in the CPA are maldevelopmental masses that arise from abnormal differentiation of the meninx primitiva (11). They are homogeneous fatty lesions surrounding and encasing normal adjacent neurovascular structures with very dense adhesions. Nevertheless, lipomas are rarely symptomatic, and conservative follow-up is often preferred to aggressive and potentially risky resection (20).
Lipomas appear as fat (Fig 4): homogeneously hypoattenuating with a negative attenuation valueon CT scans and as characteristic and suggestive homogeneous high signal intensity on T1-weighted images, which decreases on fat-suppressed images. There is no enhancement after contrast material administration. Therefore, nonenhanced T1-weighted imaging should be performed when evaluating a CPA syndrome to look for spontaneous hyperintense lesions, such as lipomas.
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Figure 4a. Lipoma in a 7-year-old boy with a polymalformation syndrome. (a) Axial CT scan shows a well-defined hypoattenuating lipoma of the left CPA. (b) Axial T1-weighted MR image shows that the lipoma has signal intensity similar to that of subcutaneous fat.
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Figure 4b. Lipoma in a 7-year-old boy with a polymalformation syndrome. (a) Axial CT scan shows a well-defined hypoattenuating lipoma of the left CPA. (b) Axial T1-weighted MR image shows that the lipoma has signal intensity similar to that of subcutaneous fat.
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Schwannoma
Although vestibular schwannomas account for 95% of intracranial schwannomas, neural sheath tumors can develop in all the other nerves of the CPA, especially the trigeminal nerve (Fig 5) and facial nerve but also cranial nerves IX to XII (Fig 6). Neuroimaging features are the same as for vestibular schwannomas, including possible cystic or hemorrhagic components (21). Keys to the diagnosis are the neuroanatomic location of the tumor, its extension following the courses of the nerves, smooth enlargement of their foramina, and fatty amyotrophy of the corresponding enervated muscles (22),(23).
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Figure 5. Schwannoma in a 34-year-old man with right trigeminal neuralgia. Contrast material-enhanced axial T1-weighted MR image shows a homogeneous, enhanced, dumbbell-shaped right trigeminal schwannoma involving the cisternal part of the nerve and Meckel cave.
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Figure 6a. Schwannoma in a 52-year-old woman with left ear pain. (a) Gadolinium-enhanced coronal T1-weighted MR image shows an enhancing schwannoma of the ninth, 10th, and 11th cranial nerves in the lowest part of the CPA. (b) Coronal T2-weighted MR image shows the extent of the schwannoma along the course of the nerves (arrows) and beneath the normal left internal auditory canal (arrowheads). (c) Axial CT scan shows smooth erosion of the skull base and especially the jugular foramen by the schwannoma.
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Figure 6b. Schwannoma in a 52-year-old woman with left ear pain. (a) Gadolinium-enhanced coronal T1-weighted MR image shows an enhancing schwannoma of the ninth, 10th, and 11th cranial nerves in the lowest part of the CPA. (b) Coronal T2-weighted MR image shows the extent of the schwannoma along the course of the nerves (arrows) and beneath the normal left internal auditory canal (arrowheads). (c) Axial CT scan shows smooth erosion of the skull base and especially the jugular foramen by the schwannoma.
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Figure 6c. Schwannoma in a 52-year-old woman with left ear pain. (a) Gadolinium-enhanced coronal T1-weighted MR image shows an enhancing schwannoma of the ninth, 10th, and 11th cranial nerves in the lowest part of the CPA. (b) Coronal T2-weighted MR image shows the extent of the schwannoma along the course of the nerves (arrows) and beneath the normal left internal auditory canal (arrowheads). (c) Axial CT scan shows smooth erosion of the skull base and especially the jugular foramen by the schwannoma.
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Aneurysms of Posterior Fossa Arteries
The vertebral and basilar arteries and some of their branches pass through the cerebellopontine cistern, where a tortuous segment or ectasia or even an aneurysm can develop. Although not neoplasms, such lesions can cause mass effect on the neural structures of the CPA and thus produce neurologic symptoms (24),(25).
High-flow aneurysms appear as oval or round masses that have no signal (flow void) with all spin-echo sequences (Fig 7). Extreme low signal intensity on T2-weighted images is very suggestive of such lesions and indicates that normal arteries need to be evaluated. When an aneurysm is thrombosed, high signal intensity on T1-weighted images due to methemoglobin could be suggestive, but the signal intensity is variable; radiologists should be aware that enhancement of the mass can be observed due to organization of the thrombus (Fig 8), thus mimicking a schwannoma (26).
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Figure 7. Asymptomatic aneurysm in a 68-year-old man with lymphoma and right trigeminal neuralgia. Axial T2-weighted MR image shows an aneurysm of the left posterior inferior cerebellar artery with typical lack of signal (arrow). Note the lymphoma in the right pterygopalatine fossa (arrowheads), which explains the neuralgia.
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Figure 8a. Aneurysm in a 75-year-old man with hypoglossal nerve palsy. (a) Axial T2-weighted MR image shows a thrombosed aneurysm of the right posterior inferior cerebellar artery with focal calcification (arrowhead). Note the normal right hypoglossal canal (arrow), a finding inconsistent with a schwannoma. (b) Contrast-enhanced coronal T1-weighted MR image shows homogeneous enhancement of the organized thrombus, which completely fills the aneurysm.
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Figure 8b. Aneurysm in a 75-year-old man with hypoglossal nerve palsy. (a) Axial T2-weighted MR image shows a thrombosed aneurysm of the right posterior inferior cerebellar artery with focal calcification (arrowhead). Note the normal right hypoglossal canal (arrow), a finding inconsistent with a schwannoma. (b) Contrast-enhanced coronal T1-weighted MR image shows homogeneous enhancement of the organized thrombus, which completely fills the aneurysm.
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Melanocytic Neoplasms
Melanocytes are normally present in the intracranial leptomeninges, usually in the posterior cerebral fossa (27), and can give rise to benign (meningeal melanocytoma) or malignant (malignant melanoma) tumors. However, metastatic malignant melanomas greatly outnumber primary melanocytic neoplasms (28). At CT, primary melanocytic lesions appear as well-circumscribed, isoattenuating to hyperattenuating extraaxial tumors (Fig 9) with homogeneous enhancement. Even if there is no hyperostosis, they can mimic a meningioma (29). MR imaging demonstrates variable signal intensity on T1- and T2-weighted images in proportion to the amount of melanin within the tumor (30). Therefore, melanocytic neoplasms are isointense or hyperintense to adjacent normal brain tissue on T1-weighted images and isointense or hypointense on T2-weighted images (31). They enhance after intravenous administration of contrast material.
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Figure 9a. Melanoma in a 58-year-old woman with a left cerebellar syndrome. (a) Axial CT scan shows a hyperattenuating melanoma of the left CPA. (b) Axial T1-weighted MR image shows a well-defined extraaxial mass at the posterior edge of the petrous bone. The high signal intensity is suggestive of melanin. (c) Gadolinium-enhanced axial T1-weighted MR image shows a normal left internal auditory canal (arrow) and lack of dural tail enhancement.
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Figure 9b. Melanoma in a 58-year-old woman with a left cerebellar syndrome. (a) Axial CT scan shows a hyperattenuating melanoma of the left CPA. (b) Axial T1-weighted MR image shows a well-defined extraaxial mass at the posterior edge of the petrous bone. The high signal intensity is suggestive of melanin. (c) Gadolinium-enhanced axial T1-weighted MR image shows a normal left internal auditory canal (arrow) and lack of dural tail enhancement.
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Figure 9c. Melanoma in a 58-year-old woman with a left cerebellar syndrome. (a) Axial CT scan shows a hyperattenuating melanoma of the left CPA. (b) Axial T1-weighted MR image shows a well-defined extraaxial mass at the posterior edge of the petrous bone. The high signal intensity is suggestive of melanin. (c) Gadolinium-enhanced axial T1-weighted MR image shows a normal left internal auditory canal (arrow) and lack of dural tail enhancement.
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Miscellaneous Meningeal Lesions
In addition to carcinomatous meningitis, many infectious or inflammatory diseases can have focal nodular meningeal involvement and manifest as a CPA mass. Leptomeningeal metastases from lung cancer, breast cancer, melanoma, or other neo-plasms can mimic benign tumors of the CPA (Fig 10) (32). Therefore, when an unusual aggressive schwannomalike mass is encountered and no primary cancer is known to be present, radiologists should be aware of the possibility of metastases and check the lungs and breast carefully. However, in most cases, there are numerous other intra- and extraaxial masses, which are very suggestive of metastases when correlated with a clinical history of carcinoma.
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Figure 10a. Metastases in a 67-year-old man with lung cancer and right-sided hypoacusia, vertigo, and ear pain. (a) Axial T2-weighted MR image shows a metastasis of the right CPA that mimics a vestibular schwannoma but with unusual associated middle ear retention (*). (b) Contrast-enhanced axial T1-weighted MR image shows intense enhancement of the lesion, which extends into the cochlea (arrow). Note the presence of another enhancing lesion at the tip of the right petrous bone (arrowhead).
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Figure 10b. Metastases in a 67-year-old man with lung cancer and right-sided hypoacusia, vertigo, and ear pain. (a) Axial T2-weighted MR image shows a metastasis of the right CPA that mimics a vestibular schwannoma but with unusual associated middle ear retention (*). (b) Contrast-enhanced axial T1-weighted MR image shows intense enhancement of the lesion, which extends into the cochlea (arrow). Note the presence of another enhancing lesion at the tip of the right petrous bone (arrowhead).
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Tuberculosis is one of the main causes of focal pachymeningeal thickening in the posterior cranial fossa, but this finding is not specific (33). Neurosarcoidosis can also appear as a dura-based mass, like meningioma. The lesions of neurosarcoidosis are hyperattenuating on CT scans and isointense to gray matter on T1-weighted images with homogeneous enhancement; however, the possible low signal intensity on T2-weighted images may suggest the diagnosis (34).
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Tumors Originating from the Skull Base with CPA Impingement
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Lesions originating from the various structures of the skull base surrounding the posterior cranial fossa, especially the temporal bone, can spread into or even mainly develop in the CPA. Usually, smooth bone erosion or even extensive destruction is seen around the site of origin of the lesion. Such extradural masses, such as extraaxial lesions, narrow the cisterns but displace neurovascular structures without invading them.
Cholesterol Granuloma
Cholesterol granulomas can occur in any obstructed air cells. They often arise from the apex of the petrous bone and may enlarge enough to expand in the posterior cranial fossa and produce nerve disturbances (35). At CT, cholesterol granulomas appear as sharply and smoothly marginated expansile lesions in the temporal bone, isoattenuating with brain tissue and nonenhancing (36). At MR imaging, they characteristically have a large central region of increased signal intensity and a thin peripheral rim of decreased signal intensity on both T1- and T2-weighted images (Table 2, Fig 11) (37). The latter finding corresponds to expanded cortical bone and hemosiderin deposits. Spontaneous and homogeneous central high signal intensity on T1-weighted images is very suggestive of a cholesterol granuloma.
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Figure 11a. Cholesterol granuloma in a 32-year-old man with right trigeminal neuralgia. (a) Axial T1-weighted MR image shows a cholesterol granuloma at the apex of the right petrous bone with typical high signal intensity. An additional suggestive feature is the thin hypointense rim (arrowheads), which represents expanded cortical bone of the petrous apex. (b) Axial T2-weighted MR image shows that the granuloma has heterogeneous signal intensity surrounded by a hypointense rim (arrowheads). (c) Contrast-enhanced coronal T1-weighted MR image shows the normal right trigeminal nerve (arrow) at the top of the mass.
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Figure 11b. Cholesterol granuloma in a 32-year-old man with right trigeminal neuralgia. (a) Axial T1-weighted MR image shows a cholesterol granuloma at the apex of the right petrous bone with typical high signal intensity. An additional suggestive feature is the thin hypointense rim (arrowheads), which represents expanded cortical bone of the petrous apex. (b) Axial T2-weighted MR image shows that the granuloma has heterogeneous signal intensity surrounded by a hypointense rim (arrowheads). (c) Contrast-enhanced coronal T1-weighted MR image shows the normal right trigeminal nerve (arrow) at the top of the mass.
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Figure 11c. Cholesterol granuloma in a 32-year-old man with right trigeminal neuralgia. (a) Axial T1-weighted MR image shows a cholesterol granuloma at the apex of the right petrous bone with typical high signal intensity. An additional suggestive feature is the thin hypointense rim (arrowheads), which represents expanded cortical bone of the petrous apex. (b) Axial T2-weighted MR image shows that the granuloma has heterogeneous signal intensity surrounded by a hypointense rim (arrowheads). (c) Contrast-enhanced coronal T1-weighted MR image shows the normal right trigeminal nerve (arrow) at the top of the mass.
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Paraganglioma
Paragangliomas (glomus tumors) that invade the CPA arise from minute bodies present in the jugular foramen along the vagus nerve (glomus jugulare tumor) or along the Jacobson nerve on the promontory of the middle ear (glomus tympanicum tumor) (38). These benign but locally aggressive tumors can destroy the petrous bone and enlarge enough to invade the CPA. At CT, paragangliomas appear as well-defined, enhanc-ing soft-tissue masses associated with moth-eaten erosion of the bony margins of the site of tumor origin (ie, jugular foramen or promontory). At MR imaging, paragangliomas appear as soft-tissue masses with hypervascularity; they demonstrate punctate and serpentine signal voids pro-duced by high-flow blood vessels. In addition, focal intratumoral hemorrhages with methemoglobin appear as high signal intensity on T1-weighted images, producing a characteristic salt-and-pepper appearance (Fig 12) (39). The tumor enhances intensively after contrast material administration.
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Figure 12a. Paraganglioma in a 54-year-old woman with right facial nerve palsy, vertigo, and tinnitus. (a) Axial T2-weighted MR image shows a huge paraganglioma destroying the petrous bone and invading the right CPA. Massive flow voids (arrowheads) reflect the hypervascularity of the lesion. Note the thin layer of trapped CSF (arrow) between the mass and the brainstem, which indicates an extraaxial origin. (b) Axial T1-weighted MR image shows the suggestive salt-and-pepper appearance of the paraganglioma. (c) Contrast-enhanced axial T1-weighted MR image shows intense enhancement of the lesion along with unusual dural tail enhancement of the meninges (arrows).
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Figure 12b. Paraganglioma in a 54-year-old woman with right facial nerve palsy, vertigo, and tinnitus. (a) Axial T2-weighted MR image shows a huge paraganglioma destroying the petrous bone and invading the right CPA. Massive flow voids (arrowheads) reflect the hypervascularity of the lesion. Note the thin layer of trapped CSF (arrow) between the mass and the brainstem, which indicates an extraaxial origin. (b) Axial T1-weighted MR image shows the suggestive salt-and-pepper appearance of the paraganglioma. (c) Contrast-enhanced axial T1-weighted MR image shows intense enhancement of the lesion along with unusual dural tail enhancement of the meninges (arrows).
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Figure 12c. Paraganglioma in a 54-year-old woman with right facial nerve palsy, vertigo, and tinnitus. (a) Axial T2-weighted MR image shows a huge paraganglioma destroying the petrous bone and invading the right CPA. Massive flow voids (arrowheads) reflect the hypervascularity of the lesion. Note the thin layer of trapped CSF (arrow) between the mass and the brainstem, which indicates an extraaxial origin. (b) Axial T1-weighted MR image shows the suggestive salt-and-pepper appearance of the paraganglioma. (c) Contrast-enhanced axial T1-weighted MR image shows intense enhancement of the lesion along with unusual dural tail enhancement of the meninges (arrows).
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Chondroma and Chondrosarcoma
Chondromatous tumors develop from embryonic cartilaginous remnants enclosed in the bones of the skull base. They often arise from the petro-occipital or spheno-occipital synchondrosis and destroy the adjacent bones (Fig 13) (40). Chondromatous tumors can be hypoattenuating at CT, possibly with a marginal high-attenuation area due to a dense matrix of hyaline cartilage or mas-sive calcification. Lytic bone erosion may be seen. At MR imaging, the tumor is hypointense on T1-weighted images and heterogeneously hyperintense on T2-weighted images; it enhances poorly due to its hypovascularity (41).
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Figure 13a. Chondrosarcoma in a 25-year-old woman with intracranial hypertension. (a) Axial T2-weighted MR image shows a well-defined chondrosarcoma with marked mass effect on the pons and fourth ventricle. (b) Axial T1-weighted MR image shows a skull base pedicle. (c) Contrast-enhanced coronal T1-weighted MR image shows punctate enhancement, which could suggest a chondromatous lesion. (d) Axial CT scan shows an eroded petro-occipital synchondrosis (*), which reflects the cartilaginous origin of the chondrosarcoma, although no calcifications are seen.
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Figure 13b. Chondrosarcoma in a 25-year-old woman with intracranial hypertension. (a) Axial T2-weighted MR image shows a well-defined chondrosarcoma with marked mass effect on the pons and fourth ventricle. (b) Axial T1-weighted MR image shows a skull base pedicle. (c) Contrast-enhanced coronal T1-weighted MR image shows punctate enhancement, which could suggest a chondromatous lesion. (d) Axial CT scan shows an eroded petro-occipital synchondrosis (*), which reflects the cartilaginous origin of the chondrosarcoma, although no calcifications are seen.
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Figure 13c. Chondrosarcoma in a 25-year-old woman with intracranial hypertension. (a) Axial T2-weighted MR image shows a well-defined chondrosarcoma with marked mass effect on the pons and fourth ventricle. (b) Axial T1-weighted MR image shows a skull base pedicle. (c) Contrast-enhanced coronal T1-weighted MR image shows punctate enhancement, which could suggest a chondromatous lesion. (d) Axial CT scan shows an eroded petro-occipital synchondrosis (*), which reflects the cartilaginous origin of the chondrosarcoma, although no calcifications are seen.
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Figure 13d. Chondrosarcoma in a 25-year-old woman with intracranial hypertension. (a) Axial T2-weighted MR image shows a well-defined chondrosarcoma with marked mass effect on the pons and fourth ventricle. (b) Axial T1-weighted MR image shows a skull base pedicle. (c) Contrast-enhanced coronal T1-weighted MR image shows punctate enhancement, which could suggest a chondromatous lesion. (d) Axial CT scan shows an eroded petro-occipital synchondrosis (*), which reflects the cartilaginous origin of the chondrosarcoma, although no calcifications are seen.
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Chordoma
Chordomas develop from remnants of the notochord and are located near the dorsum sellae, from which they can expand into the CPA (42). At CT, chordomas appear hypoattenuating with possible slight calcification at the periphery of the tumor in association with irregular bone erosion. At MR imaging, especially on T2-weighted images, chordomas usually appear as lobulated, large, hyperintense masses with septa of low signal intensity (43). Slight enhancement is present (Fig 14). The overall appearance can be quite similar to that of chondroma.
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Figure 14. Chordoma in a 61-year-old man with left trigeminal neuralgia and headaches. Contrast-enhanced axial T1-weighted MR image shows a chordoma invading the left CPA with unusual sparing of the clivus. There are suggestive enhanced septa (arrowheads).
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Endolymphatic Sac Tumor
Endolymphatic sac tumors are papillary adenomatous tumors that originate from the endolymphatic sac, which is located in the distal portion of the vestibular aqueduct of the petrous bone (44). These tumors occur sporadically but are frequent in von Hippel–Lindau disease (45). At CT, the tumor destroys the retrolabyrinthine petrous bone with geographic or moth-eaten margins, and intratumoral spiculated or reticulated bone can be seen (46). At MR imaging, endolymphatic sac tumors appear heterogeneous on both T1- and T2-weighted images with focal high signal intensity due to subacute hemorrhages (Fig 15) and low signal intensity due to calcification or hemosiderin. Blood-filled cysts and protein-filled cysts, both of which appear hyperintense on T1- and T2-weighted images, may be present (46) and suggest the diagnosis. Heterogeneous enhance-ment is seen after contrast material administration.
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Figure 15a. Endolymphatic sac tumor in a 26-year-old woman with von Hippel-Lindau disease and vertigo. (a) Contrast-enhanced axial T1-weighted MR image shows an endolymphatic sac tumor that destroys the right temporal bone and invades the CPA with hyperintense blood and a protein-filled cyst. (b) Axial T2-weighted MR image shows the cystic component of the tumor more clearly (*).
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Figure 15b. Endolymphatic sac tumor in a 26-year-old woman with von Hippel-Lindau disease and vertigo. (a) Contrast-enhanced axial T1-weighted MR image shows an endolymphatic sac tumor that destroys the right temporal bone and invades the CPA with hyperintense blood and a protein-filled cyst. (b) Axial T2-weighted MR image shows the cystic component of the tumor more clearly (*).
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Pituitary Adenoma
Rarely, aggressive and invasive pituitary macroadenomas can destroy the dorsum sellae, extend behind the clivus (47), and invade the CPA laterally. Invasive pituitary adenomas appear as enhancing soft-tissue masses centered around the pituitary fossa, which erode adjacent bony structures and invade surrounding normal structures. Rare metastases from pituitary adenoma can also be encountered in the CPA (48). In both situations, a history of pituitary adenoma may be present and is helpful in making the diagnosis.
Apex Petrositis
Apex petrositis (petrous apicitis) involves an inflammatory lesion of the petrous apex secondary to otitis media (49). In addition to ear pain, patients experience fifth and sixth cranial nerve palsy; this clinical complex constitutes Gradenigo syndrome (50). CT scans show a destructive lesion of the petrous apex, often in association with fluid in the middle ear. MR imaging can show an enhancing mass at the petrous tip (Fig 16) and extension of this mass, as well as its relationship to cranial nerves V and VI (51).
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Figure 16a. Apex petrositis in a 50-year-old woman with Gradenigo syndrome at clinical evaluation. (a) Axial T1-weighted MR image shows an irregular lesion at the tip of the petrous apex (arrow). (b) Contrast-enhanced axial T1-weighted MR image shows right-sided apex petrositis as an enhancing lesion along the courses of cranial nerves V and VI (arrow).
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Figure 16b. Apex petrositis in a 50-year-old woman with Gradenigo syndrome at clinical evaluation. (a) Axial T1-weighted MR image shows an irregular lesion at the tip of the petrous apex (arrow). (b) Contrast-enhanced axial T1-weighted MR image shows right-sided apex petrositis as an enhancing lesion along the courses of cranial nerves V and VI (arrow).
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Intraaxial Tumors with CPA Impingement
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Sometimes, intraaxial or intraventricular tumors can be pedunculated or large enough to invade the CPA or to manifest as a CPA mass. In these cases, the intraaxial origin may be almost impossible to confirm. Diagnosis is difficult, but subtle signs like narrowing of the cisterns, irregularity of the tumor-brain interface, and edema are helpful, along with the site of origin and the age and clinical history of the patient.
Pedunculated Brainstem Glioma
In young adults, brainstem gliomas can manifest as asymmetric expansion of the brainstem with a possible pedicle into the CPA and even then can mimic an acoustic neuroma (Fig 17) (52),(53). Gliomas appear as hypoattenuating masses at CT with variable enhancement depending on the glioma grade. At MR imaging, T1-weighted images show hypointense masses, whereas T2-weighted images show hyperintense tumors (Table 3) as well as hyperintense adjacent edema. In addition, the exophytic component in the cerebellopontine cistern is much better demonstrated than at CT.
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Figure 17. Brainstem glioma in a 23-year-old man with vertigo and hypoacusia. Contrast-enhanced axial T1-weighted MR image shows an unusual round grade III glioma located in front of the porus. The tumor demonstrates central enhancement.
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Table 3. MR Imaging Characteristics and Suggestive Features of Intraaxial or Intraventricular Lesions Invading the CPA
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Choroid Plexus Papilloma
Choroid plexus papillomas occur mainly in children but also occur in adults. These tumors derive from the neuroepithelial cells of the choroid plexus and recapitulate the structure of normal choroid plexus when benign (54). Therefore, papillomas can be encountered wherever choroid plexus is. In adults, choroid plexus papillomas often arise in the fourth ventricle and extend to the CPA through the foramen of Luschka, but they can also primarily develop in the CPA (55). At CT, these tumors are frequently hyperattenuating on nonenhanced scans with a possible cyst and calcification. MR imaging shows an isointense, irregular, but homogeneous mass that strongly enhances after injection of gadolinium contrast material (Fig 18) and is often associatedwith hydrocephalus. The hydrocephalus is explained in part by CSF hypersecretion by the tumor (56). There is usually no edema in the brainstem and no erosion of adjacent bony structures.
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Figure 18a. Choroid plexus papilloma in a 49-year-old woman with vertigo and intracranial hypertension. (a) Axial T2-weighted MR image shows a right CPA papilloma extending through the foramen of Luschka. The tumor contains massive hypointense calcification (arrowhead). (b) Contrast-enhanced axial T1-weighted MR image shows intense enhancement of the hypervascularized tumor. Note the normal choroid plexus in the left foramen of Luschka.
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Figure 18b. Choroid plexus papilloma in a 49-year-old woman with vertigo and intracranial hypertension. (a) Axial T2-weighted MR image shows a right CPA papilloma extending through the foramen of Luschka. The tumor contains massive hypointense calcification (arrowhead). (b) Contrast-enhanced axial T1-weighted MR image shows intense enhancement of the hypervascularized tumor. Note the normal choroid plexus in the left foramen of Luschka.
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Lymphoma
Lymphomas can be observed in the posterior cranial fossa and even partly in the CPA (57). Lymphomas appear as isoattenuating or hyperattenuating homogeneous masses on CT scans with intense enhancement after contrast agent administration (58). At MR imaging, lymphomas invad-ing the CPA have no specific imaging features; they are hypointense on T1-weighted images and hyperintense on T2-weighted images and enhance after contrast agent injection. Nevertheless, mass effect and edema are also present and may suggest the intraaxial origin (Fig 19) and therefore the correct diagnosis, especially in an immunodeficient patient.
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Figure 19a. Lymphoma in a 34-year-old man with acquired immunodeficiency syndrome, vertigo, and headaches. (a) Contrast-enhanced axial T1-weighted MR image shows a round lymphoma mimicking a vestibular schwannoma in front of the right porus. (b) Axial T2-weighted MR image shows narrowing of the cisterns and extensive edema, which suggest an intraaxial tumor.
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Figure 19b. Lymphoma in a 34-year-old man with acquired immunodeficiency syndrome, vertigo, and headaches. (a) Contrast-enhanced axial T1-weighted MR image shows a round lymphoma mimicking a vestibular schwannoma in front of the right porus. (b) Axial T2-weighted MR image shows narrowing of the cisterns and extensive edema, which suggest an intraaxial tumor.
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Hemangioblastoma
Hemangioblastomas are frequent in von Hippel–Lindau disease, but they may also occur in the absence of that disease in the posterior cranial fossa of young adults, where they rarely reach the cerebellar surface and invade the CPA. Hemangioblastoma usually appears as a large, well-circumscribed, homogeneous cystic mass with a small hypervascular mural nodule, but it can also appear as a solid mass without a cystic component (59). The nodule appears hypointense on T1-weighted images and hyperintense on T2-weighted images and enhances intensely after injection of gadolinium contrast material (60). Because of the hypervascularity, possible high-flow vessels can be observed as flow voids at the periphery of the mass (Fig 20). When present, cysts usually have smooth margins and homogeneous CSF-like signal intensity. Finally, edema is usually slight or absent around this intraaxial tumor (3).
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Figure 20a. Hemangioblastoma in a 28-year-old woman with von Hippel-Lindau disease and vertigo. (a) Axial T2-weighted MR image shows a solid hemangioblastoma in the left CPA. Note the vascular pedicle (arrowhead), which appears as a flow void with all sequences. (b) Contrast-enhanced axial T1-weighted MR image shows homogeneous enhancement of the hemangioblastoma (arrowhead). (c) Contrast-enhanced sagittal MR image shows a spotty, solid intraaxial hemangioblastoma (arrow) beneath the first one.
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Figure 20b. Hemangioblastoma in a 28-year-old woman with von Hippel-Lindau disease and vertigo. (a) Axial T2-weighted MR image shows a solid hemangioblastoma in the left CPA. Note the vascular pedicle (arrowhead), which appears as a flow void with all sequences. (b) Contrast-enhanced axial T1-weighted MR image shows homogeneous enhancement of the hemangioblastoma (arrowhead). (c) Contrast-enhanced sagittal MR image shows a spotty, solid intraaxial hemangioblastoma (arrow) beneath the first one.
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Figure 20c. Hemangioblastoma in a 28-year-old woman with von Hippel-Lindau disease and vertigo. (a) Axial T2-weighted MR image shows a solid hemangioblastoma in the left CPA. Note the vascular pedicle (arrowhead), which appears as a flow void with all sequences. (b) Contrast-enhanced axial T1-weighted MR image shows homogeneous enhancement of the hemangioblastoma (arrowhead). (c) Contrast-enhanced sagittal MR image shows a spotty, solid intraaxial hemangioblastoma (arrow) beneath the first one.
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Ependymoma
Ependymomas of the posterior cranial fossa are more frequent in children but can also occur in you
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Abstract
LEARNING OBJECTIVES FOR TEST...
