Disclosure
Background: Ependymoma is a central nervous system (CNS) neoplasm composed of glial cells that have differentiated along ependymal lines. Ependymoma occurs most commonly in the ependymal lining of the ventricles, but it also arises in the filum terminale and the central spinal canal. Radiologic imaging plays a role in both the diagnostic workup and treatment of patients with ependymoma. Patients with CNS symptoms routinely undergo cross-sectional imaging. Computed tomography (CT) is often the modality used initially to evaluate for intracranial hemorrhage, mass, or mass effect. If a tumor is suspected, magnetic resonance imaging (MRI) is the next study performed. MRI better characterizes CNS tumors, and findings often lead to a presumptive diagnosis. Final diagnosis of ependymoma, as with most CNS neoplasms, is achieved with tissue sampling. Both CT and MRI are also important in the treatment of patients with ependymoma. Imaging is essential to assess for response to therapy and recurrence. For excellent patient education resources, visit eMedicine's Cancer and Tumors Center. Also, see eMedicine's patient education article Brain Cancer. Pathophysiology: The histology of this frequently benign tumor is of ependymal cell rests forming true rosettes and perivascular pseudo-rosettes dissociated from the ependyma (Barone, 1970). The gross pathology of ependymoma reveals an irregular solid mass. Cyst formation is common. Punctate calcification, necrosis, and intratumoral hemorrhage are frequent. An aggressive anaplastic form of ependymoma, characterized by pleomorphic multinucleated cells, necrosis, and vascular changes, occurs in as many as 25% of patients (Spoto, 1990; Sun, 1999). Subependymoma is an ependymoma related benign fibrillary tumor lacking the ependymal rosettes, necrosis, and neovascularity of conventional ependymoma. These commonly occur in the caudal portion of the fourth ventricle or in the frontal horn of the lateral ventricle (Osborn, 1994). The related and highly aggressive primitive undifferentiated ependymoblastoma is grouped with primitive neuroectodermal tumors (PNETs) (Edwards-Brown, 1994). Frequency:
Mortality/Morbidity: The prognosis for patients with untreated ependymoma is dismal. Regardless of histologic type, treatment with surgery alone results in a 5-year survival rate of 17-27%.
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Anatomy: Intracranial ependymoma can occur either above or below the tentorium. The tumor arises in areas of ventricular angulation from rests of ependymal cells that extend into adjacent white matter (Sanford, 1997). Infratentorial ependymoma occurs in 60-73% of patients, and 70-80% of these posterior fossa tumors are located in the fourth ventricle (Edwards-Brown, 1997; Sun, 1999). An additional 15% arise within the cerebellopontine angle, and the remaining 5-8% arise within the cerebellar hemispheric substance (Vezina, 1994). As many as 55% of infratentorial ependymomas invade the cerebellopontine angle cisterns via the lateral recesses of the fourth ventricle. Extension is frequent from the fourth ventricular foramen of Magendie through the foramen magnum, with involvement of the upper cervical spinal cord. Of infratentorial ependymomas, 12% present with subarachnoid seeding, especially those demonstrating anaplastic histology (Han, 1984). Supratentorial tumors typically arise near the trigone of the lateral ventricle (Sun, 1999; Han, 1984). Clinical Details: Clinical presentation is related to the site of the tumor. Infratentorial tumors produce posterior fossa symptoms and signs, including nausea, vomiting, ataxia, cranial nerve palsy, dizziness, extremity weakness, and diplopia. Supratentorial tumors produce symptoms and signs related to increased intracranial pressure. Preferred Examination: Currently, MRI is the chief modality used in the study of ependymomas. CT is a useful adjunct. Before the development of cross-sectional and multiplanar imaging, angiography and pneumoencephalography were used to localize brain masses and characterize tumor vascularity. Ultrasonography, nuclear medicine studies, angiography, and radiography are of no benefit in the workup of ependymoma. The final diagnosis of ependymoma is achieved through pathology; however, when correlated with demographic and clinical features, MRI and CT findings can be strongly suggestive of ependymoma. Limitations of Techniques: A general limitation of CT is radiation exposure. Additionally, the use of iodinated contrast material may sometimes be associated with nausea, vomiting, and rare anaphylactoid reactions. Limitations of CT with respect to ependymoma include imprecise anatomic detail. General limitations of MRI include its cost and the need for patient cooperation. Patient motion is a cause of considerable artifact. Many patients, especially children and patients with claustrophobia, require sedation. Another general limitation is the incompatibility of MRI with numerous foreign and/or medically implanted objects, such as pacemakers. Finally, MRI is of limited benefit in the evaluation of cortical bone and the detection of calcium.
Astrocytoma, Brain
Colloid cyst
Findings: X-ray findings are included only for historical interest. A study by Barone and Elvidge demonstrated that in 45 pathology-proven cases of ependymoma, intracranial calcifications were present in 6 patients (Barone, 1970). The pineal gland was calcified in 4 patients, and the pineal gland was displaced from the midline in 2 patients. Separation of the sutures occurred in 12 patients. In the 43 patients in whom ventriculography was performed, 41 demonstrated hydrocephalus with identification of the site of obstruction. |
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Findings: Limitations of CT scans are especially evident in examining lesions in the posterior fossa. MRI eliminates the problems of CT streak artifact, provides superior tissue definition and contrast resolution, and allows multiplanar imaging. However, CT scanning remains a useful adjunct to MRI in the evaluation of ependymoma. CT scans help delineate areas of calcification, which tend to be punctate and focal. Calcification is present in 50% of supratentorial ependymomas and 46% of infratentorial ependymomas (Sanford, 1997). Solid components of ependymomas tend to be isoattenuating to hypoattenuating relative to gray matter. Hypoattenuating cystic features are demonstrated by 46-83% of supratentorial ependymomas and 23% of infratentorial ependymomas (Sanford, 1977). One third of supratentorial ependymomas homogeneously enhance with the intravenous administration of contrast material, whereas the remaining two thirds enhance heterogeneously. Enhancement patterns of infratentorial ependymomas are slightly different; 10% do not enhance, while the remaining 90% are divided roughly equally between homogeneous and heterogeneous enhancement. CT is useful in differentiating ependymomas from other tumors. Medulloblastomas have more homogeneous enhancement and slightly higher attenuation values than those of ependymomas. Calcification is less frequent in medulloblastoma (15%) than in ependymoma, in which 50% of tumors have calcifications. Also, ependymomas tend to fill the fourth ventricle, sometimes extending out of the foramina of Luschka, whereas medulloblastomas tend to efface the fourth ventricle. A combination of punctate calcification and cyst formation favors ependymoma over cerebellar astrocytoma. Brainstem gliomas tend to be isoattenuating, they seldom become calcified, and they have a propensity to infiltrate and expand the pons (Sanford, 1997). Degree of Confidence: Location and CT-attenuation characteristics can narrow the focus of the differential diagnosis when evaluating brain neoplasms. However, the diagnosis of ependymoma cannot be made on the basis of CT findings alone. MRI is the next most appropriate imaging modality, and final diagnosis is achieved by tissue sampling. False Positives/Negatives: No normal variants mimic this disease process.
Findings: MRI has supplanted CT scanning as the diagnostic modality of choice in the workup and follow-up observation of intracranial neoplasms, including ependymoma. The most appropriate role for MRI in the treatment of ependymoma is in the detection of tumor and direction of its resection and/or irradiation. MRI is used to monitor ongoing treatment and to survey for recurrence. Solid portions of ependymoma typically are isointense to hypointense relative to white matter on short recovery time/echo time (TR/TE) T1-weighted images. The tumor is hyperintense to white matter on long TR/TE T2-weighted images. As many as 50% of ependymomas demonstrate signal heterogeneity, which may indicate calcification, necrosis, methemoglobin, hemosiderin, or tumor vascularity (Spoto, 1990; Sun, 1999; Vezina, 1994). For example, hyperintense foci on both T1- and T2-weighted images suggest methemoglobin in subacute hemorrhage of 1-4 weeks in age. Hypointense foci on both T1- and T2-weighted images suggest hemosiderin, calcium, or necrosis. Punctate calcific foci are difficult to diagnose prospectively but are present in as many as 45% of ependymomas (Vezina, 1994; Han, 1984). Cystic changes result in high signal intensity on T2-weighted MRIs. Signal heterogeneity is a feature useful in distinguishing ependymoma from the more homogeneous medulloblastoma. Calcification and hemorrhagic foci are more typical of ependymoma than medulloblastoma. Additionally, ependymomas are more apt to extend through the foramina of Luschka and Magendie, hence the term plastic ependymoma. Similarly, choroid plexus papilloma is more homogeneous than ependymoma and lacks the typical irregular margins and surrounding edema of ependymoma. Enhancement with gadolinium is useful in differentiating tumor from adjacent vasogenic edema and normal brain parenchyma. Without intravenous contrast enhancement, T2-weighted images are more reliable in differentiating tumor margins than are T1-weighted images (Han, 1984). Some reports describe ependymomas that cause displacement of the vein of the lateral recess of the fourth ventricle on cerebral arteriography (Han, 1984). This vein normally courses from the transverse and lateral supratonsillar veins along the anterior and lateral aspect of the superior pole of the cerebellar tonsil. It then courses lateral to the cerebellopontine angle, over the brachium pontis, to join the petrosal vein. Ependymoma expanding the fourth ventricle and its lateral recesses can displace this vein posteriorly and laterally. Gadolinium-based contrast agents (gadopentetate dimeglumine [Magnevist], gadobenate dimeglumine [MultiHance], gadodiamide [Omniscan], gadoversetamide [OptiMARK], gadoteridol [ProHance]) have recently been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). For more information, see the eMedicine topic Nephrogenic Fibrosing Dermopathy. The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans. As of late December 2006, the FDA had received reports of 90 such cases. Worldwide, over 200 cases have been reported, according to the FDA. NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble moving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness. For more information, see the FDA Public Health Advisory or Medscape. Degree of Confidence: While the MRI findings can be of great help in narrowing the differential diagnosis of brain tumors, final diagnosis is achieved through histologic sampling. False Positives/Negatives: No normal variants mimic the MRI findings of this disease process.
Findings: The role of ultrasound in the evaluation of ependymoma is limited. Fetal ultrasound and pediatric transcranial sonography are used primarily as screening tools for other pathologic conditions but can detect hydrocephalus reliably. A study by Han et al demonstrated that 6 of 1528 infants undergoing transcranial ultrasonography had a pathologically proven brain neoplasm. One patient had ependymoma. Sonography demonstrated a solid echoic fourth ventricular mass with localized, well-defined, anechoic cystic areas (Han, 1984). These findings are not sensitive or specific for ependymoma.
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