ACR Codes: 1.3
Ependymoma accounts for about 9% of childhood brain tumors and 5- 6% of all intracranial gliomas. It occurs in proximity to the ventricular system, with a 2:1 fourth ventricular predominance. Three-fourths occur in children, with a mean age of 5-7 years. Hydrocephalus, cerebellar ataxia, and cranial nerve deficits occur at presentation. Leptomeningeal dissemination develops in about 11%, but extraneural metastases are rare. Histological variants include ependymoblastoma [an immature malignant form sometimes classified as a primitive neuroectodermal tumor (PNET)], malignant ependymoma, and subependymoma. Subependymoma is a benign, possibly hamartomatous or reactive form often identified incidentally or in middle aged or older adults. Primary treatment includes surgical resection and radiation therapy.
Ependymoma is hypo- to isointense on T1-weighted images and heterogeneously hyperintense on T2-weighted studies. The signal heterogeneity has been attributed to methemoglobin, hemosiderin, necrosis, cysts, calcification, or tumor vessels. On noncontrast CT, ependymoma characteristically is an isodense mass with focal calcifications in about 40% of cases. Although calcifications are difficult to identify on MR, multiplanar MR with contrast enhancement optimally demonstrates eccentricity; leptomeningeal dissemination; and extension into the adjacent brain stem, cerebellum, cerebellopontine cisterns, or upper cervical cord. Cyst formation and hemorrhage are uncommon in posterior fossa ependymoma.
Ependymoma and medulloblastoma may be indistinguishable on CT or MR. Calcification and lateral extension via the foramina of Luschka favor ependymoma. Choroid plexus papilloma or meningioma would be most unusual in this location in a child. Brain stem glioma uncommonly mimics ependymoma or medulloblastoma by exophytic extension into the fourth ventricle. Cerebellar astrocytoma characteristically displaces rather than expands the fourth ventricle.
Another typical case of posterior fossa ependymoma is shown in Films .4-.8. Note the "plastic" configuration of the mass as it extends out the foramina of Luschka and Magendie.
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Disease Topic 1444 
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