The ependyma is the differentiated epithelial lining of the ventricular system. Tumors of the ependyma are classified under the revised WHO schema as subependymoma and myxopapillary ependymoma (grade 1), ependymoma (grade 2), malignant ependymoma (grade 3) and ependymoblastoma (grade 4). Ependymoblastoma is, however, a distinctive pathological entity and is usually grouped within the category of primitive neuroectodermal tumors. The overall incidence of ependymoma in children is 2.2 per million per year. Intracranial ependy-momas are more common in the infratentorial location while spinal-cord ependymomas are rare. In the posterior fossa, ependymomas typically arise from the fourth ventricle or immediately adjacent to it from the lateral recesses (Chapter 2, Fig. 22). These tumors display a characteristic pattern of growth into the subarach-noid space, often surrounding and enveloping cranial nerves and vessels. This feature makes the complete removal of larger tumors difficult.
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