Ependymomas most often arise next to the ependymalined ventricular system, including the central canal of the spinal cord. In the first 2 decades of life, they typically occur near the fourth ventricle and constitute 5% to 10% of the primary brain tumors in this age group. In adults, the spinal cord is their most common location; tumors in this site are particularly frequent in the setting of neurofibromatosis type 2 (Chapter 22). The clinical outcome for completely resected supratentorial and spinal ependymomas is better than for those in the posterior fossa.
MORPHOLOGYInthefourthventricle,ependymomastypicallyaresolidorpapillarymassesextendingfromtheventricularfloor.Thetumorsarecomposedofcellswithregular,roundtoovalnucleiandabundantgranularchromatin.Betweenthenucleiisavariablydensefibrillarybackground.Tumorcellsmayformroundorelongatedstructures(rosettes, canals) thatresembletheembryologicependymalcanal,withlong,delicateprocessesextendingintoalumen(
Fig.23.31 );morefrequentlypresentareperivascular pseudorosettes inwhichtumorcellsarearrangedaroundvesselswithaninterveningzonecontainingthinependymalprocesses.Anaplasticependymomasshowincreasedcelldensity,highmitoticrates,necrosis,microvascularproliferation,andlessevidentependymaldifferentiation.
Far less frequent than gliomas, tumors composed of cells with neuronal characteristics are typically lower-grade lesions that often present with seizures. While some neuronal differentiation can be observed in many tumors, lesions in this group are primarily composed of cells that express neuronal markers, such as synaptophysin and neurofilaments.
• Central neurocytoma is a low-grade neoplasm found within and adjacent to the ventricular system (most commonly the lateral or third ventricle); it is characterized by evenly spaced, round, uniform nuclei and often islands of neuropil.
Fig. 23.31 Microscopicappearanceofependymoma.
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Dysembryoplastic neuroepithelial tumor is a distinctive, low-grade tumor of children and young adults that grows slowly, often manifests as a seizure disorder, and carries a favorable prognosis after resection. It typically is located in the superficial temporal lobe and consists of small, round neuronal cells arranged in columns and around central cores of processes.
Gangliogliomas are tumors with a mixture of glial elements, usually a low-grade astrocytoma and mature-appearing neurons. Most of these tumors are slow growing, and often manifest with seizures. About 20% to 50% of gangliogliomas harbor point mutations in the BRAF gene.
Some tumors of neuroectodermal origin have a primitive “small round cell” appearance that is reminiscent of normal progenitor cells encountered in the developing CNS. Differentiation is often limited, but may progress along multiple lineages. The most common is the medulloblastoma, accounting for 20% of pediatric brain tumors.
Medulloblastoma occurs predominantly in children and exclusively in the cerebellum. Neuronal and glial markers are nearly always expressed, at least to a limited extent. It is highly malignant, and the prognosis for untreated patients is dismal; however, medulloblastoma is exquisitely radiosensitive. With total excision, chemotherapy, and irradiation, the 5-year survival rate may be as high as 75%. There are a series of histologic patterns observed in medulloblastoma, which are informative about prognosis and correlate in part with the underlying genetics.