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Cerebellar astrocytoma accounts for about 11 - 30% of primary CNS tumors in children, and has a post-resection survival of up to 94% at 10 years. This tumor makes up about one-third of childhood posterior fossa tumors, typically presenting late in the first decade or in the early second decade. These well-circumscribed masses are often completely resected surgically, with no other therapy required.

Cerebellar astrocytoma tends to involve the hemispheres, although extension into or origin within the vermis or even the fourth ventricle have been described infrequently. It may be predominantly cystic with a mural nodule (as shown in a different patient, Film .4), multicystic, or predominantly solid (diffuse) on CT or MR. Recognizable cysts occur in up to 80%. Histologically, most are juvenile pilocytic astrocytomas similar to those of the third ventricle region. Diffuse infiltrating fibrillary types occur in up to 15% of cerebellar astrocytomas and have a less favorable prognosis. Frank malignancy, leptomeningeal seeding, and spontaneous hemorrhage are distinctly unusual.

Radiographically, cerebellar astrocytoma arises within the cerebellar hemisphere and less often in the vermis, and displaces the fourth ventricle without invading it. On CT, 10-25% contain foci of calcification. The mass is hypointense on T1-weighted and hyperintense on T2-weighted MR, with one or more cystic components. After contrast administration, the mural nodule and/or solid components enhance, rendering nonenhancing cysts more conspicuous.

In children, vermian or diffuse astrocytoma or those involving the fourth ventricle may mimic medulloblastoma or ependymoma. In older adolescents or adults, hemangioblastoma may present as a cystic mass indistinguishable from cerebellar astrocytoma. Metastasis is the most common posterior fossa neoplasm in adulthood. Rarely vascular malformation, abscess, or lymphoma might have a similar radiographic appearance.