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Findings: Axial T2 and T1 weighted images as well as sagittal T1 weighted images following the intravenous administration of contrast are shown. These demonstrate a large enhancing posterior fossa mass with enhancing solid component within the left cerebellar hemisphere. In addition, a large cystic component is seen. The fourth ventricle is not well visualized. There is a moderate degree of hydrocephalus.
Diagnosis: Juvenile pilocytic astrocytoma.
Discussion: The differential diagnosis for posterior fossa tumors in children include juvenile pilocytic astrocytoma, medulloblastoma as well as ependymomas. Astrocytomas and medulloblastomas account for approximately two thirds of posterior fossa tumors in the pediatric population.
Pilocytic astrocytomas usually present within the first two decades of life with a peak age of presentation at ten years old. Patients present with headache, nausea and vomiting.
The typical radiographic appearance is that of an enhancing mural nodule as well as a nonenhancing cystic component. Approximately 85% of posterior fossa juvenile pilocytic astrocytomas arise from the cerebellar vermis. Other common locations include the optic chiasm and hypothalmus as well as around the third ventricle. When occurring in the region of the vermis, obstructive hydrocephalus occurs relatively early.
When completely removed, there is a greater than 90% five year survival rate. Surgery is usually curative, but prognosis does depend on cellular morphology.
References:
Osborn AEG. Diagnostic Radiology. Mosby, St. Louis; 1994:553-558.
Taveras JM. Neuroradiology, 3rd Edition. Williams & Wilkins; 1996:676-683.
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