Radiologic Findings: A T1-weighted image (Fig. 1) shows a hypointense, approximately 7cm x 4cm mass in the right cerebellar hemisphere, with a moderate amount of mass effect on the 4th ventricle and cerebellum. The mass is hyperintense on the T2-weighted image (Fig. 3). The cystic portion of the mass has signal intensity similar to CSF on the T1W images and T2W images, but is hyperintense to CSF on the proton density weighted image (Fig. 2) due to increased protein content. Following Gadolinium administration, T1-weighted images (Fig. 4-6) show intense homogeneous enhancement of a mural nodule in the superior aspect of the mass. Although septations within the mass show faint enhancement, the cystic portion of the mass does not show enhancement.
Diagnosis: Juvenile pilocytic astrocytoma.
Discussion: Cerebellar juvenile pilocytic astrocytomas are the most common infratentorial neoplasm in the pediatric age group. Although they commonly form well-defined lobular masses which contain cysts and a vascular mural nodule, they may occasionally present a solid mass without a cystic component and may simulate other pediatric posterior fossa masses.
Juvenile pilocytic astrocytomas are one of the most benign tumors of glial origin. They have an increased incidence in type I neurofibromatosis. They have an excellent prognosis following complete resection. However, 15% of cerebellar astrocytomas are of the fibrillary type (85% are of the pilocytic type) and carry a worse prognosis due to infiltration. Fibrillary type of astrocytoma also is the predominate histological type in brainstem gliomas.
The differential diagnosis includes medulloblastoma and ependymoma in a child. Hemangioblastomas are more often seen in adults and show flow voids on MR due to hypervascularity.
References: 1. Willing, Atlas of Neuroradiology, W.B. Saunders, Philadelphia, PA, 1995, pgs. 187-188.
2. Grossman and Yousem, Neuroradiology: The Requisites, Mosby Year Book,St. Louis, MO, 1994, pg. 83.
Submitted by:Rakesh Shah, M.D.Charles F. Lanzieri, M.D. (uhrad.com).