Pilocytic Cerebellar Astrocytoma

Sandra L Mondro, MD
Richard B Schwartz, MD

May 24, 1996

Presentation

Imaging Findings

Computed tomography (CT) of the head without intravenous contrast demonstrates a predominantly cystic mass in the midline of the cerebellum, measuring 4.5 cm in greatest dimension (arrow). A solid component is located eccentrically along the left lateral margin (arrow). The mass compresses the fourth ventricle. Hydrocephalus is present.

T2-weighted fast-spin-echo axial MR images demonstrate the cystic component of the mass to have a high signal intensity (arrow). T1-weighted coronal MR images obtained following intravenous contrast show eccentric enhancement of the solid component, mimicking the CT findings (arrow).

Differential Diagnosis

Diagnosis

Discussion

Medulloblastoma is primarily a neoplasm of children but occurs in adults 30% of the time. Peak incidence is in the latter half of the first decade although a smaller second peak occurs in the early third decade. In young adults, medulloblastomas usually arise in the dorsal aspect of the lateral cerebellar hemispheres, as opposed to the characteristic origin from the cerebellar vermis in children. Medulloblastoma typically has a high CT density before intravenous contrast with dense enhancement after injection. On MRI, the long TR images usually demonstrate an isointense mass, rather than the hyperintensity seen in this case. Like other primitive neuroectodermal tumors (PNET), medulloblastomas have the propensity to disseminate into the subarachnoid space via cerebrospinal fluid (CSF) pathways. The occurrence of medulloblastomas has been associated with such heritable diseases as Gorlin's basal cell nevus syndrome, Turcot's glioma-polyposis syndrome, and ataxia-telangiectasia.

Ependymomas account for about 5% of all intracranial gliomas. They are most common in children under 5 years of age, but a smaller peak occurs at age 30 to 40 years. The fourth ventricle is the most common site, often leading to dilatation and extrusion of tumor through the various ventricular foramina. However, CSF seeding occurs less frequently compared to PNET tumors. Ependymomas appear heterogeneous on CT and MRI and often contain hemorrhagic foci. Calcification is present in about 40-50% of patients.

Hemangioblastomas of the posterior fossa usually occur spontaneously but are associated with von Hippel Lindau disease (vHL) in 4-20% of patients. With vHL, 20% of tumors are multiple and can occur anywhere in the cerebellum, brainstem, or spinal cord. Hemangioblastomas classically are cystic-appearing with an enhancing mural nodule, but they are entirely solid in 30-40% of patients. The tumors often have associated large vessels leading to the mass, best seen on gradient echo MRI.

References

2. Maroldo TV, Barkovich AJ. Pediatric brain tumors. Semin Ultrasound, CT, MR 1992;13:412-448.

3. Schwartz RB, Mantello MT. Primary brain tumors in adults. Semin Ultrasound, CT, MR 1992;13:449-472.

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