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Radiologic Findings: The first image shows a large posterior fossa mass on a nonenhanced CT. Prominent features shown are obstructive hydrocephalus, obliteration of the fourth ventricle, and marked internal calcifications. Images two and three are T1 and T2 weighted MR axial scans prior to contrast. These confirm the presence of a mass lesion and elicit several internal flow voids suggesting encasement of at least the right superior cerebellar artery. The mass extends laterally through the right foramen of Luschka. Images four and five are also T1 weighted MR scans, now following gadolinium administration and shown in axial and sagittal planes. These demonstrate heterogeneous enhancement and clearly show marked mass effect on the medulla as well as complete compromise of the foramen magnum. Following emergent ventricular decompression, the patient was taken for operative resection. The final image shows the gross intraoperative appearance with the tumor fungating out the foramen of Magiendie beneath the cerebellar hemispheres.
Diagnosis: Differential diagnostic considerations for fourth ventricular masses in children include pilocytic astrocytoma, medulloblastoma, ependymoma, brainstem glioma, and choroid plexus papilloma. No cystic components of this mass are apparent. Though this does not exclude pilocytic astrocytoma, it makes that diagnosis less likely. This mass appears separate from the brainstem as it exerts mass effect upon it rather than infiltrating. A pontine glioma is therefore unlikely. This mass lacks the typical hyperintensity on T2 weighted images seen with medulloblastoma, and again it is not cystic. Typical uniform intense contrast enhancement typical of choroid plexus tumors is not present despite the expected location and age distribution for these entities. Most likely differential possibilities are thus narrowed to medulloblastoma and ependymoma which both can produce calcifications.
Discussion: Pathologic evaluation diagnosed an ependymoma. An ependymoma is a glioma derived from differentiated ependymal cells. They therefore occur in a peri/intraventricular location intracranially. They may also arise anywhere along the course of spinal ependyma. Intracranially sixty percent are infratentorial, and of these the vast majority occur in the fourth ventricle. Typically they present in children less than five. A smaller number appear in thirty to forty year olds. The tumors are slow growing and lobulated. They tend to fill the ventricle and can extend out the exiting foramina. About one half will present with calcifications. Often they will be cystic when supratentorial. These tend to be isodense on CT, iso/hypointense on T1, and iso/hyperintense on T2. Enhancement is heterogeneous and moderate. Prognosis is moderate as many will die from recurrence or progression.
References:
Furie, DM and Provenzale, JM: Supratentorial Ependymomas and Subependymomas
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CT and MR Appearance, J Comp. Asst. Tomo. 19:518-526.
Osborne, AG: Diagnostic Neuroradiology, Mosby, pp566-571.
Tortori-Donati, P, et al.: Ependymomas of the posterior cranial fossa: CT and MR
findings, Neurorad. 37:238-243, 1995.
Willing, SJ, Atlas of Neuroradiology, Saunders, pp 200-204.
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