Pilocytic astrocytoma

Pilocytic astrocytomas represent about 2.5 to 5% of all intracranial tumors and 5-10% of all gliomas. In the pediatric population they account for one third of all gliomas.

Macroscopic appearance and general characteristics

Pilocytic astrocytomas are more circumscribed. They can be cystic with a very vascular mural nodule most frequently seen in the cerebellum. Others present as a circumscribed multilobulated mass, with local invasion of the subarachnoid space or the wall of the third ventricle. Expansion into the surrounding brain is slow, with a very rare tendency for progression to anaplasia. They occur mainly in children and young adults. They occur in the midline (chiasm, third ventricle, thalamus, brain stem) or in the cerebellum. They are slowly growing tumors with 5 year survival up to 90%. Histologically pilocytic astrocytoma corresponds to Grade I. Malignant transformation (Grade III) is very rare.

Microscopic appearance (images 2 - 3 - 4)

The tumor is composed, at least in part, of bipolar fusiform or "piloid" cells with dense fibrillation. A biphasic pattern is common, in which pilocytic areas are associated with a loosely structured microcystic component of neoplastic astrocytes.

Imaging features (see case reports)

- On CT and MRI the images reflect the dual appearance of pilocytic astrocytoma. In some cases, mostly in the cerebellum, a cyst is found with a mural nodule displaying the same density or intensity as grey matter. Enhancement of the mural nodule is marked, without enhancement of the cyst wall (image 5). Delayed enhancement of the content of the cyst with creation of a contrast-fluid level has been described. The more solid midline pilocytic astrocytomas (image 6) of the chiasm and the hypothalamus are well-defined multilobulated masses that are hypodense on CT, hyperintense on T2 weighted images and hypointense on T1 weighted images. Enhancement is moderate to marked and can be inhomogeneous.

Differential diagnosis

The cystic tumors with enhancing nodule can resemble hemangioblastoma. The more solid midline suprasellar masses have to be differentiated from germinoma, choroid plexus papilloma pituitary adenoma, meningioma of the diaphragma sellae, giant aneurysm and craniopharyngioma. Lymphoma and metastases are low on the list of differential diagnoses.

Case reports

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Pilocytic astrocytoma

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