Cerebellar Astrocytoma

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Cerebellar astrocytomas constitute 12-17% of primary CNS tumors in the pediatric population, with a peak incidence between ages 6 to 8. There is a slight male preponderance. Classically, the tumor originates within a cerebellar hemisphere and forms a cystic structure with a mural nodule. The walls of the cyst may be gliotic or neoplastic. Alternatively, the tumor may be solid and originate in the vermis possibly with hemispheric extension. Closer examination differentiates malignant from fibrillary and pilocytic subclasses. Malignant astrocytomas have characteristic features such as anaplasia, necrosis, and numerous mitotic figures. Endothelial proliferation may also be present. Fibrillary subtypes are less diffuse and resemble benign cerebral astrocytomas of adulthood. Pilocytic are the commonest subtype, comprising 70% of juvenile astrocytomas. Histologically these tumors have Rosenthal fibers (indicating a chronic gliotic process) and elongated, hair-like processes. Necrosis, mitotic figures, pleomorphism, and hypercellularity may be present yet spread through the CSF is rare.

Presentation and Diagnosis

Headache is the commonest symptom, presenting in the front initially due to hydrocephalus. Morning headaches are characteristic as high CO2 and venous congestion from recumbency increase intracranial pressure (ICP). Later, posterior headache and neck pain may arise from mass effect leading to tonsilar herniation. Other symptoms include vomiting, gait (vermis) or limb (hemisphere) ataxia, diplopia, vertigo, and personality change. Signs include papilledema, increased head circumference, cerebellar dysfunction such as dysmetria, nystagmus, and ataxia, and brainstem involvement manifested by hypereflexia or cranial nerve palsies. Plain films may show splitting of sutures or sella turcica erosion from increased ICP. CT reveals isodense or slightly hypodense nodule relative to adjacent brain that enhances with contrast. Cyst fluid is proteinaceous and therefore denser than CSF. MR is the modality of choice and shows a hyperintensity on T2-weighted imaging. Cyst wall enhancement with contrast suggests tumor invasion vs. benign reactive tissue.

Surgery

Goal of surgery is complete resection with minimal neurologic deficit. Total removal yields excellent prognosis with 90% of patients experiencing long term, recurrence free survival. Cystic vs. solid lesions and histological subclass do not affect outcome. Residual tumor, especially if solid, should be treated with radiosurgery, whereas recurrence necessitates re-operation (as an exception, recurrence within 2 years may be treated with radiation). An enhancing cyst wall should be biopsied intraoperativley; if negative for tumor, resection may not be necessary.

Reference

Beni-Adani L, Gormor M, Spektor S, et al. Cyst Wall Enhancement in Pilocytic Astrocytoma: Neoplastic or Reactive Phenomena. Pediatric Neurosurgery 2000; 32: 234-239.

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