In the current WHO classification, diffuse astrocytoma is considered grade II and anaplastic astrocytoma, grade III as in Table 30-2. The lower-grade astrocytomas constitute a minority of adult tumors but a larger proportion of (infratentorial) tumors in children. The median survival in cases of anaplastic astrocytoma is considerably longer than for glioblastoma, 2 to 5 years in adults, often longer. Favored sites of occurrence are the cerebrum in adults and in children, are cerebellum, hypothalamus, optic nerve and chiasm, and pons. Astrocytomas of the cerebral hemispheres arise mainly in adults in their third and fourth decades or earlier; astrocytomas in other parts of the nervous system, particularly the posterior fossa and optic nerves, are more frequent in children and adolescents. These tumors had been classified further according to their histologic characteristics: protoplasmic or fibrillary; gemistocytic (enlarged cells distended with hyaline and eosinophilic material); pilocytic (elongated, bipolar cells); and mixed astrocytoma-oligodendroglioma types; the current terminology is in Table 30-2 wherein these are subsumed by the larger category of astrocytoma. The most common type is composed of well-differentiated fibrillary astrocytes. The tumor cells contain glial fibrillary acidic protein (GFAP), which is a useful diagnostic marker in biopsy specimens. Some cerebral astrocytomas present as mixed astrocytomas and glioblastomas. The most common low-grade fibrillary type (grade II) is distinguished from the more benign WHO grade I pilocytic tumor (with a very good prognosis) and the rare pleomorphic xanthoastrocytoma. These distinctions correlate to a degree with the biologic behavior of the astrocytomas and therefore have prognostic importance but are eclipsed by certain genetic characteristics discussed above.
Cerebral astrocytoma is a slowly growing tumor of infiltrative character with a tendency in some cases to form large cavities or pseudocysts. Other astrocytomas are noncavitating and appear grayish white, firm, and relatively avascular, almost indistinguishable from normal white matter, with which they merge imperceptibly. Fine granules of calcium may be deposited in parts of the tumor, but calcium in a slow-growing intracerebral tumor is more characteristic of an oligodendroglioma. The CSF is acellular; the only abnormalities in some cases are the increased pressure and protein content. The tumor may distort the lateral and third ventricles and displace adjacent cerebral vessels (Fig. 30-4). It may not be possible on clinical or imaging grounds to distinguish low-grade gliomas from a number of rare tumors in childhood such as the dysembryoplastic neuroepithelioma (DNET) discussed further on.
In about two-thirds of patients with astrocytoma, the first symptom is a focal or generalized seizure, and between 60 and 75 percent of patients have recurrent seizures in the course of their illness. Other subtle cerebral symptoms follow after months, sometimes after years. Headaches and signs of increased intracranial pressure are relatively late occurrences.
On T1-weighted MRI, astrocytomas are isointense or hypointense; on T2 sequences, hyperintense, and there is variable enhancement of the solid portion of the tumor after gadolinium infusion. Cyst formation and small amounts of calcification are common, especially in cerebellar tumors. Other low-grade astrocytomas take the form of a more homogenous T1 hypointense and T2 hyperintense infiltrating mass with poorly defined borders and little or no contrast enhancement.
In children, astrocytic tumors usually arise in the cerebellum and declare themselves by some combination of gait unsteadiness, unilateral ataxia, and increased intracranial pressure (headaches, vomiting).