Pathologically, the astrocytomas may be classified into Kernahan grades I-IV, but clinically, it is more useful to subdivide them into malignant and low-grade forms.
Malignant astrocytomas embrace Kernahan grades III and IV, including glioblastoma multiforme. Peak age is 55 years and they constitute some 50% of primary intracranial tumours. Growth is rapid and infiltration may be extensive. The tumour may spread across the corpus callosum as the so-called 'butterfly' astrocytoma.
Low-grade astrocytomas embrace Kernahan grades I and II. Peak incidence is in children and young adults and account for some 15% of primary intracranial tumours. They are diffuse and slowly growing, and infiltrate the surrounding brain. There is no definitive edge or capsule. Fibrillary, protoplasmic and gemistocytic subtypes are recognised histologically. A further low grade type is the pilocytic or 'juvenile' astrocytoma which occurs in the hypothalamus, optic nerve and cerebellum. Some consider the pilocytic type to be a hamartoma rather than a true tumour.
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