Primary brain tumours occur in approximately 6 persons per 100 000 per year. Fewer patients with metastatic tumours reach a neurosurgical centre, although the actual incidence must equal, if not exceed that of primary tumours. About 1 in 12 primary brain tumours occur in children under 15 years.
In adults, the commonest tumours are gliomas, metastases and meningiomas; most lie in the supratentorial compartment.
Intracranial tumours are often described as 'benign' or 'malignant', but these terms cannot be directly compared with their extracranial counterparts:
A benign intracranial tumour may have devastating effects if allowed to expand within the rigid confines of the skull cavity. A benign astrocytoma may infiltrate widely throughout brain tissue preventing complete removal, or may occupy a functionally critical site preventing even partial removal.
A malignant intracranial tumour implies rapid growth, poor differentiation, increased cellularity, mitosis, necrosis and vascular proliferation, but metastases to extracranial sites rarely occur.
In 1979, the World Health Organisation drew up an internationally agreed classification of intracranial tumours based on the tissue of origin. This system avoids the term 'glioma' -previously encompassing astrocytoma, oligodendroglioma, ependymoma and glioblastoma multiforme. Since the cell origin of the highly malignant glioblastoma is unrecognisable, this is classified along with tumours of embryonic origin.
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