Brain tumors encompass neoplasms that originate in the brain itself (primary brain tumors) or involve the brain as a metastatic site (secondary brain tumors). The most common primary brain tumors are the gliomas, which are discussed in this section.
Although some glioma entities are histologically and biologically benign, the nature of the brain often blurs the distinction between benign and malignant. A relatively small, slow-growing, mitotically inactive tumor with little or no metastatic potential may prove lethal if located in a region of the brain that renders it less than totally resectable.
Gliomas account for the great majority of primary tumors that arise within the brain parenchyma. The term "glioma" refers to tumors that have histologic features similar to normal glial cells (ie, astrocytes, oligodendrocytes, and ependymal cells). The origin of these tumors, however, is not clear. While traditional sources favored an origin from normal glial cells, more recent data suggest that such tumors may arise from neural stem cells or progenitors.  Nonetheless, for each of these types of gliomas, there are neoplasms that span a broad spectrum of biological aggressiveness.
The classification of gliomas is discussed here. The classification of other common brain tumors (eg, meningiomas, medulloblastomas) is discussed separately. (See "Meningioma: Epidemiology, risk factors, and pathology" and "Histopathology and molecular pathogenesis of medulloblastoma".)
HISTORY OF GLIOMA CLASSIFICATION
Primary gliomas are classified by light microscopy according to their predominant cell type and graded based upon the presence or absence of standard pathologic features. Historical attempts at developing a classification system for brain tumors date back to the 1830s. The German pathologist Rudolf Virchow first introduced the term "glioma" in 1860. Virchow was also the first to attempt to correlate the microscopic and macroscopic features of central nervous system tumors .