 a glioma is a type of tumor that starts in the brain or spine. it is called a glioma because it arises from glial cells. the most common site of gliomas is the brain. gliomas make up about 30% of all brain and central nervous system tumors and 80% of all malignant brain tumors. ==classification== gliomas are classified by cell type, by grade, and by location. ===by type of cell=== gliomas are named according to the specific type of cell they share histological features with, but not necessarily originate from. the main types of gliomas are: * ependymomasâependymal cells. * astrocytomasâastrocytes (glioblastoma multiforme is a malignant astrocytoma and the most common primary brain tumor among adults). * oligodendrogliomasâoligodendrocytes. * brainstem glioma â develop in the brain stem * optic nerve glioma â develop in or around the optic nerve * mixed gliomas, such as oligoastrocytomas, contain cells from different types of glia. ===by grade=== gliomas are further categorized according to their grade, which is determined by pathologic evaluation of the tumor. thumb|right|low-grade brain glioma in a 28-year-old male. (taken on 10 july 2007) *low-grade gliomas [who grade ii] are well-differentiated (not anaplastic); these tend to exhibit benign tendencies and portend a better prognosis for the patient. however, they have a uniform rate of recurrence and increase in grade over time so should be classified as malignant. *high-grade [who grade iiiâiv] gliomas are undifferentiated or anaplastic; these are malignant and carry a worse prognosis. of numerous grading systems in use, the most common is the world health organization (who) grading system for astrocytoma, under which tumors are graded from i (least advanced diseaseâbest prognosis) to iv (most advanced diseaseâworst prognosis). ===by location=== gliomas can be classified according to whether they are above or below a membrane 
Brain_tumor