Glioma is a common type of primary brain tumor, accounting for about 33% of these tumors. Gliomas originate in the glial cells in the brain. Glial cells are the tissue that surrounds and supports neurons in the brain. These tumors arise from three different types of cells that are normally found in the brain: astrocytes, oligodendrocytes, and ependymal cells. Gliomas are called intrinsic brain tumors because they reside within the substance of the brain and often intermix with normal brain tissue.