“Glioma” is a general term used to describe any tumor that arises from the supportive (“gluey”) tissue of the brain. This tissue, called “glia,” helps to keep the neurons in place and functioning well.
There are three types of normal glial cells that can produce tumors. An astrocyte will produce astrocytomas (including glioblastomas), an oligodendrocyte will produce oligodendrogliomas, and ependymomas come from ependymal cells. Tumors that display a mixture of these different cells are called mixed gliomas.
Tumors such as “optic nerve glioma” and “brain stem glioma” are named for their locations, not the tissue type from which they originate.
Three types of normal glial cells can produce tumors—astrocytes, oligodendrocytes, and ependymal cells. Tumors that display a mixture of these cells are called mixed gliomas.
- Mixed Glioma (also called Oligoastrocytoma): These tumors usually contain a high proportion of more than one type of cell, most often astrocytes and oligodendrocytes. Occasionally, ependymal cells are also found. The behavior of a mixed glioma appears to depend on the grade of the tumor. It is less clear whether their behavior is based on that of the most abundant cell type.
- Optic Glioma: These tumors may involve any part of the optic pathway, and they have the potential to spread along these pathways. Most of these tumors occur in children under the age of 10. Grade I pilocytic astrocytoma and grade II fibrillary astrocytoma are the most common tumors affecting these structures. Higher-grade tumors may also arise in this location. Twenty percent of children with neurofibromatosis (NF-1) will develop an optic glioma. These gliomas are typically grade I, pilocytic astrocytomas. Children with optic glioma are usually screened for NF-1 for this reason. Adults with NF-1 typically do not develop optic gliomas.
- Gliomatosis Cerebri: This is an uncommon brain tumor that features widespread glial tumor cells in the brain. This tumor is different from other gliomas because it is scattered and widespread, typically involving two or more lobes of the brain. It could be considered a “widespread low-grade glioma” because it does not have the malignant features seen in high-grade tumors.
The widespread nature of gliomatosis cerebri causes enlargement of any part of the brain it involves. This may include the cerebral hemispheres, or less often, the cerebellum or brain stem.
The location of the tumor depends on the type of cells from which it originates.