primary brain tumors

How are primary brain tumors classified?

Brain tumors are classified by both the cell of the brain that makes them up, and how the tumor looks under a microscope. Primary brain tumors can arise from any of the cells in the brain. They can come from the neurons, the glial cells, the lining of the brain, or from specific structures in the brain. Glial cells support the neurons of the brain and tumors which arise from these cells are known as glial tumors. The membrane that surrounds the brain can also develop tumors and these are known as meningiomas. There are other types of tumors, which involve other structures of the brain including ependymomas, among others.

About 85% of primary brain tumors arise from the glial cells of the brain and are called gliomas. Unfortunately, a majority of these tumors (35-45%) are the most aggressive type, called Glioblastoma Multiforme. Astrocytic tumors are another type of glioma, which arise from star shaped cells called astrocytes. When a pathologist (a doctor who studies cells in the laboratory) looks at brain tumors under a microscope, he/she can get a sense of how aggressive the tumor is by the way the cells look, and based on this, assign the tumor a grade. The most common classification system is the World Health Organization (WHO) system, which classifies CNS tumors according to histology (cell appearance under the microscope) as well as tumor grade. The WHO numerical grade represents the overall biologic potential for malignancy (or aggressiveness) from I (benign) to IV (malignant).

The gliomas classified according to their WHO grading:

• Low grade astrocytomas include pilocytic astrocytomas (grade I) and diffuse astrocytomas (grade II)
• A grade III astrocytoma is called anaplastic astrocytoma.
• A grade IV astrocytoma is a glioblastoma, also called glioblastoma multiforme or GBM.

Other brain tumors include:

• Ependymomas arise from glial cells that line the open spaces in the brain and the spinal cord. They are most commonly found in children and young adults. Includes grade I and II ependymomas and grade III anaplastic ependyoma.
• Oligodendrogliomas arise from cells that make the fatty substance that covers and protects nerves. Under the microscope, the cells have a “fried egg” appearance. They are most common in middle-aged adults. These include oligodendroglioma (grade II) and anaplastic oligodendroglioma (grade III)
• Medulloblastomas (primitive neuroectodermal tumor) often arise in the cerebellum. It is the most common brain tumor in children and is grade IV.
• Meningiomas arise in the meninges, which are the tissues that cover the brain. They are typically benign and slow-growing, though there are also rare grade II and III malignant meningeal tumors.
• Pituitary Tumors and craniopharyngiomas form in the center of the brain and can occur in children and adults.