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In most cases, brain tumors are named for the specific cell types from which they originate. The general term used for tumors that originate from the glial cells that support and nourish the brain is gliomas. There are several different types of glial cells that can be differentiated by their appearance under a microscope including oligodendrocytes, ependymal cells, and astrocytes. The most common primary brain tumors are astrocytomas that originate from star-shaped glial cells called astrocytes.

The World Health Organization (WHO) classification system divides astrocytomas into the following four types or Grades:

• Grade I = Pilocytic Astrocytoma - This is a slow-growing astrocytoma that usually does not spread to other parts of the central nervous system. This is the least malignant of the four Grades of astrocytomas.

• Grade II = Low-Grade Astrocytoma - This is also a relatively slow-growing type of astrocytoma but grows faster than a pilocytic astrocytoma (Grade I). It may or may not invade the surrounding normal brain tissue. It tends to recur after treatment.

• Grade III = Anaplastic Astrocytoma - This is a malignant astrocytoma that grows faster than a low-grade astrocytoma (Grade II). It has a tendency to invade normal brain tissue and to also recur after treatment. Grade III astrocytomas are rare and account for less than 5% of all brain tumors.

• Grade IV = Glioblastoma Multiforme - This is the most malignant and fastest growing type of all of the astrocytomas. Under a microscope, several different cell types can be observed in the tumor including astrocytes and oligodendrocytes. Areas of necrosis (dead cells) can also be observed at the center of the tumor. Glioblastoma multiforme tends to grow and spread rapidly and quickly invades normal brain tissue. Glioblastoma multiforme accounts for about 25% of all primary brain tumors and occurs most commonly in adults 50 to 70 years of age.