Brain cancer, the abnormal and uncontrollable growth of cells in the brain, most often affects children between the ages of 3 and 12 and adults between the ages of 40 and 70, according to the National Cancer Institute. Because the brain serves as the control center for all the vital functions of the body, brain cancer quickly becomes life-threatening. Doctors classify brain cancer based on the location within the brain, the site of origination, the type of cells affected, the extent of the tumor and how the tumor cells appear.
Brain Structure
The brain contains three main sections: the cerebrum, the cerebellum and the brain stem. The cerebrum, which makes up 85 percent of the brain, controls voluntary movements and thinking, according to the Nemours Foundation. The cerebellum controls balance, movement and coordination. The brain stem, the portion of the brain that connects to the spinal cord, controls automatic functions such as the heartbeat, breathing and blood pressure. Brain cancer classifications often indicate the portion of the brain affected, such as a brain stem glioma.
Primary vs. Secondary
Doctors classify brain cancer into two broad categories. Primary brain cancer describes a cancerous tumor that originates from the cells within and surrounding the brain. However, cancer in other areas of the body can spread to the brain. Cells break away from the original tumor, such as a tumor in the breast, lungs or skin, and travel through the blood to the brain, as described by Cancer Research UK. When a brain tumor begins in another area of the body doctors classify it as secondary brain cancer.
Types of Brain Cancer
Once doctors determine whether the tumor originated in the brain or outside the brain, they determine the type of brain cancer based on the specific type of cell that grew out of control. Cancer that begins in the glial cells, the cells that make up the supportive tissue in the brain, accounts for approximately 65 percent of all primary brain cancers, according to the Merck Manual. Doctors further classify glial cell tumors based on the specific type of glial cell involved, such as astrocytoma or ependymoma. Cancer that forms from the membranes surrounding the brain is classified as meningioma and tumors originating in primitive nerve cells as medulloblastoma.
Tumor Grading
The World Health Organization grades brain tumors based on their growth rate, their blood supply and their ability to invade other cells. A grade I tumor describes tumors that grow slowly and appear very similar to the surrounding normal cells. Grade II tumors grow slightly faster and begin to exhibit an abnormal appearance. Grade III tumors reproduce quickly, invade surrounding tissues and frequently recur, as described by IRSA.org. Grade IV tumors, the most advanced and life-threatening, reproduce rapidly, contain dead cells within the tumor and cause new blood vessels to form to support their rapid growth rate.
Histological Classification
In addition to the grading system, doctors classify tumors based on their appearance under a microscope. A tumor classified as "well-differentiated" looks like normal cells. Tumors classified as "non-infiltrating" rarely invade the surrounding tissues. Tumors described as "anaplastic" appear very different from normal cells, according to the University of Maryland Greenebaum Cancer Center.
