Brain tumors account for 1 percent of all cancers in adults and 2.5 percent of adult cancer deaths, and are the most common solid tumors in children. Tumors of the spinal cord or nerves are more rare. Brain and nervous system tumors almost never spread beyond the nervous system.
The nervous system consists of two parts: the central nervous system, made up of the brain and spinal cord; and the peripheral nervous system, which encompasses all other nerves in the body. A series of three membranes called the meninges surround and protect the brain and spinal cord. The cerebral hemispheres are the largest part of the human brain, where reasoning takes place. The cerebellum, a smaller structure beneath the cerebrum, coordinates motion and balance. Some automatic functions essential for life, such as breathing and heartbeat, are controlled by structures within the brain stem.
Most tumors begin in the glial cells, which form the connective tissue of the nervous system, and are known collectively as gliomas. Tumors that begin in the impulse-conducting cells of the nervous system, or neurons, are much more rare. Tumors may also occur in the pineal or pituitary glands, which are situated within the brain. Tumors of the meninges are relatively common and usually benign. Although they are technically not nervous-system tumors because they are not made of the same type of tissue, meningiomas are included in this summary.
Brain tumors that are benign, meaning they do not invade other tissue, may still be deadly because they can interfere with the function of vital brain structures. When a benign tumor grows, because the skull is not flexible, the tumor can compress and damage other structures within the brain. Benign tumors may also become malignant (cancerous). For these reasons, the distinction between benign and malignant tumors is less important in brain and nervous system cancers than it is in tumors involving other parts of the body. Brain tumors are instead referred to as low-grade or high-grade, depending on how quickly they grow and spread.
Tumors that have spread to the brain from cancers originating in other parts of the body, or metastatic brain cancers, are more common than primary brain tumors, which arise first in the brain. This summary addresses primary brain tumors only.
The American Cancer Society estimates that 16,500 malignant brain tumors (9,500 in men and 7,000 in women) will be diagnosed during 2000 in the United States. Approximately 13,000 people (7,100 men and 5,900 women) will die from brain cancers.
Treating brain and spinal cord tumors is difficult for many reasons. The blood-brain barrier, which bars many drugs from entering the brain via the blood stream, means chemotherapy is often ineffective for treating cancers within the brain. Tumors may also involve vital structures within the brain and spinal cord, and thus cannot be safely removed with surgery. It is also extremely difficult to completely remove tumors surgically from the brain, nervous system and meninges. Also, radiation therapy can damage healthy tissue within the nervous system.
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