Brain tumors are masses of abnormal cells that have grown out of control. In most other parts of the body, it is very important to distinguish between benign (non-cancerous) tumors and malignant (cancerous) ones. Benign tumors in other parts of the body are almost never life threatening. The main reason cancers are so dangerous is because they can spread throughout the body.

Most brain cancers can spread through the brain tissue but rarely spread to other areas of the body. But even so-called "benign" tumors can, as they grow, compress normal brain tissue, causing damage that is often disabling and sometimes fatal. For this reason, doctors usually speak of "brain tumors" rather than "brain cancers." The major concern with brain tumors is how readily they spread through the rest of the central nervous system and whether they can be removed and not come back.

Brain and spinal cord tumors are different in adults and children. They often form in different areas, develop from different cell types, and may have a different outlook and treatment. This document refers only to adult tumors. Brain and spinal cord tumors in children are discussed in a separate document.

In order to understand brain and spinal cord tumors, it helps to know about the normal structure and function of the central nervous system.

The central nervous system

The central nervous system (CNS) is the medical name for the brain and spinal cord.

The brain is the center of thought, feeling, memory, speech, vision, hearing, movement, and much more. The spinal cord and special nerves in the head called cranial nerves help carry messages between the brain and the rest of the body. These messages tell our muscles how to move, transmit information gathered by our senses, and help coordinate our internal organs.

The brain is located within and protected by the skull. Likewise, the spinal cord is protected by the bones (vertebrae) of the spinal column.

The brain and spinal cord are surrounded and cushioned by a special fluid, called cerebrospinal fluid (CSF). Cerebrospinal fluid is made by the choroid plexus, which is located in cavities within the brain called ventricles. The ventricles as well as the spaces around the brain and spinal cord are filled with CSF.

Parts of the brain and spinal cord

The main areas of the brain include the cerebrum, cerebellum, and brain stem. Each of these parts has a special function.

Cerebrum: The cerebrum is the large, outer part of the brain. It is made up of 2 hemispheres (halves) and controls reasoning, thought, emotion, and language. It is also responsible for planned muscle movements (throwing a ball, walking, chewing, etc.) and for taking in sensory information such as vision, hearing, smell, touch, and pain.

The symptoms caused by a tumor in a cerebral hemisphere depend on the part of the hemisphere in which the tumor arises. Common symptoms include:

• seizures
• trouble speaking
• a change of mood such as depression
• a change in personality
• weakness or paralysis of part of the body
• changes in vision, hearing, or other sensations

Basal ganglia: The basal ganglia are structures deeper within the brain that help control our muscle movements. Tumors or other problems in this part of the brain typically cause weakness, but in rare cases can cause tremor, chorea (involuntary jerky movements), or athetosis (involuntary slow movements).

Cerebellum: The cerebellum controls coordination of movement. Tumors of the cerebellum can cause problems with coordination in walking, trouble with fine movements of arms and legs, impairment of swallowing or synchronized eye movements, and changes in rhythm of speech.

Brain stem: The brain stem contains bundles of very long nerve fibers that carry signals controlling muscles and sensation or feeling between the cerebrum and the rest the body. In addition, most cranial nerves (which carry signals directly between the brain and the face, eyes, tongue, mouth, and some other areas) start in the brain stem. Special centers in the brain stem also control breathing and the beating of the heart.

Tumors in this critical area of the brain may cause weakness, stiff muscles, or problems with sensation, hearing, facial movement, or swallowing. Double vision is a common early symptom of brain stem tumors, as are problems with coordination in walking. Because tumors of the brain stem often intermingle with normal nerve cells and the brain stem is so essential for life, it may not be possible to surgically remove these tumors.

Spinal cord: The spinal cord, like the brain stem, contains bundles of very long nerve fibers that carry signals controlling muscles, sensation or feeling, and bladder and bowel control. Spinal cord tumors may cause weakness, paralysis, or numbness. Because the spinal cord is such a narrow structure, tumors arising within it usually cause symptoms involving both sides of the body (for example, weakness or numbness of both legs). This is different than tumors of the brain, which usually affect only one side of the body. Moreover, most tumors of the spinal cord arise below the neck after nerves to the arms have branched off the spinal cord, so that only lower body functions -- bowel, bladder, or leg -- are affected.

Cranial nerves: Tumors may also arise from cranial nerves, which are nerves that extend directly out of the base of the brain (as opposed to coming out of the spinal cord). Tumors starting in cranial nerves may cause vision problems, trouble swallowing, hearing loss in one or both ears, facial paralysis, or facial numbness or pain.

Types of cells and body tissues in the brain and spinal cord

The brain and spinal cord contain different kinds of tissues and cells, which can result in different types of tumors. These tumors can have varying outlooks for survival and may be treated differently.

Neurons (nerve cells): These are the most important cells within the brain. They send signals through their nerve fibers (axons). Axons in the brain tend to be short, while those in the spinal cord can be as long as several feet. Electric signals carried by neurons determine thought, memory, emotion, speech, muscle movement, and just about everything else that the brain and spinal cord do. Unlike many other types of cells that can grow and divide to repair damage from injury or disease, neurons stop dividing about a year after birth (with a few exceptions). Neurons do not usually form tumors, but they are often damaged by tumors that start nearby.

Glial cells: Glial cells are the supporting cells of the brain. Most brain and spinal cord tumors develop from glial cells. They are sometimes referred to as a group called gliomas.

There are 3 types of glial cells -- astrocytes, oligodendrocytes, and ependymal cells. A fourth cell type called microglia is part of the immune system and is not truly glial in origin.

• Astrocytes help support and nourish neurons. When the brain is injured, astrocytes form scar tissue that helps repair the damage. The main tumors starting in these cells are called astrocytomas or glioblastomas.

• Oligodendrocytes make myelin, a substance that surrounds and insulates axons of the brain and spinal cord. This helps neurons transmit electric signals through axons. Tumors starting in these cells are called oligodendrogliomas.

• Ependymal cells line the ventricles (fluid-filled areas) within the central part of the brain and form part of the pathway through which cerebrospinal fluid travels. Tumors starting in these cells are called ependymomas.

• Microglia are the immune (infection fighting) cells of the central nervous system.

Neuroectodermal cells: These are primitive cells that are probably the remains of embryonic cells. They are found throughout the brain. The most common tumor that comes from these cells is the medulloblastoma, which arises in the cerebellum from forerunners of nerve cells called granule cells.

Meninges: These are tissues that line and protect the brain and spinal cord. The meninges help form the spaces through which CSF travels. The most common tumors that start in these cells are called meningiomas.

Choroid plexus: The choroid plexus is the area of the brain within the ventricles that makes CSF, which nourishes and protects the brain.

Pituitary gland and hypothalamus: The pituitary is a small gland found at the base of the brain. The hypothalamus is the part of the brain to which the pituitary gland is connected. Both help regulate the activity of several other glands. For example, they control the amount of thyroid hormone made by the thyroid gland, the production and release of milk by the breasts, and the amount of male or female hormones made by the testicles or ovaries. They also make growth hormone, which stimulates body growth, and vasopressin, which regulates water balance by the kidneys.

The growth of tumors in or near the pituitary or hypothalamus, as well as surgery and/or radiation therapy in this area, can interfere with these functions. Consequently, a person may have low levels of one or more hormones and may need hormone treatments to correct any hormone deficiency.

Pineal gland: The pineal gland is not strictly part of the brain. It is, in fact, an endocrine gland that sits between the cerebral hemispheres. Its main function is probably to make melatonin, a hormone that regulates sleep, in response to changes in light.

Blood-brain barrier: Unlike the case with most other organs, the small blood vessels (capillaries) in the brain and spinal cord have a very selective barrier between the blood and the tissues of the central nervous system. This normally keeps harmful toxins from getting into the brain. Unfortunately, it also keeps out most chemotherapy drugs that are used to kill cancer cells, which in some cases limits their usefulness.

Types of brain and spinal cord tumors

It's important to know the difference between tumors that start in the brain (primary brain tumors) and tumors that start in other organs, such as the lung or breast, and then spread to the brain (metastatic or secondary brain tumors). In adults, metastatic tumors to the brain are actually more common than primary brain tumors. These cancers are not treated the same way. For example, breast or lung cancers that spread to the brain are treated differently than cancers that start in the brain. This document is only about primary brain and spinal cord tumors, not those that have spread from elsewhere in the body.

Unlike other cancers, tumors arising within the brain or spinal cord rarely metastasize to distant organs. They cause damage because they spread locally and destroy normal brain tissue in the place where they arise. Still, tumors of the brain or spinal cord are rarely considered "benign" (non-cancerous). Unless they are completely removed or destroyed, most brain or spinal cord tumors will continue to grow and eventually lead to death.

Primary brain tumors can start in any of the different types of tissues or cells within the brain or spinal cord. Some tumors contain a mixture of cell types. Tumors in different areas of the central nervous system may be treated differently and have a different prognosis (outlook).

Gliomas

Gliomas are not a specific type of cancer. Glioma is a general term for a group of tumors that start in glial cells. A number of tumors can be considered gliomas, including glioblastoma multiforme, astrocytomas, oligodendrogliomas, and ependymomas. About 4 out of 10 of all brain tumors are gliomas. Counting only malignant tumors, about 8 out of 10 are gliomas.

Astrocytomas: Most tumors that arise within the brain itself start in glial cells called astrocytes. These tumors are called astrocytomas. About 3 out of 10 brain tumors are astrocytomas.

Most astrocytomas can spread widely throughout and blend with the normal brain tissue, which can make them very hard to remove by surgery. Sometimes they spread along the cerebrospinal fluid pathways. It is very rare for them to spread outside of the brain or spinal cord.

Astrocytomas are often classified as low grade, intermediate grade, or high grade, based on how the cells look under the microscope.

• Low-grade astrocytomas are the slowest growing.
• Intermediate-grade astrocytomas, or anaplastic astrocytomas, grow at a moderate rate.
• The highest-grade astrocytoma, known as glioblastoma multiforme (or just glioblastoma), is the fastest growing. These make up about two-thirds of astrocytomas and are the most common malignant brain tumors of adults.

Some special types of astrocytomas tend to have a particularly good prognosis. These are called non-infiltrating astrocytomas (for example, juvenile pilocytic astrocytomas). They are more common in children than in adults.

Oligodendrogliomas: These tumors start in brain cells called oligodendrocytes. Like astrocytomas, most of these can grow into (infiltrate) nearby brain tissue and cannot be completely removed by surgery. Oligodendrogliomas sometimes spread along the cerebrospinal fluid pathways but rarely spread outside the brain or spinal cord. Very aggressive forms of these tumors are known as anaplastic oligodendrogliomas. Only about 4% of brain tumors are oligodendrogliomas.

Ependymomas: These tumors arise from the ependymal cells, which line the ventricles. Ependymomas may block the exit of cerebrospinal fluid from the ventricles, causing the ventricles to become very large -- a condition called hydrocephalus. Unlike astrocytomas and oligodendrogliomas, ependymomas usually do not grow into (infiltrate) normal brain tissue. As a result, some (but not all) ependymomas can be completely removed and cured by surgery. Spinal cord ependymomas have the greatest chance of surgical cure. Ependymomas may spread along the cerebrospinal fluid pathways but do not spread outside the brain or spinal cord. Very aggressive forms of these tumors are known as anaplastic ependymomas. Only about 2% of brain tumors are ependymomas.

Meningiomas

Meningiomas arise from the meninges, the layers of tissue that surround the outer part of the brain and spinal cord. Meningiomas account for about 1 out of 3 primary brain and spinal cord tumors. They are the most common brain tumor in adults (although strictly speaking, they are not actually "brain tumors").

The risk of these tumors increases with age. They are about twice as common in women. In some cases these tumors run in families, especially in those with neurofibromatosis, a syndrome in which people develop many benign tumors of nerve tissue

Meningiomas cause symptoms by pressing on the brain or spinal cord. About 4 out of 5 meningiomas are benign, and most of these can be cured by surgery. Some meningiomas, however, are located dangerously close to vital structures within the brain and cannot be cured by surgery alone. A small number of meningiomas are malignant and may come back many times after surgery or, rarely, even spread to other parts of the body.

Medulloblastomas

Medulloblastomas are tumors that develop from neuroectodermal cells (primitive nerve cells) in the cerebellum. They are fast-growing tumors and often spread throughout the cerebrospinal fluid pathways, but they can be treated by radiation therapy and chemotherapy. Medulloblastomas occur much more often in children than in adults. They are discussed in more detail in our document, Brain and Spinal Cord Tumors in Children.

Gangliogliomas

A tumor containing both neurons and glial cells is called a ganglioglioma. These are very uncommon in adults and have a high rate of cure by surgery alone or surgery combined with radiation therapy.

Schwannomas (neurilemomas)

Schwannomas arise from Schwann cells, which are the myelin-forming part of cranial nerves and other peripheral nerves. These are usually benign tumors. They can arise from any cranial nerve. When they form from the cranial nerve responsible for balance near the cerebellum they are called vestibular schwannomas or acoustic neuromas. They may also arise from spinal nerves after they have left the spinal cord. When this is the case, they can compress the spinal cord, causing weakness, sensory loss, and bowel and bladder problems. These make up about 8% of all CNS tumors.

Other tumors that can start in or near the brain

Chordomas: These tumors start in the bone at the base of the skull or at the lower end of the spine. These tumors are not from the central nervous system, but they can cause injury to the nearby nervous system by compressing it. Typically they come back many times over 10 to 20 years, causing progressive injury. They usually do not spread to other organs.

Non-Hodgkin lymphomas: Lymphomas start in lymphocytes (one of the main cell types of the immune system). Some central nervous system (CNS) lymphomas occur in people with immune system problems, such as those infected with HIV, the virus that causes AIDS. Because of new treatments for AIDS, this type of brain lymphoma has become less common in recent years. Lymphomas of the brain are thought of as highly malignant and are often very hard to treat. Recent advances in chemotherapy, however, have improved the prognosis of people with these cancers. For more information on CNS lymphomas (including treatment), see our document, Non-Hodgkin Lymphoma.

Last Medical Review: 08/06/2008
Last Revised: 05/13/2009