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) and malignant (cancerous) tumors. Benign tumors in other parts of the body do not invade nearby tissues or spread to distant areas, so they are almost never life threatening. One of the main reasons cancers are so dangerous is because they can spread throughout the body.
Although brain tumors rarely spread to other parts of the body, most of them can spread through the brain tissue. Even so-called benign tumors can, as they grow, destroy and 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 main concerns with brain tumors are how readily they spread through the rest of the brain or spinal cord 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.
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 the actions of our internal organs.
The brain is 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 liquid, called cerebrospinal fluid (CSF). Cerebrospinal fluid is made by the choroid plexus, which is located in spaces 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 and interpreting 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 lies under the cerebrum at the back part of the brain. It helps coordinate movement. Tumors of the cerebellum can cause problems with coordination in walking, trouble with fine movements of arms and legs, problems swallowing or synchronizing eye movements, and changes in rhythm of speech.
Brain stem: The brain stem is the lower part of the brain that connects to the spinal cord. It contains bundles of very long nerve fibers that carry signals controlling muscles and sensation or feeling between the cerebrum and the rest the body. Special centers in the brain stem also help control breathing and the beating of the heart. Also, 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.
Tumors in this critical area of the brain may cause weakness, stiff muscles, or problems with sensation, eye movement, hearing, facial movement, or swallowing. Double vision is a common early symptom of brain stem tumors, as are problems with coordination in walking. Because the brain stem is a small area that is so essential for life, it may not be possible to surgically remove tumors in this area.
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 within it usually cause symptoms on both sides of the body (for example, weakness or numbness of both legs). This is different from brain tumors which usually affect only one side of the body.
Nerves that branch off the spinal cord to the arms arise in the neck (cervical spine). Nerves that branch off the spinal cord to the legs, bowel, and bladder arise in the back (thoracic and lumbar spine). Tumors of the spinal cord in the neck can cause symptoms in the arms and legs, as well as affect bowel and bladder function. Spinal cord tumors below the neck may only affect the legs and bowel and bladder function.
Cranial nerves: The cranial nerves 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, or facial paralysis, numbness, or pain.
Types of cells and body tissues in the brain and spinal cord
The brain and spinal cord have many kinds of tissues and cells, which can result in the development of different types of tumors. People with these tumors can have varying prognoses (outlooks) and may be treated differently.
Neurons (nerve cells): These are the most important cells in the brain. They carry electric signals that determine thought, memory, emotion, speech, muscle movement, and just about everything else that the brain and spinal cord do. Neurons send these 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. 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. These tumors are sometimes referred to as 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 a glial cell.
- 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 fatty substance that surrounds and insulates the nerve cell axons of the brain and spinal cord. This helps neurons send electric signals through the 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 CSF flows. 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 tumors that come from these cells develop in the cerebellum and are called medulloblastomas.
Meninges: These are layers of tissue that line and protect the brain and spinal cord. CSF travels through spaces formed by the meninges. The most common tumors that start in these tissues 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 at the base of the brain. It is connected to a part of the brain called the hypothalamus. 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. As a result, a person may have low levels of one or more hormones after treatment and may need to take hormones to correct any deficiencies.
Pineal gland: The pineal gland is not strictly part of the brain. It is, in fact, a small endocrine gland that sits between the cerebral hemispheres. It makes melatonin, a hormone that regulates sleep, in response to changes in light.
Blood-brain barrier: The small blood vessels (capillaries) in the brain and spinal cord create a very selective barrier between the blood and the tissues of the central nervous system. This normally helps maintain the brain's metabolic balance and 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 from 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 cancers that start in other parts of the body, tumors that start in the brain or spinal cord rarely spread to distant organs. They cause damage because they can grow and spread into nearby areas, where they can destroy normal brain tissue. 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 be life-threatening.
Primary brain tumors can start in almost any type of tissue or cell in 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 (also known as glioblastoma multiforme), astrocytomas, oligodendrogliomas, and ependymomas. About 3 out of 10 of all brain tumors are gliomas. Counting only malignant tumors, about 8 out of 10 are gliomas.
Astrocytomas: Most tumors that develop in the brain itself start in glial cells called astrocytes. These tumors are called astrocytomas. About 2 out of 10 brain tumors are astrocytomas.
Most astrocytomas can spread widely throughout the brain and blend with the normal brain tissue, which can make them very hard to remove by surgery. Sometimes they spread along the CSF 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 tend to be slow growing.
- Intermediate-grade astrocytomas, or anaplastic astrocytomas, grow at a moderate rate.
- The highest-grade astrocytoma, known as glioblastoma (or glioblastoma multiforme), is the fastest growing. These tumors make up about two-thirds of astrocytomas and are the most common malignant brain tumors in adults.
Some low-grade types of astrocytomas called non-infiltrating astrocytomas tend to have a particularly good prognosis. These include juvenile pilocytic astrocytomas and subependymal giant cell 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 CSF pathways but rarely spread outside the brain or spinal cord. Very aggressive forms of these tumors are known as anaplastic oligodendrogliomas. Only about 2% of brain tumors are oligodendrogliomas.
Ependymomas: These tumors arise from ependymal cells, which line the ventricles. They can range from fairly low-grade (less aggressive) tumors to higher grade ones, which are called anaplastic ependymomas.
Ependymomas may spread along the CSF pathways but do not spread outside the brain or spinal cord. Ependymomas may block the exit of CSF 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. But because they can spread along ependymal surfaces and CSF pathways, treating them can sometimes be difficult. Spinal cord ependymomas have the greatest chance of surgical cure, but treatment can cause side effects related to nerve damage. Only about 2% of brain tumors are ependymomas.
Meningiomas
Meningiomas begin in 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 tumors in adults (although strictly speaking, they are not actually "brain tumors").
The risk of these tumors increases with age. They occur about twice as often 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 are often assigned a grade, based on how the cells look under the microscope.
- Grade I (benign) tumors have cells that look the most like normal cells. They make up about 80% of meningiomas. Most of these can be cured by surgery, but some grow very close to vital structures within the brain or cranial nerves and cannot be cured by surgery alone.
- Grade II (atypical) meningiomas have cells that look slightly more abnormal. About 15% to 20% of meningiomas are grade II. They are more likely to come back (recur) after surgery.
- Grade III (anaplastic) meningiomas have cells that look the most abnormal. They make up only about 1% to 3% of meningiomas. They tend to grow quickly and are the most likely to come back after treatment. Some may 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 part of a class of tumors called primitive neuroectodermal tumors (PNETs) that can also start in other parts of the central nervous system. 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 can usually be cured by surgery alone or surgery combined with radiation therapy.
Schwannomas (neurilemmomas)
Schwannomas arise from Schwann cells, which are the myelin-forming part of cranial nerves and other 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 start in parts of spinal nerves outside of the spinal cord. Schwannomas make up about 9% of all CNS tumors.
Craniopharyngiomas
These slow-growing tumors start above the pituitary gland but below the brain itself. They may compress the pituitary gland and the hypothalamus, causing hormonal problems. Because they are very close to the optic nerves, they can also cause vision problems. Craniopharyngiomas are more common in children, but they are sometimes seen in older adults.
Other tumors that can start in or near the brain
Chordomas: These rare tumors start in the bone at the base of the skull or at the lower end of the spine. Chordomas don't start in the central nervous system, but they can injure the nearby area of the nervous system by compressing it. These tumors are treated with surgery if possible, often followed by radiation therapy, but they tend to come back in the same area after treatment, causing more and more damage. They usually do not spread to other organs. For more information on chordomas, see our document, Bone Cancer.
Non-Hodgkin lymphomas: Lymphomas are cancers that start in cells called lymphocytes (one of the main cell types of the immune system). Most lymphomas start in other parts of the body, but some may start in the CNS. These lymphomas are more common in people with immune system problems, such as those infected with HIV, the virus that causes AIDS. Because of new treatments for AIDS, CNS lymphomas have become less common in recent years. These lymphomas often grow quickly and can be 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.
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