A brain tumor is a mass formed by an overgrowth of abnormal cells. Something triggers a single cell's genes to change, causing it to divide out of control. If a tumor originates in the brain, the brain's coverings, or its nerves, it's known as a primary brain tumor. Most brain tumors in children are primary tumors. In adults, most brain tumors are metastatic or secondary tumors, meaning the cancer has spread to the brain from the breast, lung, or other part of the body. Nearly 1 in 4 people with cancer will get a secondary brain tumor. Historically, the prognosis for patients with these tumors was bleak; people were expected to survive only several weeks after diagnosis. More aggressive surgical treatments, innovative radiation approaches, and new approaches to chemotherapy and other drugs now allow survival with a good quality of life that is measured in several months to years.
Brain tumors are often classified as benign or malignant. Truly benign tumors are composed of noncancerous cells that don't invade brain or other tissues. A malignant brain tumor may contain benign-appearing cells that invade normal tissue or contain cancerous cells either from the brain or other body cancers. The latter are typically life-threatening tumors, as they can spread throughout the brain or to the spinal cord. (Brain cancer rarely spreads outside the central nervous system).
Even some benign tumors can be life-threatening. Because the brain and spinal cord are housed within bony structures that can't expand—the skull and spinal column—tumors made of benign cells can grow large enough to put pressure on sensitive areas of the brain, impair function, and lead to coma and death.
Brain tumors are grouped by their location within the brain and the type of brain cells where the cancer began. The most common primary brain tumors in adults are pituitary tumors, meningiomas, and glioblastomas. The most common primary brain tumors in children are medulloblastomas, astrocytomas, ependymomas, and brain stem gliomas. The chance of recovery and choice of treatment depend on the type of brain tumor and the patient's general state of health.
More than 180,000 brain tumors (malignant and benign) are diagnosed each year in the United States. Of those, about 36,000 are primary brain tumors. Brain tumors can occur at any age, but they're most common in adults between the ages of 40 and 70 and in children between the ages of 3 and 12. Primary brain tumors account for only 2 percent to 3 percent of all new cancer cases in adults. In children, however, brain tumors account for 25 percent of all cancers. About 2,900 children under age 20 are diagnosed with brain tumors each year in the United States.
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The brain is divided into three main parts: the cerebrum, the cerebellum, and the brain stem. The cerebrum is the largest part of the brain and is responsible for most higher-level brain functions, including language, consciousness, emotions, integration of sensory stimuli, and some motor functions. The second-largest structure in the brain is the cerebellum. Located under the cerebrum at the back of the brain (just above the back of the neck), this portion of the brain governs coordination, balance, and some other aspects of movement.
Finally, the brain stem, the smallest part, attaches the brain to the spinal cord and head and includes the midbrain, pons, and medulla oblongata. It is among the most "primitive" parts of the brain and controls many of the body functions that occur automatically, including heartbeat, blood pressure, and wakefulness.
Each part of the brain breaks down into smaller regions that control various body functions and behaviors.
Protected by the skull and three thin membranes called meninges, the brain floats and is cushioned by cerebrospinal fluid. This watery liquid flows between the meninges and other spaces called ventricles.
A network of nerves made up of nerve cells, or neurons, transmits signals from the brain to other parts of the body. A different group of brain cells, called glial cells, serves as support for the neurons-feeding them, insulating them, clearing dead neurons away. The most common primary brain tumors, called gliomas, begin in glial cells or their stem cells.
It is usually impossible to determine the cause of a brain tumor in any given individual. A brain tumor begins when certain genes on the chromosomes of a cell no longer function properly. These genes normally regulate how often the cell divides (or if it divides at all) and repair other genes. A defective gene might then be programmed to tell a damaged cell to self destruct.
Some people might be born with partial defects in one or more of these genes, and environmental factors (such as radiation and chemicals) might lead to further damage. In other people, environmental factors might be the only cause of damage to the gene function.
Once a cell is dividing too rapidly and can't check its own growth, it can grow into a tumor. If a tumor cell begins to grow, the body's immune system should detect the abnormal cell and kill it. Many tumors, however, produce substances that block the immune system from recognizing the abnormal tumor cells. Over time, the tumor cells overpower all of the body's defenses against their growth.
Scientists have linked several factors with an increased risk of developing a primary brain tumor. Nevertheless, most people who have these risk factors do not develop brain tumors, and many of those who do get brain tumors have none of the risk factors. These factors include the following:
- Males are more likely to get brain tumors than females. Meningiomas, however, are more common in females.
- Caucasians tend to get more brain tumors than do people of other races.
- Having a family member with a glioma may mean you are slightly more likely to develop the disease.
If you've been exposed to radiation—working in the nuclear industry, for example—you may have an increased risk of developing a brain tumor. Also, people who have received radiation therapy to the head may have an increased risk of developing meningiomas later in life.
Pathologists and embalmers who work with formaldehyde have a greater risk of developing brain cancer. But scientists have not found a link with other workers who are exposed to formaldehyde. Workers who make plastics may be exposed to vinyl chloride, which may increase the risk of brain tumors. Textile and plastic workers may be exposed to acrylonitrile, which could also increase the chance of brain cancer.
Scientists are researching whether cellphone use is a risk factor, but so far studies have not found convincing evidence of an increased risk of brain tumors among cellphone users. Researchers are also investigating if head injuries are a risk factor, but so far, studies have not shown an increased incidence of brain tumors.
What type of tumor a brain tumor is depends on where it started, how fast it grows, and whether or not it contains cancer cells. A tumor that begins in the brain or adjacent tissue is called a primary brain tumor. These are named for the type of cells in which they begin. For instance, tumors that start in the glial cells are called gliomas. A secondary brain tumor, which has spread from another part of the body, has the same kind of abnormal cells and the same name as the primary cancer.
Common benign primary brain tumors include those of the lining of the skull (meningiomas), those of the nerves (schwannomas), and those of the pituitary gland (pituitary adenomas). However, benign brain tumors can become malignant and, once removed, can also grow back unless nearly every tumor cell was destroyed or removed during treatment.
With few exceptions, most primary brain tumors involving the brain tissue (gliomas) are cancerous. These cancers can range from slow-growing, noninvasive tumors (low-grade tumors) to rapidly growing, highly destructive tumors (high-grade tumors) , such as glioblastomas. (Grading is based on the tumor's tendency to spread as well as its growth rate). In the past, low-grade tumors were often referred to as being benign. It is now widely recognized that even these low-grade tumors are rarely cured and usually change into higher-grade tumors over time.
Brain tumors in children often arise from tissues that are different from those most commonly affected in adults. Common pediatric tumors include tumors of primitive cells that have not matured into adult cell types (primitive neuroectodermal tumors or PNETs) , tumors of the lining of the brain's fluid sacs (ependymomas), and benign tumors of the supporting cells of the brain (juvenile astrocytomas).
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Gliomas are the most common primary brain tumors. There are a number of types, distinguished by the type of cells they affect; some are slow growing, and some grow quickly and are difficult to treat. Mixed gliomas are brain tumors that occur in more than one type of brain cell.
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Astrocytomas make up nearly 60 percent of all tumors. They resemble astrocytes (star-shaped cells that help feed neurons by drawing nutrients from blood vessels) and arise either from astrocytes or from their stem cells. Astrocytes also remove waste products, which are carried away from the brain by the blood. In adults, astrocytomas most often occur in the cerebrum. Astrocytomas in the cerebellum are more common in children.
Low-grade infiltrating astrocytomas (grade II) are tumors that grow slowly but usually mix with the brain around them. Anaplastic or grade III astrocytomas are faster growing and start to displace or replace local brain, while spreading into adjacent brain. Glioblastoma multiformes, also called grade IV astrocytomas, usually grow very rapidly, are highly destructive, and spread even more quickly to other parts of the brain.
The majority of these tumors in children are low grade, while the majority in adults are high grade. With few exceptions—pilocytic astrocytomas (grade I) are truly benign and usually can be completely removed through surgery—Grades II-IV astrocytomas are rarely curable with currently available treatments.
Ependymomas begin in the ependymal cells that line the cavities and passageways of the brain (ventricles) and spinal cord. They most often occur in the cerebellum or brain stem. About 85 percent of ependymomas are low grade.
Well-differentiated ependymomas have cells that look like normal cells and grow slowly. Anaplastic ependymomas grow more quickly than well-differentiated ependymal tumors. Ependymoblastomas are rare cancers that usually occur in children. They may grow very quickly.
Low-grade ependymomas grow slowly and usually do not spread to the spine or other areas of the brain. They often can be successfully treated with surgery. High-grade tumors tend to grow more quickly and to invade surrounding tissue. Treatment of these tumors is less successful, and they often recur. In some cases, cells from ependymomas "break off" from the tumor and spread to the spine through the cerebrospinal fluid. This process is called CSF seeding.
Ependymomas are relatively uncommon, accounting for about 6 percent of all tumors affecting the central nervous system. Ependymomas that occur in the brain are more common in children, and ependymomas that begin in the central canal of the spinal cord are more common in adults. Ependymomas are the third-most-common brain tumor in children, occurring most often between the ages of 1 and 5. In adults, ependymomas most often occur between the ages of 30 and 35.
Oligodendrogliomas begin in the oligodendrocytes (or their stem cells), cells that make a fatty substance called myelin that provides support and nourishment for the neurons, or cells that transmit nerve impulses. Oligodendrogliomas usually occur in the cerebrum. Most are slow-growing, low-grade tumors that spread into surrounding brain tissue and often cause seizures. They account for 10 to 20 percent of all primary brain tumors, and they occur mostly in middle-aged adults. More rare are anaplastic oligodendrogliomas, which grow quickly and invade nearby tissue.
Brain stem gliomas are located in the brain stem, which attaches the cerebrum to the rest of the head and spinal cord. Most brain stem gliomas occur in the pons and are called pontine gliomas. The pons (Latin for " bridge " ) is a band of nerve fibers linking the medulla oblongata and cerebellum to the upper portions of the brain. Brain stem gliomas rarely spread to other areas of the brain or to the spinal cord. Approximately three fourths of patients with brain stem gliomas are younger than 20. In the United States, brain stem gliomas make up about 2 percent of all brain tumors in adults and about 10 percent to 15 percent of all childhood brain tumors. Treatment of brain stem gliomas in the midbrain or medulla is generally more successful than gliomas affecting the pons.
Medulloblastomas begin in the lower part of the brain. This type of cancer may spread to the spine or to other parts of the body. Medulloblastomas are considered to be primitive neuroectodermal tumors or PNETs because they contain nervous system cells that did not develop properly at a very early stage of life. Although they can affect people of any age, 75 percent of all cases occur in children. About 1 in 5 childhood brain tumors is a medulloblastoma, with 350 new cases diagnosed each year in the United States.
Pineal region tumors are found in or around the pineal gland, a small structure located near the center of the brain that produces melatonin. There are many types of pineal region tumors, and they can be slow growing (pineocytomas) or fast growing (pineoblastomas).
Germ cell tumors arise from primitive cells that normally develop into the testicles and ovaries but also can occur in other parts of the body, including the brain. There are different kinds of germ cell tumors, including germinomas, embryonal carcinomas, choriocarcinomas, and teratomas.
Craniopharyngiomas are tumors that occur near the pituitary gland, which is responsible for many body functions, such as growth, metabolism, and reproduction. These are most common in children.
Meningiomas are slow-growing tumors in the meninges, or membranes covering the brain. They are usually benign but can be difficult to remove through surgery and can recur. They generally do not recur if they are completely removed. Meningiomas most often occur as single tumors but can occur as multiple growths. Malignant meningiomas are rare tumors that grow more quickly than other meningiomas. Meningiomas account for 15 percent to 20 percent of all primary brain tumors. They most often occur in people between the ages of 40 and 60. They are more common in women than in men and are rare in children.
Choroid plexus tumors start in the choroid plexus, tissue located in the brain's ventricles. The choroid plexus makes the fluid that fills the ventricles and surrounds the brain and spinal cord. Tumors of the choroid plexus can grow slowly (choroid plexus papilloma) or more rapidly (anaplastic choroid plexus papilloma) . The rapidly growing tumors are more likely to spread to other places in the brain and to the spinal cord.
Schwannomas are tumors that originate in the schwann cells of nerves. Vestibular schwannomas (also known as acoustic neuromas) are tumors of the nerve that controls balance and hearing in the inner ear. The tumor occurs most often in adults.
Childhood supratentorial primitive neuroectodermal tumors occur in the upper part of the brain. Sometimes called cerebral neuroblastomas, they can spread to other parts of the brain and spinal cord.
Last reviewed on 10/9/09
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