[Editor's note: This article is from 2007. Some newer methods of diagnosis and treatment are not listed here. See further information on BrainWeb]
sections include: symptoms of brain tumors, what causes brain tumors?, diagnosing a brain tumor, types of brain tumors, treatment
The term brain tumor is used to describe any tumor growing within the skull, though a more accurate term might be intracranial tumor. Only some of these growths arise directly from brain tissue. Others grow from the other tissues inside the skull, such as pituitary tumors. In contrast to these primary brain tumors, which arise within the skull, another group consists of tumors that spread to the head from another source, such as lung or breast cancer; these are secondary brain tumors, and they are much more common. There are a great many different types of brain tumors, each with its own specific biology and treatment, but all cause similar symptoms.
Both primary and secondary tumors exist on a spectrum, from high to low grade. In most high-grade tumors, also called malignant, or cancerous, the cells are very different from normal cells, grow relatively quickly, and can spread (metastasize) easily to other locations. However, malignant brain tumors differ from malignant tumors that arise elsewhere in the body because they do not spread to other organs.
Tumors are usually called low-grade, benign, or noncancerous if their cells are similar to other, normal cells in the body, grow relatively slowly, and remain confined to one location. In most areas of the body, these growths cause little damage, and surgical removal is usually a cure. The same is true for many benign intracranial tumors that arise outside the brain, such as meningiomas and pituitary tumors. However, benign tumors arising within the brain can quickly become harmful and are very difficult to treat or cure. Most of the human body is soft and yielding, but our hard skulls mean that any abnormal growth inside can squeeze sensitive tissues. A benign tumor growing next to vital brain structures does not have to grow very large before it can seriously impair function and threaten health. Furthermore, if a benign tumor is growing deep inside the brain, surgeons have to cut through important areas, causing irrevocable damage, to remove it. Over time, some benign tumors can also become malignant.
Symptoms of Brain Tumors
A common feature of many intracranial tumors is the swelling that they incite in the surrounding brain. The growth irritates the brain and triggers a process called edema, in which fluid from the blood leaks into the surrounding tissue. This fluid can disrupt brain function, and the symptoms caused by a brain tumor are usually due to the combination of the tumor itself and the edema surrounding it. The fluid causes pressure that disrupts the function of surrounding brain regions. Shrinking the edema with steroids will frequently relieve symptoms such as weakness, numbness, or impaired speech.
Seizures are a common initial symptom of brain tumors. They may be either generalized (grand mal), in which case an individual loses consciousness, or limited to only a portion of the body (for example, involuntary shaking of one arm or leg). Other symptoms of a brain tumor may include progressive weakness of one side of the body, or such language difficulties as trouble finding the correct words to express one’s thoughts. People may display personality changes, such as loss of initiative, or apathy; loved ones may mistake this change as depression, but it can be related to a brain tumor. Nausea and vomiting, vision or hearing difficulties, balance problems, and other behavioral and cognitive symptoms, such as trouble with thinking and memory or psychotic episodes, can all occur. About 40 percent of patients with brain tumors suffer headaches; however, they almost always accompany other symptoms.
With the exception of abrupt seizures, these symptoms all usually develop slowly over several weeks or even months. This contrasts with the same symptoms when caused by other common neurological problems. In a stroke, symptoms appear suddenly (ischemic, hemorrhagic). In Alzheimer’s disease, they take many years to develop.
What Causes Brain Tumors?
Each year, doctors in the United States diagnose approximately 17,000 new primary brain tumors and 100,000 new secondary brain tumors. Overall, brain tumors are slightly more common in men than in women—except for meningiomas, which are more common in women. There is no known ethnic predisposition for the most common types of brain tumors. In adults, the incidence of brain tumors increases with age. There is concern that the incidence of brain tumors has been rising, but it is difficult to know whether that indicates a real increase in tumors or is the result of better diagnosis with modern imaging techniques.
There is no known behavioral or environmental factor that leads to brain tumors. Fears about cellular phones, microwave ovens, foods or food additives, and other rumored brain carcinogens have no scientific basis. The only definitive risk factor is having had radiation therapy to the head, usually for another condition; this can increase the risk of most common types of brain tumors, but it may take one or two decades before a brain tumor appears. Dental X rays do not carry an increased risk. Brain tumors are associated with some genetic syndromes, such as neurofibromatosis, but these conditions account for a small fraction of all brain tumors in the United States.
Diagnosing a Brain Tumor
The best way to establish the diagnosis of a brain tumor is with an imaging test, such as a computed tomography (CT) or magnetic resonance imaging (MRI) scan. In general, MRI is superior to CT, but in many instances a CT scan can accurately reveal the underlying problem. For the most part, imaging is the only test necessary to diagnose a brain tumor.
Neurologists are the specialists who typically diagnose brain tumors. A subspecialty of neurology is neuro-oncology. Neuro-oncologists are doctors who specialize in the treatment of brain tumors; they should be consulted by most patients with brain tumors.
The next step is figuring out what sort of brain tumor a person has. How malignant is it? From what sort of cells did it grow? Answers to these questions are crucial to planning treatment. In many cases, this identification process requires removal of the tumor or a small piece of the tumor for biopsy, which can require surgery. Frequently this can be done with a needle through a small hole in the skull using radiologic coordinates, a procedure known as stereotactic biopsy.
Occasionally, physicians use other tests to plan treatment. For instance, before surgery to remove a meningioma, they may do an angiogram, injecting a dye into the blood vessels to outline the tumor. A functional MRI may reveal how close a tumor is to critical brain areas—for example, those that control language. These examinations enable surgeons to operate aggressively but safely. Doctors may also test a person’s functioning to determine how much harm a tumor is causing; for example, visual-field testing can reveal impairment from tumors pressing on the optic nerve.
Types of Brain Tumors
The most common types of primary brain tumors in adults are gliomas and meningiomas. (Tumors most likely to occur in children are discussed here.) Cures for gliomas are almost nonexistent, unless they are extremely low grade—and these typically evolve to become malignant.
About half of all primary brain tumors are gliomas, meaning that they grow from glial cells. Within the brain, gliomas usually occur in the cerebral hemispheres but may also strike other areas, especially the optic nerve, the brain stem, and—particularly among children—the cerebellum. Gliomas are classified into several groups because there are different kinds of glial cells.
The most common type of glioma is the astrocytoma, named for its star-shaped cells. Tumors of this sort are usually classified in one of four grades, depending on their growth rate and ease of treatment. A grade I astrocytoma is a rare type of tumor called a pilocytic astrocytoma and is usually seen in children. Grade II astrocytomas are also called “well-differentiated,” grade III “anaplastic,” and grade IV “glioblastoma multiforme.” Lower-grade astrocytomas can be treated with surgery, sometimes followed by radiation or chemotherapy. In contrast, glioblastoma multiforme is among the most common and devastating primary brain tumors that strike adults. It grows rapidly, invades nearby tissue, and contains very malignant cells. . It is usually treated with the same methods, but its treatment has a lower success rate.
Among other types of gliomas, ependymomas develop from glial cells that line the hollow cavities of the brain and the canal of the spinal cord. They usually occur in children, and about 85 percent are benign. Treatment usually includes surgery followed by radiation therapy.
Oligodendrogliomas, which make up about 5 percent to 15 percent of all gliomas, occur most often in young adults, within the brain’s cerebral hemispheres. Doctors often treat these tumors with surgery followed by radiation therapy or chemotherapy.
The rarest form of glioma, ganglioneuroma, grows relatively slowly and may arise in the brain or spinal cord. These tumors are usually treated with surgery.
Brain stem gliomas are most common in children and young adults. Surgeons usually cannot treat brain stem gliomas because of their vulnerable location. Radiation therapy sometimes helps to reduce symptoms and improve survival by slowing tumor growth.
Optic nerve gliomas are found on or near the nerves that travel between the eye and the brain’s vision centers. They are particularly common in individuals who have neurofibromatosis and are usually treated with surgery or radiation.
Mixed gliomas contain more than one type of glial cell, usually astrocytes and other glial cells. Treatment focuses on the most malignant type of cell found within the tumor.
Meningiomas develop from the thin membranes, or meninges, that cover the brain and spinal cord. They account for about 15 percent of all brain tumors. They affect people of all ages, but are most common among those over 40. Meningiomas usually grow slowly, generally do not invade surrounding normal tissue, and rarely spread to other parts of the body or central nervous system. Surgery is the preferred treatment for accessible meningiomas and is more successful for these tumors than for most others.
Other Types of Brain Tumors
Tumors involving tissue from the pineal and pituitary glands are often treated by surgery, and some pituitary tumors may be treated with drugs, such as cabergoline.
Primitive neuroectodermal tumors (PNETs) usually affect children and young adults. PNETs are very malignant, and because their cells spread in a scattered pattern, they are difficult to remove through surgery. Thus, surgery is followed in most cases by high doses of radiation and chemotherapy.
Schwannomas are tumors that arise from the cells that form a protective sheath around nerve fibers. Usually benign, they are removed surgically when possible. Vascular tumors are rare, noncancerous tumors that arise from the blood vessels of the brain and spinal cord. The most common vascular tumor is the hemangioblastoma, which rarely spreads and is usually treated by surgery.
As noted above, treatment varies from one tumor type to the next but often involves a combination of surgery, radiotherapy, and chemotherapy. Early treatment improves the chance of a good outcome for some types.
Some tumors, such as meningiomas, can be completely cured by surgical removal. Others can be better controlled for a time if the surgeon removes all of the area that appears diseased on an MRI. This is true for almost all of the gliomas, but these tumors are rarely cured by surgery alone. Even low-grade gliomas cannot be cured by surgery alone because individual cells migrate great distances within the brain tissue, and a surgeon cannot remove large portions of the brain without causing unacceptable damage.
Depending on the type of tumor, therefore, oncologists may recommend radiation after surgery. For most tumor types, radiation is administered only to the involved area of the brain. Occasionally, the entire brain must be irradiated because there are multiple tumors. Radiation often controls tumor growth for a period and can occasionally cure tumors. The short-term side effects are nausea and vomiting; in the long term, there may be cognitive dysfunction. Newer technologies, such as stereotactic radiosurgery, employ highly focused beams of radiation, allowing treatment to be delivered to the tumor but sparing normal surrounding brain tissue. Radiosurgery can be accomplished by several techniques, including the gamma knife, cyber knife, linear accelerator, or proton techniques. These techniques are fairly equivalent, although proton therapy—which makes use of small nuclear particles instead of electromagnetic radiation—is the most focused technique and is being employed primarily for the treatment of pediatric tumors. Stereotactic radiosurgery is particularly useful for the treatment of tumors such as meningiomas and brain metastases. It is less useful and rarely employed for the treatment of gliomas, which typically extend beyond the visible margins seen on neuroimaging.
Chemotherapy, or drug treatment, is given to improve survival or, in the case of lymphoma, cure the tumor. However, drug treatments for brain tumors can be hampered by the blood-brain barrier—the combination of physical and functional characteristics that prevent most chemicals in the bloodstream from reaching the brain. This is a normal protective mechanism that prevents many toxic substances from harming our brains. Fortunately, there are several chemotherapeutic agents whose normal properties allow them to penetrate the barrier, including temozolomide, which is taken by mouth and is particularly useful for the treatment of malignant gliomas. In addition, many tumors have an abnormal blood-brain barrier, allowing agents to enter them but not to reach areas of normal brain. These characteristics permit us to use chemotherapy to treat some brain tumors. Other options include implanting chemotherapy-impregnated wafers, such as Gliadel, which slowly releases the drug carmustine into the surrounding brain tumor after a surgeon has removed as much as safely possible. This technique is used as part of the initial treatment of glioblastoma and has been shown to slightly increase survival, but it has also been linked to difficulties in wound healing and can cause local tissue injury. Other new therapeutic approaches include the use of agents that block the growth of new blood vessels into the tumor, particularly bevacizumab, an antibody that targets the vascular endothelial growth factor (VEGF), a protein that assists in cell growth. This is effective in patients with recurrent malignant glioma, achieving a response in about 60% of patients and prolonging survival a median of about eight months.
A number of medications are available to treat the symptoms of brain tumors. For seizures, there are a variety of anticonvulsants; some people require more than one. Glucocorticoids (steroids) help reduce the swelling, or edema, around brain tumors. This often results in improved function within one or two days. Such steroids as dexamethasone (Decadron) are very beneficial, but their prolonged use often leads to side effects: weight gain, insomnia, muscle weakness, and diabetes.
Research on genetic abnormalities in brain tumors has identified persistent activation of growth factor receptors in brain gliomas. There also appears to be activation of some oncogenes. Scientists have also found a surprising amount of genetic variability in glioblastomas, with specific mutations occurring infrequently. Instead, changes seem to cluster in certain pathways related to cell growth and development, including pathways that affect the behavior of nearby cells for the tumor’s benefit. This has led to the development of experimental models of brain gliomas, which should hasten the development of new therapies.
Over the years scientists and doctors have found many ways to improve tumor treatments, resulting in longer and healthier lives for many people. Continuing research into the unique traits of brain tumors will lead us to more answers and improved treatments for the thousands of people facing this frightening disease.
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