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Gliomas and Astrocytomas

Glial cells, the most common cells in the brain, make up the brain's supportive tissue and are the main source of central nervous system tumors. Tumors that arise from these cells are called gliomas. The most common kind of glioma is called an astrocytoma because it arises from a kind of glial cell called an astrocyte. Half of all primary brain tumors are glial cell tumors, and three quarters of gliomas are astrocytomas.

Symptoms

The initial symptoms of brain tumors, such as headache and nausea, usually are the result of increased intracranial pressure caused by the bulk of the tumor or a backup of the cerebrospinal fluid that surrounds the brain and spinal cord. The glial cells are widely distributed throughout the central nervous system, so these tumors can occur in a wide variety of locations, and therefore can cause a wide variety of other symptoms. Depending on the location of the mass, gliomas may cause seizures, weakness or numbness in the limbs, impairments in language function, blurred or double vision, gradual changes in mood or personality, and memory loss.

Diagnosis

Imaging studies are the key component in the diagnosis of gliomas. Currently, magnetic resonance imaging (MRI) is the best available imaging modality. Computed tomography (CT) scans also are used. For either study, an agent that provides contrast in the image is administered intravenously so neurological surgeons can visualize the tumor against the normal brain in the background. In some cases, neurological surgeons may employ an MRI scan with frameless stereotactic guidance. For this study, a contrast MRI is performed after special markers (called fiducials) are placed on the patient's scalp. The fiducials are processed by a computer, which calculates the location of the tumor and creates a three-dimensional reconstruction. This image then is used at the time of surgery to help locate the tumor precisely, maximize tumor removal, and minimize injury to the surrounding brain.

Treatment

Surgery for gliomas involves the resection of the tumor to decrease the pressure it exerts. For most gliomas, however, surgery will not provide a cure by itself. When a tumor is removed, it can be examined under a microscope to provide an accurate diagnosis so the next steps in treatment, which may include radiation therapy or chemotherapy, can be determined. In addition, some smaller tumors may be treated effectively with stereotactic radiosurgery, which involves the use of a highly focused beam of radiation to target the cancer cells specifically and leave the surrounding brain unaffected. The choice of treatment usually is made based on the grade of the tumor, which is a measure of the tumor's malignancy.

Types

Low-Grade Astrocytomas
Low-grade astrocytomas are slow-growing tumors; their symptoms develop over an extended period of time. Representing approximately 10 to 15 percent of all gliomas, they generally are found in young patients and have a more favorable prognosis. Unfortunately, low-grade astrocytomas also occur less frequently than their malignant counterparts.

Surgery is the treatment of choice, and the primary goal of the procedure is to remove the mass effect caused by the tumor. Post-operative radiation is sometimes indicated in cases of incomplete removal of ordinary low-grade astrocytomas. These tumors may also be amenable to stereotactic radiosurgery as well. Not all of the cells in a low-grade astrocytoma are the same, so half of these low-grade tumors eventually will evolve into higher-grade ones. This evolution to more malignant forms is the most common cause of death in patients with low-grade astrocytomas.

High-Grade Astrocytomas
There are two primary forms of high-grade (malignant) astrocytomas, anaplastic astrocytomas and glioblastoma multiforme (GBM). Both varieties are more common than low-grade astrocytomas. GBMs, the most malignant astrocytomas, are the most common adult primary brain tumors, and make up approximately half of all astrocytomas. Anaplastic astrocytomas comprise approximately 30 percent of all astrocytomas. High-grade astrocytomas can occur at any age, but they are more common in older patients.

In addition to the symptoms common to all brain tumors, high-grade astrocytomas are characterized by seizures and specific neurological deficits. GBMs may be large before they begin to cause symptoms. The tumors are diagnosed with imaging studies and tumor biopsy. In a biopsy, GBM can be characterized by the presence of necrosis, or cell death, which is not present in anaplastic astrocytomas.

High-grade astrocytomas require aggressive, intense therapy. Surgery again is the primary form of treatment, but its utility is limited because these tumors tend to penetrate healthy brain tissue, making complete resection unlikely and recurrence almost certain. As a result, surgery almost always is followed by radiation therapy, and then chemotherapy. Stereotactic radiosurgery also may be used in cases of tumor recurrence or to target areas missed during the surgical resection. New therapies, such as gene therapy and new chemotherapy drugs, are being examined. The prognosis for patients with malignant astrocytomas is not good.

Oligodendrogliomas
Oligodendrogliomas are another type of glial cell tumor. They arise from oligodendrocytes, glial cells that form insulation around neurons that helps them conduct electrical impulses. These uncommon, usually slow-growing, tumors occur most often in middle-aged patients.

The symptoms of oligodendrogliomas are similar to those of other gliomas, but seizures in particular are the most common presenting symptom. And like other brain tumors, these tumors are diagnosed with radiological imaging. Ninety percent of these tumors show some degree of hardening or calcification.

Surgery again is the most common initial treatment, but it is especially important in the treatment of oligodendrogliomas because the benefits of radiation on these masses is not clear. As always, more complete tumor resection is associated with better outcomes. Recently, the role of chemotherapy as a viable treatment for some oligodendrogliomas has been established.

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