Radiation therapy uses high-energy rays or particles to kill cancer cells. This type of treatment is given by a doctor called a radiation oncologist. Radiation therapy may be used in different situations:
- After surgery to try to kill any remaining tumor cells
- As the main treatment if surgical removal is not a good option and medicines are not effective
- To help prevent or relieve symptoms, especially for spinal cord tumors
Types of radiation therapy
In most cases, the radiation is focused precisely on the tumor from a source outside the body. This is called external beam radiation therapy (EBRT). This type of radiation therapy is much like getting an x-ray, but the dose of radiation is much higher.
Before your treatments start, the radiation team will measure carefully to determine the correct angles for aiming the radiation beams and the proper dose of radiation. In most cases, the total dose of radiation is divided into daily fractions (usually given Monday through Friday) over several weeks. At each session, you lie on a special table while a machine delivers the radiation from a precise angle. The treatment is not painful. Each session lasts about 15 to 30 minutes. Much of that time is spent making sure the radiation is aimed correctly. The actual treatment time each day is much shorter.
High doses of radiation therapy can damage normal brain tissue, so doctors try to deliver high doses of radiation to the tumor with the lowest possible dose to normal surrounding brain areas. Several newer techniques help doctors focus the radiation more precisely:
Three-dimensional conformal radiation therapy (3D-CRT): 3D-CRT uses the results of imaging tests such as MRI and special computers to map the location of the tumor precisely. Several radiation beams are then shaped and aimed at the tumor from different directions. Each beam alone is fairly weak, which makes it less likely to damage normal tissues, but the beams converge at the tumor to give a higher dose of radiation there.
Intensity modulated radiation therapy (IMRT): IMRT is an advanced form of 3D therapy. It uses a computer-driven machine that actually moves around the patient as it delivers radiation. In addition to shaping the beams and aiming them at the tumor from several angles, the intensity (strength) of the beams can be adjusted to minimize the dose reaching the most sensitive normal tissues. This may let the doctor deliver a higher dose to the tumor. Many major hospitals and cancer centers now use IMRT.
Conformal proton beam radiation therapy: Proton beam therapy is related to 3D-CRT and uses a similar approach. But instead of using x-rays, it focuses proton beams on the tumor. Protons are positive parts of atoms. Unlike x-rays, which release energy both before and after they hit their target, protons cause little damage to tissues they pass through and then release their energy after traveling a certain distance. Doctors can use this property to deliver more radiation to the tumor and do less damage to nearby normal tissues. This approach may be more helpful for brain tumors that have distinct edges, but it is not clear if this approach will be useful with tumors that are infiltrative or mixed with normal brain tissue. The machines needed to make protons are expensive, and there are only a handful of proton beam centers in the United States at this time.
Stereotactic radiosurgery/stereotactic radiotherapy: This type of treatment delivers a large, precise radiation dose to the tumor area in a single session (radiosurgery) or in a few sessions (radiotherapy). (There is no actual surgery involved in this treatment.) It may be useful for some tumors in parts of the brain or spinal cord that can't be treated with surgery or when a patient's health does not permit surgery.
First, a head frame is attached to the skull to help precisely aim the radiation beams. Once the exact location of the tumor is known from CT or MRI scans, radiation may be delivered in one of two ways.
In one approach, radiation beams from a machine are focused at the tumor from hundreds of different angles for a short period of time. An example of such a machine is the Gamma Knife.
Another approach uses a movable linear accelerator (a machine that creates radiation) that is controlled by a computer. Instead of delivering many beams at once, this machine moves around the head to deliver radiation to the tumor from different angles. Several machines with names such as X-Knife, CyberKnife, and Clinac, deliver stereotactic radiosurgery in this way.
Stereotactic radiosurgery typically delivers the whole radiation dose in a single session, though it may be repeated if needed. Sometimes doctors give the radiation in several treatments to deliver the same or a slightly higher dose. This is called fractionated radiosurgery or stereotactic radiotherapy.
Brachytherapy (interstitial radiotherapy): Unlike the external radiation approaches above, brachytherapy involves inserting radioactive material directly into or near the tumor. The radiation given off travels a very short distance, so it affects only the tumor. This technique is most often used along with external radiation. It provides a high dose of radiation at the tumor site, while the external radiation treats nearby areas with a lower dose.
Whole brain and spinal cord radiation therapy (craniospinal radiation): If tests like an MRI scan or lumbar puncture find the tumor has spread along the spinal cord meninges, or into the surrounding fluid, then radiation may be given to the whole brain and spinal cord. Some tumors such as ependymomas and medulloblastomas are more likely to spread this way and often require craniospinal radiation.
Possible side effects of radiation therapy
Radiation is more harmful to tumor cells than it is to normal cells. Still, normal brain tissue is also damaged by radiation.
Some people may become irritable and fatigued during the course of radiation therapy. Nausea, vomiting, and headaches are also possible but are uncommon. Sometimes dexamethasone (Decadron), a cortisone-like drug, can help relieve these symptoms.
A person may lose some brain function if large areas of the brain receive radiation. Problems can include memory loss, personality changes, and trouble concentrating. There may also be other symptoms depending on the area of brain treated and how much radiation was given. These risks must be balanced against the risks of not using radiation and having less control of the tumor.
Rarely after radiation therapy, a large mass of dead (necrotic) tissue forms at the site of the tumor. This occurs months to years after radiation is given and is called radiation necrosis. Though this can often be controlled with corticosteroid drugs, surgery may be needed to remove the necrotic tissue in some instances.
Radiation can damage genes in normal cells. As a result, there is a small risk of developing a second cancer in an area that got radiation — for example, a meningioma of the coverings of the brain, another brain tumor, or less likely a bone cancer in the skull. If this does occur, it is usually many years after the radiation is given. This small risk should not prevent those who need radiation from getting treatment.
For more information on radiation therapy, see our document, Understanding Radiation Therapy: A Guide for Patients and Families.
Feedback
