Radiation Therapy According To Cancer Types
Brain, nervous system
Radiation Therapy for Adult Brain and Spinal Cord Tumours
Radiation therapy uses high-energy rays or small 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 tumour cells
- As the main treatment if Surgery is not a good option and medicines are not effective
- To help prevent or relieve symptoms from the tumour
Types of radiation therapy
Most often, the radiation is focused on the tumour 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 determine the correct angles for aiming the radiation beams and the proper dose of radiation. This planning session, called simulation, usually includes getting imaging tests such as CT or MRI scans.
In most cases, the total dose of radiation is divided into daily amounts (usually given Monday through Friday) over several weeks. At each treatment session, you lie on a special table while a machine delivers the radiation from precise angles. The treatment is not painful. Each session lasts about 15 to 30 minutes, and 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 the radiation to the tumour while giving the lowest possible dose to normal surrounding brain areas. Several techniques can 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 tumour precisely. Several radiation beams are then shaped and aimed at the tumour from different directions. Each beam alone is fairly weak, which makes it less likely to damage normal tissues, but the beams converge at the tumour 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 moves around the patient as it delivers radiation. Along with shaping the beams and aiming them at the tumour from several angles, the intensity (strength) of the beams can be adjusted to limit the dose reaching the most sensitive normal tissues. This may let the doctor deliver a higher dose to the tumour. Many major hospitals and cancer centres now use IMRT.
Volumetric modulated arc therapy (VMAT): This newer technique is similar to IMRT. For this treatment, the patient lies on a table, which passes through the machine delivering the radiation. The source of the radiation (the linear accelerator) rotates around the table in an arc, delivering the beams from different angles. A computer controls the intensity of the beams to help keep the radiation focused on the tumour. It’s not yet clear if this approach results in better outcomes than IMRT, although it does allow the radiation to be given over less time in each treatment session.
Conformal proton beam radiation therapy: Proton beam therapy uses an approach similar to 3D-CRT. But instead of using x-rays, it focuses proton beams on the tumour. 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. This lets doctors deliver more radiation to the tumour and do less damage to nearby normal tissues.
This approach may be more helpful for brain tumours that have distinct edges (such as chordomas), but it is not clear if it will be useful for tumours that typically grow into or mix with normal brain tissue (such as astrocytomas or glioblastomas). There are a limited number of proton beam centres in the United States at this time.
Stereotactic radiosurgery (SRS)/stereotactic Radiation therapy (SRT): This type of treatment delivers a large, precise radiation dose to the tumour area in a single session (SRS) or in a few sessions (SRT). (There is no actual Surgery in this treatment.) It may be used for some tumours in parts of the brain or spinal cord that cannot be treated with Surgery or when a patient is not healthy enough for Surgery.
A head frame might be attached to the skull to help aim the radiation beams. (Sometimes a facemask is used to hold the head in place instead.) Once the exact location of the tumour is known from CT or MRI scans, radiation is focused at the tumour from many different angles. This can be done in 2 ways:
- In one approach, thin radiation beams are focused at the tumour from hundreds of different angles for a short period of time. Each beam alone is weak, but they all converge at the tumour to give a higher dose of radiation. An example of a machine that uses this technique 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 tumour from many different angles. Several machines with names such as X-Knife, CyberKnife, and Clinac deliver Stereotactic Radiosurgery in this way.
SRS typically delivers the whole radiation dose in a single session, though it may be repeated if needed. For SRT (sometimes called fractionated radiosurgery), doctors give the radiation in several treatments to deliver the same or a slightly higher dose. Frameless techniques are now available to make this more comfortable.
Image-guided Radiation therapy (IGRT): For IGRT, an imaging test such as a CT scan is done just before each treatment to help better guide the radiation to its target. IGRT is typically used along with some of the more precise techniques for delivering radiation described above. It is most useful when the radiation needs to be delivered very precisely, such as when a tumour is very close to vital structures.
Brachytherapy (internal radiation therapy): Unlike the external radiation approaches above, brachytherapy involves inserting radioactive material directly into or near the tumour. The radiation it gives off travels a very short distance, so it affects only the tumour. This technique is most often used along with external radiation. It provides a high dose of radiation at the tumour 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 tumour has spread along the covering of the spinal cord (meninges) or into the surrounding cerebrospinal fluid, radiation may be given to the whole brain and spinal cord. Some tumours 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 tumour cells than it is to normal cells. Still, radiation can also damage normal brain tissue, which can lead to side effects.
Side effects during or soon after treatment: Some people become irritable and tired during the course of Radiation therapy. Nausea, vomiting, and headaches are also possible side effects but are uncommon. Sometimes dexamethasone (a corticosteroid) or other drugs can help relieve these symptoms. Some people might have Hair loss in areas of the scalp that get radiation. Other side effects are also possible, depending on where the radiation is aimed.
Problems with thinking and memory: A person may lose some brain function if large areas of the brain get 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 tumour.
Radiation necrosis: Rarely after radiation therapy, a mass of dead (necrotic) tissue forms at the site of the tumour in the months or years after radiation treatment. This can often be controlled with corticosteroid drugs, but Surgery may be needed to remove the necrotic tissue in some instances.
Increased risk of another tumour: 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 tumour, or less likely a bone cancer in the skull. If this develops, it is usually many years after the radiation is given. This small risk should not prevent those who need radiation from getting treatment.