External beam radiation is the most widely used type of radiation therapy. The radiation comes from a machine outside the body and is focused on the cancer. It is much like getting an x-ray, but for longer. This type of radiation is most often given by machines called linear accelerators (linacs).
The radiation is aimed at the tumor, but it also affects the normal tissue it passes through on its way into and out of the body. External beam radiation can be used to treat large areas of the body. It also can treat more than one area, such as the main tumor and nearby lymph nodes. External radiation is usually given daily over several weeks. It is given in an outpatient clinic or treatment center, so you do not have to stay in the hospital.
Types of external beam radiation
Three-dimensional conformal radiation therapy (3D-CRT) uses imaging scan pictures and special computers to very precisely map the location of a tumor in 3 dimensions. The patient is fitted with a plastic mold or cast to keep the body part still during treatment. The radiation beams are matched to the shape of the tumor and delivered to the tumor from several directions. By aiming the radiation more precisely, it may be possible to reduce radiation damage to normal tissues and better fight the cancer by increasing the radiation dose to the cancer. A drawback of 3D-CRT is that it can be hard to see the full extent of some tumors on imaging tests, and any part not seen will not get treated with this therapy.
Intensity modulated radiation therapy (IMRT) is an advanced form of external radiation therapy that uses photon beams. As with 3D-CRT, computer programs are used to precisely map the tumor in 3 dimensions. But along with aiming photon beams from several directions, the intensity (strength) of the beams can be adjusted. This gives even more control over the dose, decreasing the radiation reaching sensitive normal tissues while delivering higher doses to the tumor. Because of its precision, it is even more important that a person remain in the right place and be perfectly still during treatment. A special cast or mold may be made to keep the body in place during treatment. Again, miscalculations in tumor size and exact location can mean missed areas will not get treated. IMRT is available mainly in major cancer centers. Because IMRT uses a higher total dose of radiation, it may slightly increase the risk of second cancers later on. This is something researchers are looking into.
A related technique, intensity modulated proton therapy (IMPT), uses proton beams instead of x-rays. Protons are parts of atoms that cause little damage to tissues they pass through but are very good at killing cells at the end of their path. Also, very little of the dose goes beyond the tumor at the end of the beam’s path. This means that proton beam radiation may be able to deliver more radiation to the cancer while reducing side effects on nearby normal tissues. It is a good choice for tumors near critical body structures, such as the eye, the brain, and the spine. Protons can only be put out by a special machine called a cyclotron or synchrotron. This machine costs millions of dollars and requires expert staff. Because of this, proton beam therapy is expensive, and very few treatment centers in the United States offer it. More studies are needed to compare outcomes between proton and photon treatment so that each is used for the cancer type for which it works best.
Stereotactic radiosurgery or stereotactic radiation therapy delivers a large, precise dose of radiation to a small, well-defined tumor. The term “surgery” may be confusing because no cutting is involved. Some tumors that start in or spread to the brain are treated with this technique, but it is used for tumors in other places, too, such as the spine, liver, pancreas, kidney, lung, and prostate.
In some cases, a head frame or shell is used to hold the skull still and allow for precise aiming of radiation beams.
Once the exact location of the tumor is mapped (using imaging scans), narrow radiation beams from a machine called a Gamma Knife® are focused at the tumor from hundreds of different angles for a short time. The process may be repeated if needed. Another approach that is much like this uses a movable linear accelerator controlled by a computer. Instead of delivering many beams at once, the linear accelerator moves around to deliver radiation to the tumor from different angles. Several machines, with names such as X-Knife®, CyberKnife®, and Clinac® work in this way.
Intraoperative radiation therapy (IORT) delivers radiation to the cancer during surgery. The radiation may be given externally or internally, and it is often used along with a course of external radiation given before or after the operation.
IORT is useful for abdominal (belly area) or pelvic cancers that cannot be completely removed (such as those that have grown close to vital body parts) and for cancers that tend to grow back after treatment. As much tumor is removed as possible, then one large dose of radiation is directed right at the tumor site. Normal tissues can be moved out of the way and protected, so IORT reduces the amount of tissue that is exposed to radiation. This allows a higher dose of radiation to reach the cancer. IORT is delivered in a special operating room lined with radiation-shielding walls.
Treatment planning for external beam radiation
The process of planning external beam radiation therapy has many steps and may take several days to complete. But it is important for the success of your radiation treatment. The radiation team will design a treatment just for you. The treatment will give the strongest dose of radiation to the cancer but will spare normal tissue as much as possible.
The first part of treatment planning is called simulation. It is sometimes referred to as a “marking session.” You will be asked to lie still on a table while the health care team works out the best treatment position for you and how to keep you in that position (tape, headrests, casts, body molds, or foam pillows may be used). They will then mark the radiation field (also called the treatment port), which is the exact place on your body where the radiation will be aimed. The marks may be done with permanent markers or with tattoos that look like tiny freckles. Your doctor may use imaging tests to check the size of the tumor, figure out where it is more likely to spread, outline normal tissues in the treatment area, take measurements, and develop the treatment plan. Photos may also be taken and are used to make the daily treatment set-up easier.
Through a complex process called dosimetry, computer programs are used to find out how much radiation the nearby normal structures would be exposed to in order to deliver the prescribed dose to the cancer. The doctor and dosimetrist will work together to decide on the amount of radiation you will get and the best ways to aim it at the cancer. They base this on the size of the tumor, how sensitive the tumor is to radiation, and how well the normal tissue in the area can withstand the radiation.
Dosing and treatment with external beam radiation
The total amount of radiation you will get is measured in units called Gray (Gy). Often the dose is expressed in centigray (cGy), which is one-hundredth of a Gray.
For external radiation, the total dose is often divided into several smaller doses (called fractions) that are most often given over a number of weeks. This allows the best dose to be given with the least damage to normal tissues. In most cases, treatments are given 5 days a week, for about 5 to 8 weeks.
For some cancers, patients may be treated more often than once a day.
- Hyperfractionated radiation divides the daily dose into 2 treatment sessions without changing the length of the treatment. In this case, you would be treated twice a day for several weeks.
- Accelerated radiation gives the total dose of radiation over a shorter period of time by giving the same dose of radiation more frequently (more than once a day).
These types of schedules can make the radiation work better for some tumors. The down side is that radiation side effects are seen earlier and may be worse.
It is important that you are treated in exactly the same way each time so the right amount of radiation will be given to the right area. For external radiation, small, long-lasting or permanent (tattoo) marks may be put on your skin to show where treatment is to be focused. You will need to stay very still and in the same position during each treatment, which can last up to 30 minutes. Sometimes a special mold or cast of the body part to be treated will be used to hold you in a certain position. This helps make sure you’re in the right place and helps you stay still. Your health care team may also need to make special blocks or shields to protect certain parts of your body from radiation during treatment.
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