Radiation Therapy Principles

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External beam radiation

External beam radiation is the most widely used type of radiation therapy, and it most often uses photon beams. The radiation comes from a machine outside the body and is focused on the cancer. It’s a lot like getting an x-ray, but for longer. This type of radiation is most often given by machines called linear accelerators (linacs).

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’s given in an outpatient clinic or treatment center, so you don’t have to stay in the hospital. The radiation is aimed at the cancer, but in most cases it affects the normal tissue it passes through on its way into and out of the body. (Intensity modulated proton therapy works differently, but is not used very often. See the next section for more information).

Special ways to deliver external beam radiation

Three-dimensional conformal radiation therapy (3D-CRT)

This technique uses imaging scan pictures and special computers to map the location of a tumor very precisely 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. Careful aiming of the radiation beam may help reduce radiation damage to normal tissues and better fight the cancer by increasing the radiation dose to the tumor. Photon beams or particles (like protons) can be used in this way. 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)

This is an advanced form of external radiation therapy. 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.

A variation of IMRT is called volumetric modulated arc therapy. It uses a machine (called RapidArc®) that delivers the radiation quickly as it rotates once around the body. This allows each treatment to be given over just a few minutes. Although this can be more convenient for the patient, it’s not yet clear if it’s more effective than regular IMRT.

Because of its precision, it’s even more important that a person remain in the right position 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.

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.

Image-guided radiation therapy (IGRT) is an option on some newer radiation machines that have imaging scanners built into them. This advance lets the doctor take pictures of the tumor and make minor aiming adjustments just before giving the radiation. This may help deliver the radiation even more precisely. It might result in fewer side effects, but more research is needed to prove this.

Intensity modulated proton therapy (IMPT) is IMRT using proton beams instead of photon beams. Protons are parts of atoms that in theory can deliver radiation to the area that they are aimed at (like the cancer), while doing less damage to nearby normal tissues. Still, there have been no studies showing that proton beam radiation is better than the more common photon beam in terms of cancer outcomes or side effects. In fact, a 2012 study of proton beam therapy used to treat localized prostate cancer did not show fewer side effects compared to the more common photon beam radiation. More study on this is needed. Meanwhile, IMPT is often used for tumors near critical body structures such as the eye, the brain, and the spine.

Protons can only be sent out by a special machine called a cyclotron or synchrotron. This machine costs millions of dollars and requires expert staff to use and maintain it. Because of this, proton beam therapy is expensive, and very few treatment centers in the United States offer it. Many 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 (SRS) and fractionated stereotactic radiotherapy

These use advanced image-guided techniques to deliver a large, precise dose of radiation to a small, well-defined tumor. The term “surgery” may be confusing because no cutting is involved. This technique is used to treat tumors that start in or spread to the brain or head and neck region. If the radiation is given as a single dose, it’s called stereotactic radiosurgery. If the radiation is spread out over several doses, it’s called fractionated stereotactic radiotherapy.

When the radiation is aimed at the head, a frame or shell is used to hold the head still and allow for precise aiming of radiation beams.

A related term, stereotactic body radiation therapy (SBRT), is used to describe this technique when it’s used for tumors in other parts of the body, for instance, the spine, liver, pancreas, kidney, lung, and prostate.

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’s 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 like X-Knife®, CyberKnife®, and Clinac® work in this way.

Intraoperative radiation therapy (IORT)

With this technique, radiation is given during surgery. The radiation may be given using a machine for external beam radiation (a linear accelerator). Another option is to put a radioactive substance into the treatment area for a short time (like brachytherapy). IORT is often used along with a course of external radiation given before or after the operation.

IORT is useful for cancers that are deep inside the body, because normal tissues can be moved aside during surgery, exposing the cancer. After as much tumor is removed as possible, one large dose of radiation is directed straight at the cancer without going through normal tissues. Shielding can also be used to further protect the nearby normal tissues. IORT is given in a special operating room lined with radiation-shielding walls.

IORT is most often used 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. This technique is not widely available.

Electromagnetic-guided radiation therapy

This is another way of aiming the radiation beam that can be used with 3D and IMRT. It uses tiny electromagnetic implants (called transponders) that are placed into the area being treated. These implants send out radio waves to tell the radiation therapy machines where to aim. This lets the machine compensate for movement (like during breathing) and may help keep some of the radiation from going to normal tissues. It also helps to refocus radiation beams as organs shift or cancer shrinks over time. It’s sometimes known as 4-D therapy, because it includes time in the radiation planning formula. One such system is marketed under the brand name Calypso®. In theory, better focusing radiation could lower side effects. So far, though, studies have not found this type of radiation to be better for patients than other approaches.

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’s a key part of successful radiation treatment. The radiation team will design a treatment just for you. The treatment will give the strongest dose of radiation to the cancer while sparing normal tissue as much as possible.

The first part of treatment planning is called simulation. It’s sometimes referred to as a “marking session.” You’ll 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. If you don’t want to be tattooed, ask beforehand how your radiation marking will be done and what your options are.

Your doctor may use imaging tests to check the size of the tumor, figure out where it’s most likely to have spread, outline normal tissues in the treatment area, take measurements, and plan your treatment. 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 if the prescribed dose were delivered to the cancer. The doctor and dosimetrist will work together to decide on the amount of radiation you need to 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’ll 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 smaller doses (called fractions) that are typically given over a number of weeks. This allows the best dose to be given with the least damage to normal tissues. Treatments are usually given 5 days a week, for about 5 to 8 weeks.

Some cancers 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. In other words, giving more frequent doses (more than once a day) to get the same total dose of radiation; it may shorten the course of treatment by a week or two.
  • Hypofractionated radiation breaks radiation into fewer doses, so that each dose is larger. Sometimes, this could mean it’s given less often 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, even though it doesn’t increase the radiation’s late effects.

It’s important that you are in the correct position each time so the right amount of radiation will be given to the right area. The marks on your skin will show where treatment is to be focused. You’ll 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.


Last Medical Review: 10/27/2014
Last Revised: 10/27/2014