ACS :: External Radiation Therapy

Choosing the type of radiation to use depends on the type of cancer you have and where it is in your body.

After a physical exam and a review of your medical history and test results, the doctor will pinpoint the area to be treated. This is done a few days before starting radiation therapy in a process called simulation. You will be asked to lie still on a table while the radiation therapist uses a special x-ray machine to define your treatment field (also called treatment port). These are the exact places on your body where the radiation beams will be aimed.

To be sure the radiation beams are aimed correctly, special molds or casts of parts of your body may be made to help you be still during your treatment. They will also help get you in the same position for future treatments. The radiation therapist may mark the treatment field with freckle-sized dots of semi-permanent ink. The marks will likely fade away over time, but they are needed until your treatment is finished. Don’t use soap on or scrub these marks. Sometimes the area may be marked with permanent dots like a tattoo. (These can later be removed with a laser if you like.)

Based on the simulation, other tests, and your medical diagnosis, the doctor will decide how much radiation is needed, how it will be given, and how many treatments you should have.

External radiation therapy is usually given with a machine called a linear accelerator (called linac for short), which is described below.

In most cases the total dose of radiation needed to kill a tumor can’t be given all at once. This is because it is very hard to give radiation therapy without harming some normal cells nearby. A dose of radiation given all at once can cause more damage to normal tissues, and as a result more side effects, than giving the same dose over many treatments.

This is why the total dose of external radiation therapy is usually divided into smaller doses called fractions. The most common way to give it is daily, 5 days a week (Monday through Friday) for 5 to 8 weeks. Weekend rest breaks allow time for normal cells to recover. The total dose of radiation and the number of treatments can vary, based on:

Other radiation schedules may be used in certain cases. For example, radiation therapy may last only a few weeks (or less) when it’s being given to relieve symptoms, because the overall dose of radiation is lower. In some cases, radiation might be given as 2 or more treatments each day (called hyperfractionated radiation therapy). Or it might be given as split-course therapy, which allows for several weeks off in the middle of treatments to allow the body time to recover while the cancer shrinks.

External radiation treatment is painless. It is much like having a regular x-ray. The treatment itself takes only a few minutes. But each session can last 15 to 30 minutes because of the time it takes to set up the equipment and place you in the correct position.

Depending on the area being treated, you may need to undress, so wear clothes that are easy to take on and off. You will be asked to lie on a treatment table next to the radiation machine (called a linear accelerator or linac). The machine has an arm that extends over the table. The radiation comes out of this arm. The machine can move around the table to change the angle of the radiation, if needed.

The radiation therapist may put special heavy shields between the machine and parts of your body that are not being treated to help protect normal tissues and organs.

Once you are in the correct position, the radiation therapist will go into a nearby room to operate the machine and watch you on a TV monitor. You will be able to talk with the therapist over an intercom. You will be asked to lie still during the treatment. You do not have to hold your breath -- just breathe normally.

The radiation therapy machine will make clicking and whirring noises and may sometimes sound like a vacuum cleaner as it moves to aim the radiation beam from different angles. The radiation therapist controls the movement and checks to be sure it is working properly. If you are concerned about anything that happens in the treatment room, ask the therapist to explain. If you feel ill or uncomfortable during the treatment, tell the therapist right away. The machine can be stopped at any time.

External radiation therapy affects cells in your body only for a moment. Because there is no radiation source in your body, you are not radioactive at any time during or after treatment.

Today, scientists have developed newer, more precise ways of giving external radiation therapy. These approaches allow doctors to focus the radiation more directly on the tumors.

These newer forms of radiation do less damage to normal tissues, and this allows doctors to use higher doses directed only at the tumors. These methods though are still fairly new, and their long-term effects are still being studied.

Newer machines allow doctors to shape (conform) the radiation beam to match the shape of the tumor. With conformal radiation, a special computer uses imaging scans (such as CT scans) to map the location of the cancer in the body in 3 dimensions (3 dimensional or 3-D). Radiation beams can then be shaped (or conformed) to match the shape of the cancer. This helps to better protect nearby tissues and the parts of the body in between the radiation beam and the cancer.

Three-dimensional conformal radiation therapy (3D-CRT) delivers shaped beams at the cancer from different directions. Patients are fitted with a mold or cast to keep the body part still so the radiation can be aimed more accurately. 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 tumor.

Intensity modulated radiation therapy (IMRT) is a newer method much like 3D-CRT. It conforms to the tumor shape like 3D-CRT, but also allows the strength of the beams to be changed in some areas to lessen damage to normal body tissues. This provides even more control in reducing the radiation reaching normal tissue while getting a higher dose to the tumor. It may result in even fewer side effects.

A newer form of IMRT, called helical tomotherapy, uses a linear accelerator inside a large "donut" that spirals around the body while you lie on the treatment table. It can deliver radiation from many different angles around the body. This may allow for even more precisely focused radiation.

Conformal proton beam radiation therapy is much like conformal therapy but it 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. This means that proton beam radiation may be able to deliver more radiation to the tumor while reducing side effects on nearby normal tissues. 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. This is why proton beam therapy can be expensive and it is not yet found in many radiation treatment centers. More studies are needed to determine whether proton radiation produces better results in specific cancers than the usual photon radiation treatment.

Intraoperative radiation therapy (IORT) is external radiation given directly to the tumor or tumors during surgery. It may be used if the tumors cannot be totally removed or if there is a high chance of the tumor coming back in the same area. While the patient is under anesthesia (asleep), the surgeon finds the cancer. Normal tissues can be moved out of the way and protected with special shields, so IORT can allow the doctor to give one large dose of radiation to the cancer and not affect nearby tissues. IORT is usually given in a special operating room lined with radiation-shielding walls.

Stereotactic radiosurgery is not really surgery but a type of radiation treatment that gives a large dose of radiation to a small tumor area in a single session. It is mostly used for brain tumors and other tumors inside the head. 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 the CT or MRI scans, radiation is sent to the area from a machine. The radiation is very precise and nearby tissues are affected as little as possible.

Stereotactic radiosurgery typically uses one session to deliver the whole radiation dose, though it may be repeated if needed. Sometimes doctors give the radiation in many smaller treatments to deliver the same or slightly higher dose (fractionation). This is sometimes called fractionated radiosurgery or stereotactic radiotherapy.

Clinical trials are being done to look at how well stereotactic radiosurgery and stereotactic radiotherapy work alone and when used along with other types of radiation therapy.