Radiotherapy, also called radiation therapy, is the
treatment of cancer and other diseases with ionizing radiation. Ionizing
radiation deposits energy that injures or destroys cells in the area being
treated (the "target tissue") by damaging their genetic material,
making it impossible for these cells to continue to grow. Although radiation
damages both cancer cells and normal cells, the latter are able to repair
themselves and function properly. Radiotherapy may be used to treat localized
solid tumors, such as cancers of the skin, tongue, larynx, brain, breast, or
uterine cervix. It can also be used to treat leukemia and lymphoma (cancers of
the blood-forming cells and lymphatic system, respectively).
One type of radiation therapy commonly used involves
photons, "packets" of energy. X-rays were the first form of photon
radiation to be used to treat cancer. Depending on the amount of energy they
possess, the rays can be used to destroy cancer cells on the surface of or
deeper in the body. The higher the energy of the x-ray beam, the deeper the
x-rays can go into the target tissue. Linear accelerators and betatrons are
machines that produce x-rays of increasingly greater energy. The use of
machines to focus radiation (such as x-rays) on a cancer site is called
external beam radiotherapy.
Gamma rays are another form of photons used in
radiotherapy. Gamma rays are produced spontaneously as certain elements (such
as radium, uranium, and cobalt 60) release radiation as they decompose, or
decay. Each element decays at a specific rate and gives off energy in the form
of gamma rays and other particles. X-rays and gamma rays have the same effect
on cancer cells.
Another technique for delivering radiation to cancer
cells is to place radioactive implants directly in a tumor or body cavity.
This is called internal radiotherapy. (Brachytherapy, interstitial
irradiation, and intracavitary irradiation are types of internal
radiotherapy.) In this treatment, the radiation dose is concentrated in a
small area, and the patient stays in the hospital for a few days. Internal
radiotherapy is frequently used for cancers of the tongue, uterus, and cervix.
Several new approaches to radiation therapy are being
evaluated to determine their effectiveness in treating cancer. One such
technique is intraoperative irradiation, in which a large dose of external
radiation is directed at the tumor and surrounding tissue during surgery.
Another investigational approach is particle beam
radiation therapy. This type of therapy differs from photon radiotherapy in
that it involves the use of fast-moving subatomic particles to treat localized
cancers. A very sophisticated machine is needed to produce and accelerate the
particles required for this procedure. Some particles (neutrons, pions, and
heavy ions) deposit more energy along the path they take through tissue than
do x-rays or gamma rays, thus causing more damage to the cells they hit. This
type of radiation is often referred to as high linear energy transfer (high
LET) radiation.
Scientists also are looking for ways to increase the
effectiveness of radiation therapy. Two types of investigational drugs are
being studied for their effect on cells undergoing radiation. Radiosensitizers
make the tumor cells more likely to be damaged, and radioprotectors protect
normal tissues from the effects of radiation. Hyperthermia, the use of heat,
is also being studied for its effectiveness in sensitizing tissue to
radiation.
Other recent radiotherapy research has focused on the
use of radiolabeled antibodies to deliver doses of radiation directly to the
cancer site (radioimmunotherapy). Antibodies are highly specific proteins that
are made by the body in response to the presence of antigens (substances
recognized as foreign by the immune system). Some tumor cells contain specific
antigens that trigger the production of tumor-specific antibodies. Large
quantities of these antibodies can be made in the laboratory and attached to
radioactive substances (a process known as radiolabeling). Once injected into
the body, the antibodies actively seek out the cancer cells, which are
destroyed by the cell-killing (cytotoxic) action of the radiation. This
approach can minimize the risk of radiation damage to healthy cells. The
success of this technique will depend upon both the identification of
appropriate radioactive substances and determination of the safe and effective
dose of radiation that can be delivered in this way.
Radiation therapy may be used alone or in combination
with chemotherapy or surgery. Like all forms of cancer treatment, radiation
therapy can have side effects. Possible side effects of treatment with
radiation include temporary or permanent loss of hair in the area being
treated, skin irritation, temporary change in skin color in the treated area,
and tiredness. Other side effects are largely dependent on the area of the
body that is treated. More information about the side effects associated with
radiotherapy can be found in the NCI booklet Radiation Therapy and You.
National Cancer Institute Information Resources
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