- What is radiation therapy?
Radiation therapy (also called radiotherapy,
x-ray
therapy, or irradiation)
is the use of a certain type of energy (called ionizing radiation) to kill
cancer cells and shrink tumors. Radiation therapy 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 and divide.
Although radiation damages both cancer cells and normal cells, most normal
cells can recover from the effects of radiation and function properly. The
goal of radiation therapy is to damage as many cancer cells as possible, while
limiting harm to nearby healthy tissue.
There are different types of radiation and different ways to deliver the
radiation. For example, certain types of radiation can penetrate more deeply
into the body than can others. In addition, some types of radiation can be
very finely controlled to treat only a small area (an inch of tissue, for
example) without damaging nearby tissues and organs.
Other types of radiation are better for treating larger areas.
In some cases, the goal of radiation treatment is the complete destruction
of an entire tumor. In other cases, the aim is to shrink a tumor and relieve
symptoms.
In either case, doctors plan treatment to spare as much healthy tissue as
possible.
About half of all cancer patients receive some type of radiation therapy.
Radiation therapy may be used alone or in combination with other cancer treatments,
such as chemotherapy
or surgery.
In some cases, a patient may receive more than one type of radiation therapy.
- When is radiation therapy used?
Radiation therapy may be used to treat almost every type of solid
tumor, including cancers of the brain, breast,
cervix,
larynx,
lung,
pancreas,
prostate,
skin, spine, stomach,
uterus,
or soft
tissue sarcomas. Radiation can also be used to treat leukemia
and lymphoma
(cancers of the blood-forming
cells and lymphatic
system, respectively). Radiation dose
to each site depends on a number of factors, including the type of cancer
and whether there are tissues and organs nearby that may be damaged by radiation.
For some types of cancer, radiation may be given to areas that do not have
evidence of cancer. This is done to prevent cancer cells from growing in the
area receiving the radiation. This technique is called prophylactic
radiation therapy.
Radiation therapy also can be given to help reduce symptoms such as pain
from cancer that has spread to the bones or other parts of the body. This
is called palliative radiation therapy.
- What is the difference between external radiation therapy,
internal radiation therapy (brachytherapy),
and systemic
radiation therapy? When are these types used?
Radiation may come from a machine outside the body (external radiation),
may be placed inside the body (internal radiation), or may use unsealed radioactive
materials that go throughout the body (systemic radiation therapy). The type
of radiation to be given depends on the type of cancer, its location, how
far into the body the radiation will need to go, the patient's general health
and medical history, whether the patient will have other types of cancer treatment,
and other factors.
Most people who receive radiation therapy for cancer have external radiation.
Some patients have both external and internal or systemic radiation therapy,
either one after the other or at the same time.
- Will radiation therapy make the patient radioactive?
Cancer patients receiving radiation therapy are often concerned that the
treatment will make them radioactive. The answer to this question depends
on the type of radiation therapy being given.
External radiation therapy will not make the patient radioactive. Patients
do not need to avoid being around other people because of the treatment.
Internal radiation therapy (interstitial, intracavitary, or intraluminal)
that involves sealed implants emits radioactivity, so a stay in the hospital
may be needed. Certain precautions are taken to protect hospital staff and
visitors. The sealed sources deliver most of their radiation mainly around
the area of the implant, so while the area around the implant is radioactive,
the patient's whole body is not radioactive.
Systemic radiation therapy uses unsealed radioactive materials that travel
throughout the body. Some of this radioactive material will leave the body
through saliva,
sweat, and urine
before the radioactivity decays, making these fluids
radioactive. Therefore, certain precautions are sometimes used for people
who come in close contact with the patient. The patient's doctor or nurse
will provide information if these special precautions are needed.
- How does the doctor measure the dose of radiation?
The amount of radiation absorbed by the tissues is called the radiation dose
(or dosage). Before 1985, dose was measured in a unit called a “rad”
(radiation absorbed dose). Now the unit is called a gray (abbreviated as Gy).
One Gy is equal to 100 rads; one centigray (abbreviated as cGy) is the same
as 1 rad.
Different tissues can tolerate various amounts of radiation (measured in
centigrays). For example, the liver
can receive a total dose of 3,000 cGy, while the kidneys
can tolerate only 1,800 cGy. The total dose of radiation is usually divided
into smaller doses (called fractions) that are given daily over a specific
time period. This maximizes the destruction of cancer cells while minimizing
the damage to healthy tissue.
The doctor works with a figure called the therapeutic ratio. This ratio compares
the damage to the cancer cells with the damage to healthy cells. Techniques
are available to increase the damage to cancer cells without doing greater
harm to healthy tissues. These techniques are discussed in Questions 8,
9, and 15.
- What are the sources of energy for external radiation therapy?
The energy (source of radiation) used in external radiation therapy may come
from the following:
- X-rays
or gamma
rays, which are both forms of electromagnetic
radiation. Although they are produced in different ways, both use photons
(packets of energy).
- X-rays are created by machines called linear accelerators.
Depending on the amount of energy the x-rays have, they can be used to
destroy cancer cells on the surface of the body (lower energy) or deeper
into tissues and organs (higher energy). Compared with other types of
radiation, x-rays can deliver radiation to a relatively large area.
- Gamma rays are produced when isotopes of certain elements
(such as iridium and cobalt
60) release radiation energy as they break down. Each element breaks
down at a specific rate and each gives off a different amount of energy,
which affects how deeply it can penetrate into the body. (Gamma rays produced
by the breakdown of cobalt 60 are used in the treatment called the “gamma
knife,” which is discussed in Question 8).
- Particle beams use fast-moving subatomic particles instead
of photons. This type of radiation may be called particle beam radiation
therapy or particulate radiation. Particle beams are created by linear
accelerators, synchrotrons, and cyclotrons, which produce and accelerate
the particles required for this type of radiation therapy. Particle beam
therapy uses electrons, which are produced by an x-ray tube (this may be
called electron-beam radiation); neutrons, which are produced by radioactive
elements and special equipment; heavy ions (such as protons
and helium); and pi-mesons (also called pions), which are small, negatively
charged particles produced by an accelerator and a system of magnets. Unlike
x-rays and gamma rays, some particle beams can penetrate only a short distance
into tissue. Therefore, they are often used to treat cancers located on
the surface of or just below the skin.
- Proton beam therapy is a type of particle beam radiation
therapy. Protons deposit their energy over a very small area, which is
called the Bragg peak. The Bragg peak can be used to target high doses
of proton beam therapy to a tumor while doing less damage to normal tissues
in front of and behind the tumor. Proton beam therapy is available at
only a few facilities in the United States. Its use is generally reserved
for cancers that are difficult or dangerous to treat with surgery (such
as a chondrosarcoma
at the base of the skull), or it is combined with other types of radiation.
Proton beam therapy is also being used in clinical trials for intraocular
melanoma (melanoma
that begins in the eye), retinoblastoma
(an eye cancer that most often occurs in children under age 5), rhabdomyosarcoma
(a tumor of the muscle tissue), some cancers of the head and neck, and
cancers of the prostate, brain, and lung.
- What are the sources of energy for internal radiation?
The energy (source of radiation) used in internal radiation comes from the
radioactive isotope in radioactive
iodine (iodine 125 or iodine 131), and from strontium 89, phosphorous,
palladium, cesium, iridium, phosphate, or cobalt. Other sources are being
investigated.
- What are stereotactic
radiosurgery and stereotactic radiotherapy?
Stereotactic (or stereotaxic) radiosurgery
uses a large dose of radiation to destroy tumor tissue in the brain. The procedure
does not involve actual surgery. The patient's head is placed in a special
frame, which is attached to the patient'skull. The frame is used to aim high-dose
radiation beams directly at the tumor inside the patient's head. The dose
and area receiving the radiation are coordinated very precisely. Most nearby
tissues are not damaged by this procedure.
Stereotactic radiosurgery can be done in one of three ways. The most common
technique uses a linear accelerator to administer high-energy photon
radiation to the tumor (called “linac-based stereotactic radiosurgery”).
The gamma
knife, the second most common technique, uses cobalt 60 to deliver
radiation. The third technique uses heavy charged particle beams
(such as protons and helium ions) to deliver stereotactic radiation to the
tumor.
Stereotactic radiosurgery is mostly used in the treatment of small benign
and malignant
brain tumors (including meningiomas,
acoustic
neuromas,
and pituitary cancer). It can also be used to treat other conditions (for
example, Parkinson
disease and epilepsy).
In addition, stereotactic radiosurgery can be used to treat metastatic
brain tumors (cancer that has spread to the brain from another part of the
body) either alone or along with whole-brain radiation therapy. (Whole-brain
radiation therapy is a form of external radiation therapy that treats the
entire brain with radiation).
Stereotactic radiotherapy uses essentially the same approach
as stereotactic radiosurgery to deliver radiation to the target tissue. However,
stereotactic radiotherapy uses multiple small fractions of radiation as opposed
to one large dose. Giving multiple smaller doses may improve outcomes and
minimize side
effects. Stereotactic radiotherapy is used to treat tumors in the brain
as well as other parts of the body.
Clinical trials are under way to study the effectiveness of stereotactic
radiosurgery and stereotactic radiotherapy alone and in combination with other
types of radiation therapy.
- What other methods are in use or being studied to improve
external radiation therapy?
A number of refinements and techniques are in use or under study to improve
the effectiveness of external radiation therapy. These are described below:
- Three-dimensional (3–D)
conformal radiation therapy. Traditionally, the planning of radiation
treatments has been done in two dimensions (width and height). Three-dimensional
(3–D) conformal radiation therapy uses computer technology to allow
doctors to more precisely target a tumor with radiation beams (using width,
height, and depth). Many radiation
oncologists use this technique. A 3–D image of a tumor can be
obtained using computed tomography
(CT), magnetic
resonance imaging (MRI),
positron emission tomography (PET), or single photon emission computed
tomography (SPECT).
Using information from the image, special computer programs design radiation
beams that “conform” to the shape of the tumor. Because the
healthy tissue surrounding the tumor is largely spared by this technique,
higher doses of radiation can be used to treat the cancer. Improved outcomes
with 3–D conformal radiation therapy have been reported for nasopharyngeal,
prostate, lung, liver, and brain cancers.
- Intensity-modulated radiation therapy (IMRT). IMRT is
a new type of 3–D conformal radiation therapy that uses radiation
beams (usually x-rays) of varying intensities to deliver different doses
of radiation to small areas of tissue at the same time. The technology allows
for the delivery of higher doses of radiation within the tumor and lower
doses to nearby healthy tissue. Some techniques deliver a higher dose of
radiation to the patient each day, potentially shortening the overall treatment
time and improving the success of the treatment. IMRT may also lead to fewer
side effects during treatment.
The radiation is delivered by a linear accelerator that is equipped with
a multileaf collimator (a collimator helps to shape or sculpt the beams
of radiation). The equipment can be rotated around the patient so that
radiation beams can be sent from the best angles. The beams conform as
closely as possible to the shape of the tumor. Because IMRT equipment
is highly specialized, not every radiation oncology
center uses IMRT.
This new technology has been used to treat tumors in the brain, head
and neck, nasopharynx,
breast, liver, lung, prostate, and uterus. However, IMRT is not appropriate
or necessary for every patient or tumor type. Long-term results following
treatment with IMRT are becoming available.
- What are low-LET and high-LET radiation?
Linear energy transfer (LET) describes the rate at which a type of radiation
deposits energy as it passes through tissue. Higher levels of deposited energy
cause more cells to be killed by a given dose of radiation therapy. Different
types of radiation have different levels of LET. For example, x-rays, gamma
rays, and electrons are known as low-LET radiation. Neutrons, heavy ions,
and pions are classified as high-LET radiation.
Most high-LET radiation is investigational
treatment. The cost of the equipment and the amount of specialized training
needed to perform high-LET radiation therapy restrict its use to only a few
facilities in the United States.
- Who plans and delivers the radiation treatment to the
patient?
Many health care providers help to plan and deliver radiation treatment to
the patient. The radiation therapy team includes the radiation oncologist,
a doctor who specializes in using radiation to treat cancer; the dosimetrist,
who determines the proper radiation dose; the radiation
physicist, who makes sure that the machine delivers the right amount of
radiation to the correct site in the body; and the radiation
therapist, who gives the radiation treatment. Often, radiation treatment
is only one part of the patient's total therapy. Combined modality
therapy, the use of radiation with drug
therapy, is commonly used.
The radiation oncologist also works with the medical or pediatric
oncologist, surgeon,
radiologist
(a doctor who specializes in creating and interpreting pictures of areas inside
the body), pathologist
(a doctor who identifies diseases by studying cells and tissues under a microscope),
and others to plan the patient's total course of therapy. A close working
relationship between the radiation oncologist, medical or pediatric oncologist,
surgeon, radiologist, and pathologist is important in planning the total therapy.
- What is treatment planning, and why is it important?
Because there are so many types of radiation and many ways to deliver it,
treatment planning is a very important first step for every patient who will
have radiation therapy. Before radiation therapy is given, the patient's radiation
therapy team determines the amount and type of radiation the patient will
receive.
If the patient will have external radiation, the radiation oncologist uses
a process called simulation to define where to aim the radiation.
During simulation, the patient lies very still on an examining table while
the radiation therapist uses a special x-ray machine to define the treatment
port or field—the exact place on the body where the radiation will be
aimed. Most patients have more than one treatment port. Simulation may also
involve CT scans
or other imaging
studies to help the radiation therapist plan how to direct the radiation.
The simulation may result in some changes to the treatment plan so that the
greatest possible amount of healthy tissue can be spared from receiving radiation.
The areas to receive radiation are marked with either a temporary or permanent
marker, tiny dots or a “tattoo” showing where the radiation should
be aimed. These marks are also used to determine the exact site of the initial
treatments if the patient should need radiation treatment later.
Depending on the type of radiation treatment, the radiation therapist may
make body molds or other devices that keep the patient from moving during
treatment. These are usually made from foam, plastic, or plaster. In some
cases, the therapist will also make shields that cannot be penetrated by radiation
to protect organs and tissues near the treatment field.
When the simulation is complete, the radiation therapy team meets to decide
how much radiation is needed (the dose of radiation), how it should be delivered,
and how many treatments the patient should have.
- What are radiosensitizers
and radioprotectors?
Radiosensitizers and radioprotectors are chemicals that modify a cell's
response to radiation. Radiosensitizers are drugs that make cancer cells more
sensitive to the effects of radiation therapy. Several compounds are under
study as radiosensitizers. In addition, some anticancer drugs, such as 5-fluorouracil
and cisplatin,
make cancer cells more sensitive to radiation therapy.
Radioprotectors (also called radioprotectants) are drugs that protect normal
(noncancerous) cells from the damage caused by radiation therapy. These agents
promote the repair of normal cells that are exposed to radiation. Amifostine
(trade name Ethyol®) is the only drug approved by the U.S. Food and Drug
Administration (FDA) as a radioprotector. It helps to reduce the dry mouth
that can occur if the parotid glands
(which help to produce saliva and are located near the ear) receive a large
dose of radiation. Additional studies are under way to determine whether amifostine
is effective when used with radiation therapy to treat other types of cancer.
Other compounds are also under study as radioprotectors.
- What are radiopharmaceuticals? How are they used?
Radiopharmaceuticals,
also known as radionucleotides, are radioactive
drugs used to treat cancer, including thyroid cancer, cancer that recurs
in the chest
wall, and pain caused by the spread of cancer to the bone (bone
metastases). The most commonly used radiopharmaceuticals are samarium
153 (Quadramet®) and strontium 89 (Metastron™). These drugs
are approved by the FDA to relieve pain caused by bone metastases. Both agents
are given intravenously (by injection
into a vein), usually on an outpatient basis; sometimes they are given in
addition to external beam radiation. Other types of radiopharmaceuticals,
such as phosphorous 32, rhodium 186, and gallium
nitrate, are not used as frequently. Still other radiopharmaceuticals
are under investigation.
- What are some new approaches to radiation therapy?
Hyperthermia, the use of heat, is being studied in conjunction with radiation
therapy. Researchers have found that the combination of heat and radiation
can increase the response
rate of some tumors.
Researchers are also studying 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 seek out cancer
cells, which are destroyed by the radiation. This approach can minimize the
risk of radiation damage to healthy cells.
The success of this technique depends on identifying appropriate radioactive
substances and determining the safe and effective dose of radiation that can
be delivered in this way. Two radioimmunotherapy treatments, ibritumomab tiuxetan
(Zevalin®) and tositumomab
and iodine 131 tositumomab (Bexxar®), have been approved for advanced
adult non-Hodgkin lymphoma (NHL). Clinical trials of radioimmunotherapy are
under way with a number of cancers, including leukemia, NHL, colorectal cancer,
and cancers of the liver, lung, brain, prostate, thyroid, breast, ovary, and
pancreas.
Scientific advances have led to the discovery of new targets that are being
investigated to attract radioactive materials directly to cancer cells. Laboratory
and clinical research is in progress using the new molecular therapeutic agents,
such as gefitinib
(Iressa®) and imatinib
mesylate (Gleevec®), with radiation therapy.
- Where can people find more information about radiation
therapy?
The National Cancer Institute (NCI) booklet Radiation Therapy and You:
Support for People With Cancer has more information about this topic.
This publication is available from the NCI Publications Locator Web site at
http://www.cancer.gov/publications
on the Internet, and from the NCI's Cancer Information Service (see below).
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