http://www.answers.com/topic/radiati...apy?cat=health [excerpt]
Radiation Therapy
Definition
Radiation therapy, sometimes called radiotherapy, x-ray therapy radiation
treatment, cobalt therapy, electron beam therapy, or irradiation uses high
energy, penetrating waves or particles such as x rays, gamma rays, proton
rays, or neutron rays to destroy cancer cells or keep them from
reproducing.
Purpose
The purpose of radiation therapy is to kill or damage cancer cells.
Radiation therapy is a common form of cancer therapy. It is used in more
than half of all cancer cases. Radiation therapy can be used:
* alone to kill cancer
* before surgery to shrink a tumor and make it easier to remove
* during surgery to kill cancer cells that may remain in surrounding
tissue after the surgery (called intraoperative radiation)
* after surgery to kill cancer cells remaining in the body
* to shrink an inoperable tumor in order to and reduce pain and
improve quality of life
* in combination with chemotherapy
For some kinds of cancers such as early-stage Hodgkin's disease,
non-Hodgkin's lymphoma, and certain types of prostate, or brain cancer,
radiation therapy alone may cure the disease. In other cases, radiation
therapy used in conjunction with surgery, chemotherapy, or both, increases
survival rates over any of these therapies used alone.
Precautions
Radiation therapy does not make the person having the treatments
radioactive. In almost all cases, the benefits of this therapy outweigh
the risks. However radiation therapy can have has serious consequences, so
anyone contemplating it should be sure understand why the treatment team
believes it is the best possible treatment option for their cancer.
Radiation therapy is often not appropriate for pregnant women, because the
radiation can damage the cells of the developing baby. Women who think
they might be pregnant should discuss this with their doctor.
Description
Radiation therapy is a local treatment. It is painless. The radiation acts
only on the part of the body that is exposed to the radiation. This is
very different from chemotherapy in which drugs circulate throughout the
whole body. There are two main types of radiation therapy. In external
radiation therapy a beam of radiation is directed from outside the body at
the cancer. In internal radiation therapy, called brachytherapy or implant
therapy, where a source of radioactivity is surgically placed inside the
body near the cancer.
How radiation therapy works
The protein that carries the code controlling most activities in the cell
is called deoxyribonucleic acid or DNA. When a cell divides, its DNA must
also double and divide. High-energy radiation kills cells by damaging
their DNA, thus blocking their ability to grow and increase in number.
One of the characteristics of cancer cells is that they grow and divide
faster than normal cells. This makes them particularly vulnerable to
radiation. Radiation also damages normal cells, but because normal cells
are growing more slowly, they are better able to repair radiation damage
than are cancer cells. In order to give normal cells time to heal and
reduce side effects, radiation treatments are often given in small doses
over a six or seven week period.
External radiation therapy
External radiation therapy is the most common kind of radiation therapy.
It is usually done during outpatient visits to a hospital clinic and is
usually covered by insurance.
Once a doctor, called a radiation oncologist, determines the proper dose
of radiation for a particular cancer, the dose is divided into smaller
doses called fractions. One fraction is usually given each day, five days
a week for six to seven weeks. However, each radiation plan is
individualized depending on the type and location of the cancer and what
other treatments are also being used. The actual administration of the
therapy usually takes about half an hour daily, although radiation is
administered for only from one to five minutes at each session. It is
important to attend every scheduled treatment to get the most benefit from
radiation therapy.
Recently, trials have begun to determine if there are ways to deliver
radiation fractions so that they kill more cancer cells or have fewer side
effects. Some trials use smaller doses given more often. Up-to-date
information on voluntary participation in clinical trials and where they
are being held is available by entering the search term "radiation
therapy" at the following web sites:
* National Cancer Institute. <http://cancertrials.nci.nih.gov> or
(800) 4-CANCER.
* National Institutes of Health Clinical Trials.
<http://clinicaltrials.gov>.
* Center Watch: A Clinical Trials Listing.
<http://www.centerwatch.com>.
The type of machines used to administer external radiation therapy and the
material that provides the radiation vary depending on the type and
location of the cancer. Generally, the patient puts on a hospital gown and
lies down or sits in a special chair. Parts of the body not receiving
radiation are covered with special shields that block the rays. A
technician then directs a beam of radiation to a pre-determined spot on
the body where the cancer is located. The patient must stay still during
the administration of the radiation so that no other parts of the body are
affected. As an extra precaution in some treatments, special molds are
made to make sure the body is in the same position for each treatment.
However, the treatment itself is painless, like having a bone x-rayed.
Dosage
The amount of radiation used in radiation therapy is measured in grays
(Gy), and varies depending on the type and stage of cancer being treated.
For curative (radical) cases, the typical dose for a solid epithelial
tumor ranges from 60 to 80 Gy, while lymphoma tumors are treated with 20
to 40 Gy. Preventative (adjuvant) doses are typically around 45 - 60Gy in
1.8 - 2 Gy fractions (for Breast, Head and Neck cancers respectively.)
Many other factors are considered by radiation oncologists when selecting
a dose, including whether the patient is receiving chemotherapy, whether
radiation therapy is being administered before or after surgery, and the
degree of success of surgery.
Fractionation
The total dose is fractionated (spread out over time) in order to give
normal cells time to recover. In the USA and Europe, the typical
fractionation schedule for adults is 1.8 to 2 Gy per day, five days a
week. In the northern United Kingdom, fractions are more commonly 2.67 to
2.75 Gy per day, which eases the burden on thinly spread resources in the
National Health Service. For children, a typical fraction is 1.5 to 1.7 Gy
per day, reducing the chance and severity of late-onset side effects.
In some cases, two fractions per day are used near the end of a course of
treatment. This schedule, known as a concomitant boost regimen and/or
hyperfractionation, is used on tumors that regenerate more quickly when
they are smaller. In particular, tumors in the head and neck demonstrate
this behavior.
One of the best-known alternative fractionation schedules is Continuous
Hyperfractionated Accelerated Radiotherapy (CHART). CHART, used to treat
lung cancer, consists of three smaller fractions per day. Although
reasonably successful, CHART can be a strain on radiation therapy
departments.
Implants can be fractionated over minutes or hours, or they can be
permanent seeds which slowly deliver radiation until they become inactive.
Mechanism of action
Radiation therapy works by damaging the DNA of cells. The damage is caused
by a photon, electron or proton beam directly or indirectly ionizing the
atoms which make up DNA chain. Indirect ionization happens as a result of
the ionization of water, forming free radicals, notably hydroxyl radicals,
which then damage the DNA. In the most common forms of radiation therapy,
most of the radiation effect is through free radicals. Because cells have
mechanisms for repairing DNA damage, breaking the DNA on both strands
proves to be the most significant technique in modifying cell
characteristics. Because cancer cells generally are undifferentiated and
stem cell-like, they reproduce more, and have a diminished ability to
repair sub-lethal damage compared to most healthy differentiated cells.
The DNA damage is inherited through cell division, accumulating damage to
the cancer cells, causing them to die or reproduce more slowly. Proton
radiotherapy works by sending protons with varying kinetic energy to
precisely stop at the tumor.
One of the major limitations of radiotherapy is that the cells of solid
tumors become deficient in oxygen. This is because solid tumours usually
outgrow their blood supply, causing a low-oxygen state known as hypoxia.
The more hypoxic the tumours are the more resistant they are to the
effects of radiation because oxygen makes the radiation damage to DNA
permanent. Much research has been devoted to overcoming this problem
including the use of high pressure oxygen tanks, blood substitutes that
carry increased oxygen, hypoxic cell radiosensitizers such as misonidazole
and
metronidazole, and hypoxic cytotoxins, such as tirapazamine. There is
also interest in the fact that high LET particles such as carbon or neon
ions may have an antitumour effect which is independent of tumour hypoxia.
Types of radiation therapy
Three main divisions of radiotherapy are external beam radiotherapy (EBRT
or XBRT) or teletherapy, brachytherapy or sealed source radiotherapy and
unsealed source radiotherapy.
The differences relate to the position of the radiation source; external
is outside the body, while sealed and unsealed source radiotherapy has
radioactive material delivered internally. Brachytherapy sealed sources
are usually extracted later, while unsealed sources may be administered by
injection or ingestion. Proton therapy is a special case of external beam
radiotherapy where the particles are protons.
Roughly half of the 2500 worldwide radiotherapy clinics are in the US (as
of 2001).
Conventional external beam radiotherapy
This is the mainstay of external beam radiotherapy in most of the world.
Conventional refers to the way the treatment is planned or simulated on a
specially calibrated conventional diagnostic x-ray machine (or sometimes
by eye), and to the usually well established arrangements of the radiation
beams to achieve a desired plan. The aim of simulation is to accurately
target or localize the volume which is to be treated. This technique is
well established, and is generally quick and reliable. The worry is that
some high-dose treatments may be limited by the radiation toxicity to
normal structures which lay close to the target volume. An example of this
problem is seen in radical radiotherapy to the prostate gland, where the
sensitivity of the adjacent rectum can limit the dose which can safely be
prescribed to such an extent that tumor control may not be achievable with
any degree of confidence. For this reason, conformal radiotherapy is
becoming the standard treatment for a number of tumor sites.
Virtual simulation, 3-dimensional conformal radiotherapy,Tomotherapy and
intensity-modulated radiotherapy
The planning of radiotherapy treatment has been revolutionized by the
ability to delineate tumors and adjacent normal structures in three
dimensions using specialized CT scanners and planning software.[2] Virtual
simulation, the most basic form of planning, allows more accurate
placement of radiation beams than is possible using conventional X-rays,
where soft-tissue structures are often difficult to assess and normal
tissues difficult to protect.
An enhancement of virtual simulation is 3-Dimensional Conformal
Radiotherapy (3DCRT), in which the profile of each radiation beam is
shaped to fit the profile of the target from a beam's eye view (BEV) using
a multileaf collimator (MLC) and a variable number of beams. When the
treatment volume conforms to the shape of the tumour, the relative
toxicity of radiation to the surrounding normal tissues is reduced,
allowing a higher dose of radiation to be delivered to the tumor than
conventional techniques would allow.
An enhancement of 3DCRT is intensity-modulated radiotherapy (IMRT),
employing dynamic multileaf collimation not only to shape the profile of
the beam, but also to vary the intensity of the beam over its area. The
goal is to achieve greater conformality than 3DCRT provides. IMRT also
improves the ability to conform the treatment volume to concave tumour
shapes, for example when the tumour is wrapped around a vulnerable
structure such as the spinal cord or a major organ or blood vessel.
3DCRT is used extensively. Use of IMRT is growing but is limited by its
need for additional time from medical personnel. Proof of improved
survival benefit from either of these techniques over conventional
radiotherapy is limited to a few tumor sites, but the ability to reduce
toxicity is generally accepted. Both techniques may enable dose
escalation, potentially increasing usefulness. There has been some
concern, particularly with IMRT, about increased exposure of normal
tissues to radiation and the consequent potential for secondary
malignancy. Overconfidence in the accuracy of imaging may increase the
chance of missing lesions that are invisible on the planning scans (and
therefore not included in the treatment plan) or which may move between
treatments or during a treatment (for example, due to respiration or
inadequate patient immobilization). New techniques are being developed to
better control this uncertainty — for example, real-time imaging combined
with real-time adjustment of the therapeutic beams. This new technology is
called image-guided radiation therapy (IGRT) or four-dimensional
radiotherapy. Tomotherapy is one type of IGRT.
Radiation Therapy 
Linear Mode
