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Back to Oncology Procedures
Radiation therapy
Radiation therapy (or radiotherapy) is the medical use of ionizing
radiation as part of cancer treatment to control malignant cells (not
to be confused with radiology, the use of radiation in medical imaging
and diagnosis). Although radiotherapy is often used as part of
curative therapy, it is occasionally used as a palliative treatment,
where cure is not possible and the aim is for symptomatic relief.
Other rare uses are to wipe out the immune system prior to transplant
to reduce the incidence of tissue rejection, called TBI or Total Body
Irradiation; to calm hyperactive muscles -- such as might cause
twitchy eyes -- with mild superficial treatments; and to form scar
tissue around a stent to reinforce the vascular wall.
Application
Radiotherapy is commonly used for the treatment of
malignant tumors. It may be
used as the primary therapy. It is also common to combine radiotherapy
with surgery and/or chemotherapy and/or hormone therapy. The most
common tumors treated with radiotherapy are
breast cancer,
prostate
cancer, lung cancer,
colorectal cancer,
head & neck cancers,
gynecological tumors, bladder cancer and
lymphoma, although the
cancer's stage (progress) and invasion into lymph nodes, as well as
and other health and (unfortunately) monetary factors affect which
treatment will have the greatest possibility of success.
Radiation therapy is commonly applied just to the localized area
involved with the tumor. Often the radiation fields also include the
draining lymph nodes. It is possible but uncommon to give radiotherapy
to the whole body, or entire skin surface.
Side effects
Although the actual treatment is painless, using external radiation
(see below) to tackle tumors inevitably leads to side effects. The
side effects can occur during treatment (acute side effects such as
soreness and redness over the affected area; nausea and vomiting) or
long after treatment has finished (late side effects reflecting
permanent organ damage). Implanting radioactive sources has the usual
side effects associated with invasive procedures.
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Dosage
Radiation therapy is usually given daily for up to 35-38 fractions (a
daily dose is a fraction); ~2 Gy (Gray (unit)) per fraction. These
small frequent doses allow healthy cells time to grow back, repairing
damage inflicted by the radiation. The total number of fractions can
vary significantly depending on disease site and state from as low as
1 to a theoretically infinite number (in the case of LDR (low dose
rate) brachytherapy).
How it works
Radiation therapy works by damaging the DNA of cells. The damage is
caused by an electromagnetic, electron or proton beam ionizing the
bases in the DNA breaking the DNA chain. Because cells have mechanisms
for repairing DNA breakage, where the DNA is broken on both strands of
the DNA are the most significant in modifying cell characteristics.
Because cancer cells generally are undifferentiated and stem
cell-like, they reproduce more, and have an diminished ability to repair
sublethal 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. Being researched is antiproton
radiotherapy which would require fewer treatments than proton
radiotherapy.
Implications
Tumors don't repair the radiation damage as well as non-cancerous
tissue.
Most cells, however, die only during a specific phase of cellular
reproduction, which has many curious implications:
Some slowly growing tumors (for example, prostate) may be treated
best by not treating them at all, since the patient will likely die
from other causes, such as old age, before the cancer kills.
It is thought that tumors which outgrow their blood supply, causing a
low-oxygen state known as hypoxia, are more resistant to the effects
of radiation because they reproduce less frequently, and are not
subject to indirect damage caused by free radicals produced by the
ionization of oxygen.
Some brain tumors do not die at extremely high doses. It is an open
subject as to the mechanism by which they survive, but perhaps they do
not reproduce in the usual way.
Three main divisions of radiotherapy are external beam radiotherapy (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 are injected into the body.
Roughly half of the 2500 worldwide radiotherapy clinics are in the US
(as of 2001).
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