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.
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.
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.
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.
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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