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 tumours. It may be used as the primary therapy. It is also common to combine radiotherapy with surgery and/or chemotherapy. The most common tumours treated with radiotherapy are breast cancer, prostate cancer, lung cancer, colorectal cancer, head & neck cancers, gynaecological tumours, 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 localised area involved with the tumour. 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 tumours 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 radiactive sources has the usual side effects associated with invasive proceedures.
Dosage
Radiation therapy is usually given daily for up to 35-38 fractions (a daily dose is a 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 diesease 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 the particles of the 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 charactaristics. Because cancer cells reproduce more than most normal cells, the DNA damage is inhereted through cell division. The accumulated damage to the cancer cells causes them to die or reproduce more slowly.
Implications (?)
Tumours 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 tumours (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 tumours which outgrow their blood supply, causing a low-oxygen state known as hypoxia, reproduce less frequently and are therefore more resistant to the effects of radiation.
- Some brain tumours 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 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 & 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).
Further information:
- http://www.rtog.org/ - The Radiation Therapy Oncology Group
- http://search.dmoz.org/cgi-bin/search?search=radiotherapy - directory of clinics, resources and other information
- Online Radiology Resources
- See also: Cancer -- Chemotherapy -- Surgical Oncology -- Gene Therapy -- Cancer Vaccine