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 total body irradiation (TBI); 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.

Implications

Tumors don't repair the radiation damage as well as nonmalignant tissue.

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Most cells, however, die only during a specific phase of cellular reproduction, which has many curious implications:

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• 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 ionisation 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.