Radiotherapy, also called radiation therapy, the use of radiation in the treatment of disease, especially cancer. Radiation can be administered by means of X-ray therapy machines, cobalt machines, and betatrons, which develop very high voltage. It can also be administered through radium or radioisotopes, which are radioactive forms of elements. Radioisotopes, such as radioactive iodine, gold, and phosphorus, are introduced into the body, most often to treat tissues that have an affinity for a special element. For example, if the thyroid is treated with radioactive iodine, most of the element accumulates in this gland rather than in other body tissues.
Radiation has two effects on tissues. In sufficient doses it interferes with or destroys cells that are multiplying. This effect is the chief reason for using radiotherapy in cancer, because cancer cells multiply rapidly. Secondly, radiation kills cells by interfering with their enzyme systems and blocking their metabolism. Because radiation may injure or destroy healthy, as well as abnormal, tissues, precautions are taken to protect patients during radiotherapy. For example, high-energy radiation is generally used, because it penetrates deeply and may reach a tumor within the body without seriously affecting the skin and other overlying tissues. The radiation is also delivered in a concentrated beam so that it does not spread over surrounding tissue. Another protective measure is to deliver the radiation from several directions in such a way that the various beams converge on the area under treatment. The tumor receives the total radiation from all the beams, while other tissues receive only the radiation from the single beam that passes over them. In addition, parts of the body not under treatment may be shielded from radiation.
At one time, radiotherapy was widely used to treat conditions ranging from enlarged tonsils and arthritis to malignant tumors. However, it was discovered that in addition to its other harmful effects, radiation can cause genetic mutations. For this reason, high-energy radiation is limited almost entirely to the treatment of cancer. On the other hand, radioisotopes emit considerably less radiation and the radiation reaches a limited area. For this reason, radioisotopes are more widely used to treat noncancerous conditions, such as hyperthyroidism, in which the thyroid is overactive.