Radiation therapy is a common technique used to treat cancer by employing intense doses of radiation to destroy cancer cells. This is commonly performed either externally, or internally by injection of a targeting radioisotope or implantation of a radioactive source inside the tumour.
External beam-radiation therapy is performed by using a very strong source of radiation (such as cobalt-60 or a machine called a linear accelerator) to deliver intense doses to the tumour from the outside. The radiation beam is applied from many angles and shaped carefully to match the precise contour of the targeted tissue, so that the radiation dose to healthy tissue is limited. Discovering the exact extent of the tumour is one very important use of the detailed images provided by diagnostic nuclear imaging. The use of cobalt-60 for radiation therapy was pioneered in Canada in 1951 by Harold E. Johns and Roy Errington.
Injections are typically used only to treat thyroid cancers or other thyroid conditions though the injection of iodine-131, which naturally concentrates in the desired zone in the same way as isotopes are injected for diagnostic purposes.
Implantation (also called brachytherapy), involves the surgical implantation of one or several radioactive sources. This can be done temporarily by catheter, with several treatments over a series of hours or days; or permanently, with the radioactive sources left to naturally decay in the tumour. Though invasive, this is also a highly effective method of radiation therapy.
At high doses, radiation can cause mutations in cells that can affect their health and potentially cause cancer. However, at the extremely high doses used in radiation therapy, the damage caused is so severe that the affected cells can no longer reproduce, or are simply destroyed. For this reason, radiation therapy is delivered carefully and precisely, to limit the extent to which healthy tissue is affected.
Two natural differences between cancer cells and healthy tissue give further advantage to this treatment technique. First, cells that grow more quickly (undergo division by mitosis more often) are more sensitive to radiation. Because cancer cells grow very quickly, it is possible to give them a lethal dose while sparing the nearby healthy tissue. Second, cancer cells often have ineffective repair mechanisms, so an amount of radiation damage that a normal cell would ordinarily recover from will often destroy a cancer cell.
Radiation therapy is an extremely effective option for cancer treatment, and is rapidly advancing in sophistication. For more detailed information on the technical considerations involved in radiation therapy, please visit the National Cancer Institute’s information page.