The term "radiation therapy" generally refers to the use of ionizing radiation. While ionizing radiation is sometimes used in the case of an overactive thyroid, most applications are in radiation oncology. Ionizing radiation can hinder the reproduction of fast-growing cancer cells.
Ionizing radiation, particularly x-rays and gamma rays, has proved to be a useful therapeutic ttool for the treatment of cancer. This may seem paradoxical after discussing radiation hazards and the possibility of a radiation-caused cancer, but the risk of a cancer-causing mutation is quite small and the most probable result of the interaction of radiation with a cell is that the cell will fail to reproduce. Since the basic problem with a cancer is that the cells reproduce uncontrollably, then irradiation would seem to be an excellent tool to use against it. The drawback is that there is no way to irradiate the cancer without also damaging the normal tissue surrounding it. The value of radiation therapy hinges upon our our ability to find ways to damage the cancer more than the normal tissue, or at least to limit the damage to the normal tissue so that there is no unacceptable loss of function. The ratio of cancer cells killed to normal cells killed is called the "therapeutic ratio". The effectiveness of the therapy depends upon making the therapeutic ratio as high as possible.
It has been observed that undifferentiated cells and rapidly dividing cells are more sensitive to radiation than well differentiated and slowly reproducing cells. Accordingly, radiation is observed to have a greater effect upon rapidly dividing cancer cells than upon the surrounding normal cells. However, a number of types of normal cells also rapidly reproduce and must be carefully monitored during radiation therapy; examples are the white blood cells and blood platelets. Intestinal membrane cells also reproduce rapidly.
Nave & Nave
Radiation Therapy, National Cancer Institute