Radiation therapy involves either the use of a beam of gamma rays or X-rays directed at a cancerous tumour (eg Gamma rays from Cobalt-60, see below), or the insertion of a radioactive source into a tumour (eg Iodine-131 used for Thyroid Cancer). Radiotherapy does not always kill all the cancerous cells, and often may damage healthy cells nearby. It is particularly difficult to use if the cancerous cells are not well localised in one area.
Use of Cobalt-60 in Medicine
The stable isotope Cobalt-59 is bombarded with neutrons to create Cobalt-60, an unstable isotope with a half life of around 5.3 years, which will then decay by beta decay to Nickel-60 which is stable and will not decay further.
The Cobalt-60 is used for a number of medical purposes, such as sterilisation of medical equipment and as a radiation source for radiotherapy. In radiotherapy, the gamma rays given off in the decay from Cobalt-60 to Nickel-60 are directed at the tumour with the intention of killing off the cancer cells. The gamma rays are also used in food irradiation to increase storage time of fruit and dried herbs by killing off bacteria and viruses that cause decay (note that the food itself is not irradiated)