The cobalt radioisotope used in medicine is cobalt-60. This is not a naturally occurring form of cobalt, but is produced by irradiating pure cobalt-59 with a slow neutron source such as the artificial californium, or in a nuclear reactor by using cobalt rods. The most commonly used method these days is inside a nuclear reactor.
Cobalt-60 is a very toxic substance. It is a source of gamma rays and even a short exposure time can lead to death, from immediate radiation poisoning or long term effects such as cancers. Small amounts can be ingested and will be excreted in the faeces, but some will be integrated into our bones, liver and kidneys where it can cause cancer in later years. The powdered form is particularly dangerous and for this reason it is usually used in medicine in the form of small discs. Cobalt-60 has a relatively long half-life (over five years) and so continues to produce gamma rays and beta particles for a long time, increasing its toxicity. If it is so toxic why then would we use it in medicine?
Controlled , directed, exposure to small doses of radiation is used to kill cancer cells. Cancer cells are fast growing and dividing cells that are more vulnerable to radiation than our healthy tissues. Our healthy tissues should out last the irradiated cancer cells and be better able to recover. Careful targeting of the radiation improves this outcome.
Cobalt-60 may be used to target a beam of gamma rays from an external source, or occasionally by the implantation of small rods. The toxicity of cobalt-60 means that its use in nuclear medicine is more limited than it once was.
Radioisotopes with shorter half-lives are now used in imaging procedures. Other isotopes have been found to be more accurate in organ targeting, for instance Iodine-131 for thyroid cancer and Iridium-192 brain tumors. Targeted external radiation is more controlled from linear accelerators.
Cobalt-60 is still used in less wealthy or developed countries as a targeted radiation source but use will drop off as linear accelerators become more widely available.
The major remaining use of Cobalt-60 is in the sterilization of medical instruments and products. As the gamma rays emitted by cobalt-60 are not high energy they do not leave the treated articles radioactive.
For radioactive tracers caesium-37 is preferred as it has a longer half- life (over thirty years).