Cobalt-60
Vitamin B₁₂ is a cobalt compound. Normally the few milligrams of B₁₂ in the body are stored in the liver and released to the blood stream as needed. In pernicious anemia, a potentially fatal but curable disease, the B₁₂ content of the blood falls from the usual level of 300-900 micromicrograms per milliliter (ml) to 0 to 100 micromicrograms per ml. The administration of massive doses of B₁₂ is the only known remedy for this condition.
If the B₁₂ is labeled with radioactive cobalt, its passage into the blood stream may be observed by several different methods. The simplest is to give the B₁₂ by mouth, and after about 8 hours study the level of cobalt radioactivity in the blood. Cobalt-60 has been used for several years, but recently cobalt-58 has been found more satisfactory. It has a half-life of 72 days while ⁶⁰Co has a 5.3-year half-life. This reduces greatly the amount of radiation to the patient’s liver by the retained radioactivity.