Diagnostic and Therapeutic uses of Radioisotopes of Cobalt

Three radioisotopes of Cobalt used in medical practice are Cobalt-57 (⁵⁷Co), Cobalt-58 (⁵⁸Co), and Cobalt-60 (⁶⁰Co). Cobalt-60 (⁶⁰Co) has a half life of 5.27 years and it emits gamma rays (g-rays). Half life of Cobalt-57 (⁵⁷Co) is 270 days and that of Cobalt-58 (⁵⁸Co) is 71 days only. It is important to note that the radioisotopes with shorter half life are always good for diagnostic use whereas with longer half life are good for radiotherapy or radiation therapy. Cobalt-60 (⁶⁰Co) is a preferred source of radiation for radiotherapy at present; though earlier it was in dual use. Cobalt-57 (⁵⁷Co) and Cobalt-58 (⁵⁸Co) are preferred for diagnostic applications due to shorter half life.

Use of Cobalt-57 and Cobalt-58 in Diagnosis:

The most common use of Cobalt-57 (⁵⁷Co) or Cobalt-58 (⁵⁸Co) is in the diagnosis of pernicious anemia (fetal anemia caused by vitamin B-12 deficiency due to poor absorption of vitamin B-12 by small intestine due to intrinsic factor defect). Cyanocobalamin (vitamin B-12) and hydroxycobalamin labeled with ⁵⁷Co or ⁵⁸Co are commercially available for diagnostic use. The dose is 0.5 microCurie (mCi) to 2 microCurie (mCi). The administration is oral after 24 hours of fasting and the patient is not allowed to eat for 2 hours following the administration of vitamin B-12 labeled with radioisotope. The patient does not need hospitalization for this investigation. The excretion of radioactive is measured in the 24 hour urine specimen of the patient after the administration of radioactive labeled vitamin B12.

Use of Cobalt-60 in Therapy:

The gamma rays (g-rays) emitted by Cobalt-60 (⁶⁰Co) are like high energy x-rays. The radiotherapy or radiation therapy is helpful in some types of cancers. The patients are usually hospitalized for radiotherapy with Cobalt-60 (⁶⁰Co). The exposure time is worked out by the Radiation Physicist. Smaller radiation sources of Cobalt-60 (⁶⁰Co) are called seeds and needles, and are used for intra-cavity and intra-interstitial radiation therapy. The seeds or needles of Cobalt-60 (⁶⁰Co) are removed from the body cavities or interstitial tissue after the optimal time of exposure. The large sources of Cobalt-60 (⁶⁰Co) are used for 'teletherapy' or external radiation like high energy x-rays.