The following points highlight the three main purposes of radioisotopes in medical science. The purposes are: 1. Experimental 2. Diagnostic 3. Therapeutic.
Purpose # 1. Experimental:
(i) The research work needs radioisotopes to investigate absorption, mobilization, storage, and distribution of different substances in the body.
(ii) The substance under investigation is ‘tagged’ with a radioactive isotope which acts as a tracer substance. The tracer substance is detected in the body by the radiation it emits. The best method used for the detection of the radioactive isotope is Geiger-Muller counter and the less sensitive is auto-radiograph method.
(iii) The absorption, mobilization, and transport of iron have been studied by the use of radio-active iron (Fe59).
(iv) Iodine clearance of blood is calculated by radio-active iodine (I131).
(v) The radio-active phosphorus (P32) has been used for studying the formation of antibody in the reticulo-endothelial system.
Purpose # 2. Diagnostic:
(i) The volume of fluid compartments of the body is measured by radioactive isotopes.
(ii) Thyroid function is determined by the use of radio-iodine (I131).
(iii) The radioactive cobalt (Co60) is used for diagnosing pernicious anemia.
(iv) The radioactive sodium (Na24) has been used for circulator}’ studies and diagnosis of arterial diseases.
(v) The diagnosis for the localisation of brain tumours has been done by the use of radioactive di-iodo-fluorescein.
Purpose # 3. Therapeutic:
(i) The isotopes which are selectively taken up by certain tissues and emit mainly β- rays without causing any damage to the surrounding healthy tissues are only used for this purpose.
The most important radio-isotopes for the purpose are radio-iodine (I131), radio gold (Au198), and radio- phosphorus (P32).
(ii) The radioactive (I131) as a solution of Nal131 is administered in the body after a light breakfast.
The effect of a dose is observed within 3 to 4 weeks and maximum effect within 3 to 4 months.
The second dose, if required, is given at the interval of 2 to 3 months. It acts by emitting β- and γ- radiations.
This is used in the treatment of hyperthyroidism and thyroid cancers. The usefulness of I131 depends on the capacity of the thyroid gland to capture and concentrate iodine from the blood stream.
Nal131, in a very high dose, is used in the treatment of congested heart failure or angina pectoris.
I125 is more convenient than I131 because its half-life is 7 times greater than that of I131, the half-life of I125 is 57.4 days, whereas the half-life of I131 is 8.04 days.
I125 can be stored for more periods with losing lesser activity and this causes lesser damage to the gland because it does not emit P-radiation.
(iii) Radioactive phosphorus (P32) is administered in the form of sodium phosphate, either orally or intravenously.
It emits p-radiation and its half-life is 14.3 days. Its end product is S32. P32 is distributed to all tissues of the body as it has high metabolic role.
When administered, it is rapidly taken up by multiplying cells, bone marrow, spleen, and lymph nodes.
It is used in the treatment of various types of cancers. Especially, it is used in polycythemia vera and chronic lymphatic leukemia.
(iv) Radioactive gold (Au198) emits β- and γ-rays and its half-life is 2.69 days. Its end product is Hg198. It is chemically and biologically inert.
It is deposited in tissue area when injected in the body. It produces local irradiation since very little of it enters the blood.
The p-rays emitted by it are completely trapped by tissues. It is used in the treatment of malignant pleural and peritoneal effusions.
(v) Radioactive cobalt (Co60) is the most important isotope in clinical medicine. It emits both P- and y-rays and its half-life is 5.3 years.
It can be used as a substitute for sodium in interstitial surface. It has been found to be cheaper and does not require any closed container.