Radioisotopes & Radiopharmaceuticals

Isotopes Isotopes are atoms of an element having the same number of protons and different number of neutrons. Alternatively Isotopes may be defined as the atoms of an element which have the same atomic number but different atomic masses or mass numbers. Example: Hydrogen has three isotopes. Two major types of isotopes are found in nature. Stable isotopes maintain their elemental integrity and do not decompose to other isotopic or elemental forms. Unstable or radioactive isotopes decompose or decay by emission of nuclear particles, into other isotopes of the same or different elements. Carbon has five isotopes. Two are stable: Three are radioactive: .

Nature of Radio active Isotopes All elements having atomic weights above 222 are radioactive. All radioactive compounds have the common property of emitting rays or particles Natural radio active isotopes emit alpha particles, beta particles and gamma rays. Artificial radio active isotopes emit mainly beta and gamma rays. An isotope is said to decay when radiations are given off. These are classified as follows: Alpha particles Beta particles - Negative particles or Negatrons - Positive particles or Positrons Gamma rays K capture Decay

Units of Radioactivity The fundamental unit of radioactivity is the Curie(Ci) Curie is defined as the amount of radioactive material in which the number of disintegrations per second is 3.7  1010 1 Curie (Ci) = 3.7  1010 disintegrations per second (dps) 1 Mill curie (mCi) = 3.7  107 disintegrations per second (dps) 1 Micro curie (Ci) = 3.7  104 disintegrations per second (dps) Rutherford unit = 1  1010 disintegrations per second (dps) Mache unit = of 1 Micro curie of radium emanation/water In the SI unit system, the radio activity unit is 1 nuclear disintegration per time unit(s), 1 dps, and is called Becquerel (Bq).The unit 1 disintegration per minute, dpm, is often used. 1 Curie (Ci) = 3.7  1010 Bq.

Biological Effects of Radiation Cellular effects: Essential enzymes are inactivated Proteins are coagulated Nucleic acids in the genetic apparatus are damaged and Histamine like substances are produced. Skin Damage: Erythema Brittleness and dryness of skin Loss of hair and Burns Somatic effects: Cataract, severe anemias, leukemia and cancer. Genetic effects: Damage chromosomes Increase the frequency of gene mutation

Criteria of Safe Isotopes The safest radioisotopes to use and handle are those in which-  The particles and rays have relatively low energy  The physical half life is short e.g. (2.3h) (Technetium) (6h)  The biological half life is short.

Toxicity of Radioactive Isotopes Radionuclides have been classified according to their toxicity by the International Atomic Energy Authority (IAEA) in the following ways: Class 1 Radionuclides (High Toxicity) This includes alpha-emitters. Because of the high linear energy transfer from alpha particles considerable damage is produced in tissues near to the site of the ingested isotope. Radium and Plutonium are examples. Class 2 Radionuclides (Medium Toxicity-Upper Sub-group A) Isotopes with a relatively long effective half-life e.g. Isotopes that concentrate in a particular organ or tissue and therefore irradiate it heavily, e.g.: which concentrates in the thyroid gland. which are deposited in bone.

Toxicity of Radioactive Isotopes Class 3 Radionuclides (Medium Toxicity-Lower Sub-group B) This includes most of the radioactive isotopes used in medicine. e.g.: Class 4 Radionuclides (Low Toxicity) This includes isotopes with very weak emissions or a short effective half-life. 91Y

Half-Life of Radioisotopes  Physical Half Life The time taken for half of the radioactive nuclei to disintegrate (i.e. for the activity to fall to half of its original value) is known as the half-life (t1/2).  Biological Half Life Biological half-life is a term used to describe the rate of elimination of a radionuclide from the body. It is the time taken for 50% of the radio active nuclei to be eliminated.  Effective Half Life The term effective half-life is some times used in connection with the duration of radioactivity in the body following intake of an isotope. It describes the decrease in activity due to both physical and biological half-lives.

Isotopes in Medicine  Radioisotopes are used in medicine in two different ways: - as radiation sources or - as radioactive tracers  As radiation sources their principal role is in therapy.  As radioactive tracers they are used mainly for diagnostic purposes.  A radiopharmaceutical is a preparation, intended for in vivo use, containing a radionuclide in the form of an element, a simple salt or a complex and may exist in the form of a solid, liquid or gas.  Radiopharmaceuticals are used for diagnosis and therapy.  For diagnostic applications, a radiopharmaceutical should not be pharmacologically active in that it should not produce a physiological effect.  For therapy,the radiopharmaceutical preparation contains enough radioactivity to produce the intended specific changes in tissue.

Radio Pharmaceutical Preparations Chromium-51 Sodium Chromate Cr-51 Injection: It is used as a biological tracer to measure circulating red cell volume, red cell survival time and whole blood volume. Its usual dose (intravenous) is 100 to 200 Ci. Cobalt –57 and Cobalt-60 Cyanocobalamin Co 57 capsules and solution Cyanocobalamin Co 60 capsules and solution They are used as diagnostic aid to study the absorption and deposition of vitamin B12 in normal individuals and in patients with megaloblastic anemias.usual oral dose is 0.5 Ci. Iron-59 Ferrous Citrate Fe-59 Injection It is used as a diagnostic aid for the evaluation of the kinetics of iron metabolism.The usual intravenous dose is 5 to 10 Ci.

Radio Pharmaceutical Preparations Iodine - 125 and Iodine - 131 Sodium Iodide 125 Solution Sodium Iodide 131 Capsules and Solution They are used as diagnostic aid to determine thyroid function. The usual oral dose is 50 to 100 Ci. Technetium - 99m Technetium Tc 99m Injections They are used for the diagnostic study of lungs,bone imaging and renal imaging.Usual intravenous dose is 10-15 Ci.

Radiopaque Contrast Media Radiopaque media are chemical compounds containing elements of high atomic number which will stop the passage of X-rays. These types of compounds are used as diagnostic aids in radiology. The most common radiopaques contain barium and iodine. Barium sulfate is the agent of choice in radiographic studies of the gastrointestinal tract. The major criterion of Barium sulfate is it’s insolubility in acidic gastric juice. The major side effect associated with the use of Barium sulfate is constipation. The usual oral dose of Barium sulfate is 200-300g.

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