|Definition of Radiopharmaceuticals|
Radiopharmaceuticals have been defined as radioactive drugs that, when used for the purpose of diagnosis or therapy, typically elicit no physiological response from the patient. This definition is strongly supported by the Nuclear Medicine community's collective experience in administering radiopharmaceuticals: most practitioners, in their entire careers, have not observed a physiological response or an adverse reaction following administration of a radiopharmaceutical.
The design of these compounds is based solely upon physiological function of the target organ. Unlike radiographic procedures, which depend almost entirely upon tissue density differences, external imaging of radiopharmaceuticals is essentially independent of the density of the target organ. The mechanism of localization of a radiopharmaceutical in a particular target organ can be as simple as the physical trapping of particles or as sophisticated as an antigen-antibody reaction or chemisorption of an inorganic phosphate on the hydroxyapatite crystals deposited in an acute myocardial infarction.
There is a significant difference between a radioisotope (a radionuclide whose chemical form is unknown) and a radiopharmaceutical whose chemical form is usually precisely known. For example, I-123 is a radioisotope with a characteristic physical half-life. Reference to a biological half-life or an effective half-life for I-123 is meaningless since we don't know the chemical form. On the other hand, I-123 NaI is a compound with known biodistribution and clearance rates and is associated with both biological and effective half-lives.
The correct answer to the question, "What is the radiation dose delivered by I-131?" Is, "It is impossible to determine the radiation dose of a radioisotope. What is the chemical form of the I-131 compound?" Dosimetry can be calculated only if biodistribution and clearance rates are known.