Radioactivity, internal emitters

It is usefiil to calculate doses using the tissue weighting factor in protracted exposures (exposures over significant periods of time). Most nonexperimental companion or domestic animals do not receive protracted exposures. A major exception to this is the animal receiving repeated fractionated doses as radiation therapy, or the relatively rare animals in which radiation has been implanted so that it acts as an internal emitter of radioactivity (Harley, 2001). 

Once radioactivity is placed internally it becomes an internal emitter. Internal emitters are used infrequently either therapeutically (actually implanted in a tumor) or with accidental exposure via ingestion or inhalation or penetration through skin internal emitters may be removed only with varying degrees of difficulty (Harley, 2001). Usually, they cannot be removed simply or easily and an irreversible committed equivalent dose is said to exist. 

While the consequences of surface charging are reasonable if some electronic theory of catalysis is valid, it is less clear that charging is likely to occur in such different ways as to explain the different results of internal jS radioactivity and external irradiation with electrons. The decay of a jS emitter on the surface will, to be sure, leave behind a positive charge, but so will an ionization event produced by an external electron or proton. The exact states of the ions formed will differ at the... 

Since emission tomography requires some type of internal radioactive sources, a large number of radionuclides have been developed and produced specifically for these applications. Gamma emitters are used for SPECT, while positron emitters are used for PET. The chemical form of a radionuclide is tailored specifically for a given target (tissue, bone, or organ). Compounds labeled with radionuclides for administration to patients are known as radiopharmaceuticals. 

Studies with radioactive isotopes have been performed to facilitate a therapeutic effect - a strategy called radioimmunotherapy [124]. Radiolabels include -emitters such as Sc, and [125] (see Part V, Chap-ters 4 and 5). When compared to immuno-scintigraphy, the applied doses are higher in order to establish a cytotoxic effect. Even cells within the tumor that are inaccessible to the antibody can be killed without prior internalization (the bystander effecf). 

Heimbuch, A. M., Gee, H. Y., DeHaan, A. Jr., and Leventhal, L. 1965. The assay of alpha-and beta-emitters by means of scintillating ion-exchange resins in Radioactive Sample Measurement Techniques in Medicine and Biology, pp. 505-519. Vienna, Austria International Atomic Energy Agency. 

Only individuals likely to receive within 1 year more than 10% of the allowable dose limits are required to be monitored by the licensee. However, unless the dose is monitored, it is difficult to establish with certainty that an active user of radioisotopes may not have exceeded the 10% limit. Many licensees do monitor most users of radioactive materials by providing personnel dosimeters to measure external exposures, excluding those who only work with weak beta emitters. In order to monitor internal exposures, the licensee can perform measurements of (1) concentrations of radioactive materials in the air in the workplace, (2) quantities of radionuclides in the body, (3) quantities of radionuclides excreted from the body, or (4) combinations of these measurements. 

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