Radioactive material gains entry into the body by three principal routes: inhalation, ingestion, or absorption from contaminated mucous membranes or abraded skin. Misadministration of a radiopharmaceutical is a potential source of internal contamination that can occur in the hospital setting. Internal contamination becomes a major concern of the population if large amounts of radioactive material are released into the atmosphere as a result of nuclear weapon detonation, large-scale nuclear power plant accident, or even a volcanic eruption. Such events may result in inhalation of airborne radioactive material or ingestion of radioactive material deposited onto agricultural land with subsequent transfer into the food chain.
Internal contamination is medically significant because internally deposited radioactive material will continue to irradiate tissues until it decays to a stable isotope or is biologically eliminated. As with other types of radiation exposure, the dose received depends on the amount and energy of the radioactive material and the time exposed. Additionally, the route of intake into the body, the biochemical form, and the physical and biological half-life of the particular radionuclide affect the dose received from internal contamination. The biochemical nature of the radionuclide determines if it is disseminated throughout the body or concentrated in a specific organ. The term critical organ is used to describe the organ that receives the highest dose of radiation or is the site of the most significant biological damage.
IDENTIFICATION AND MEASUREMENT In contrast to external contamination, identification of the specific radionuclides that become internalized is important for determining the method of treatment. When the substance is not known, laboratory identification is possible. Internally deposited radionuclides are identified by radioanalysis of substances excreted from the body. Radiochemistry laboratories identify and quantify the specific radionuclides by analyzing swabs from nares, oropharynx, and wounds, as well as sputum, urine, and fecal specimens. This technique of internal dose assessment is referred to as an excretion method or bioassay measurement. Internal dose assessment by this method requires that all body excreta from internally contaminated patients be collected. The collection should continue for several days because repeated measurements are used to monitor the excretion rates of the contamination.
Radioactivity within the body can also be measured in vivo by a device called a whole-body counter. These detectors predominately measure gamma-emitting radiation and some high-energy beta radiation. Internalized alpha and lower-energy beta-emitting radiation do not escape from the body and are not detected by these devices. Whole-body counters are very sensitive and give false measurements if external contamination is present on the body. For this reason and because these detectors are not readily accessible, whole body counters are generally not practical in the acute setting of a radiation accident.
GENERAL TREATMENT When the contaminating radionuclide is not known, radiochemical laboratory identification of radioisotopes may take several days. Prior to obtaining identification, methods can be initiated for removal of the most commonly encountered radionuclides that are suspected for the particular accident type. As with other hazardous material, treatment is aimed at reducing absorption or hastening elimination. After all swabs have been obtained, wounds should be irrigated with physiological saline.
Pulmonary clearance of inhaled radioactive particles is not effectively enhanced by medications. If large quantities of insoluble radioactive material have been inhaled, bronchopulmonary lavage may be considered. This treatment carries the associated risk of general anesthesia and is performed more commonly in Britain than in this country.
Reduction in gastrointestinal absorption may be accomplished with gastric lavage, emetics, and purgatives. Additionally, antacids containing aluminum cause many metals to precipitate as insoluble hydroxides. Cathartics can then be administered to decrease the transit time of these precipitants.
DECORPORATION TREATMENT Once the radioactive material crosses into the extracellular fluid, incorporation has occurred and elimination is more difficult. Methods of decorporation include blocking agents, isotopic dilution, displacement, mobilizing agents, and chelation. Treatment with blocking agents reduces the uptake of a radioisotope at an organ or metabolic site by saturating the site with a stable form of the isotope. Isotopic dilution therapy involves administering large quantities of a stable form of the isotope, thus diluting the concentration of the radioisotope. In displacement treatment, a different but similar stable element is administered that will act as a competitor with the radioisotope for an uptake site. Mobilizing agents induce body tissues to release radioisotopes by increasing the natural turnover process. Chelating agents are organic compounds that provide an ion exchange matrix. The exchanging of inorganic ions results in stable nonionized ring complexes that can be excreted.
Frequently treated causes of internal contamination are radioactive forms of iodine, plutonium, cesium, and hydrogen ( T§.bl§..,19.9.-.8). Of particular importance and effectiveness are the treatments discussed below for radioiodine and alpha-emitting contamination such as plutonium.
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