radguide

27 Radiation for Radionuclide Users Routes of Exposure There are two principal routes by which humans are exposed to ionizing radiation, external and internal . External Exposures External exposures are those which are due to sources located outside of the body which emit radiation of a type and energy sufficient to reach and penetrate the body. Examples of penetrating radiations include gamma rays, x-rays, and high energy beta particles (e.g., as from 32 P). External exposures cannot result from sources which emit only alpha particles or low energy beta particles since these radiations do not penetrate the dead outer cell layer of the skin. As with all radiation exposures, the size of the dose resulting from an external exposure is a function of: (1) the activity of the source; (2) the time or duration of the exposure; (3) the distance from the source to the tissue being irradiated; and (4) the amount of shielding between the source and the tissue. In contrast to the situation for internal exposures, these factors can be altered for a particular external exposure situation with a resultant increase or decrease in the dose received. Internal Exposures Internal exposures are those that result from radioactive material which has been taken up by the body as a result of ingestion , inhalation , injection , or absorption through the skin. Such exposures are of concern for all radioactive materials regardless of the type of ionizing radiation emitted. Of particular concern, however, are radioactive materials which emit alpha and beta particles. These radiations cause significant damage to tissue when depositing their energy along highly localized paths. In contrast to the situation for external exposures, the source-to- tissue distance, exposure duration, and source strength cannot be altered for internal sources. Instead, once a quantity of radioactive material has been taken up by the body, an individual is essentially “committed” to the dose which will result from the quantities and forms of the particular radionuclides involved. In general, radionuclides taken up by the body do not distribute equally throughout the body’s tissues. Often a radionuclide concentrates in a particular organ or tissue type. For example, 131 I and 125 I concentrate in the thyroid, 45 Ca and 32 P in the bone, and 59 Fe in the spleen. In such cases, the affected tissue or organ sustains a