Radiation Exposure from Radioiodine Compounds in Pediatrics

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Abstract
The increasing use of radionuclides in diagnostic procedures and the importance of minimizing radiation exposure to infants and children necessitate accurate and detailed information concerning the doses sustained during these procedures. Although abundant data are available for dosages to adults (1.1, 35, 36) only an occasional reference is found to those received by children of various ages (12). Furthermore, much of the available information deals with exposure to the in-organic form of the radionuclide, while today many of the administered radionuclides are “tagged” to various organic molecules. Since these tagged substances are not handled by the body in the same manner as the inorganic compound, greater or less radiation exposure may result. This situation is particularly pertinent in the case of radioiodine which, because of its abundance, ease of detection, and chemical properties, has been incorporated as a tag for many organic compounds. Table III records the average whole-body radiation doses received by normal newborn infants, children of varying ages, and “standard man” for diagnostic tests employing various forms of radioiodine and, for comparison, those received during selected diagnostic radiographic procedures. Doses received by certain organs are shown in Tables IV and V. Since the amount of radioiodine used in a given test is likely to vary widely in different laboratories, the doses are calculated per micro-curie of administered compound. A short discussion of the metabolic fate of each of the compounds considered is presented, together with some necessary assumptions. Method of Calculation The radiation dose to a tissue is the sum of that received from beta and gamma radiation emanating from the radioactive material within the tissue, as well as that received from gamma radiation from sources distant from the tissue. Dose from radioactivity within the tissue: If an essentially uniform distribution of radionuclide within a tissue is assumed, and the volume of distribution is much larger than the range of the beta particles, the average absorbed dosages from beta radiation (Dβ) and from gamma radiation (Dγ) are calculated as follows (13); where 73.8 and 0.0346 are conversion factors to appropriate energy units E̅β is the average β-ray energy per disintegration in Mev. Iγ is the γ-ray dose rate constant in air in r/hr. at 1 cm. from 1 mc. ρ is the density of tissue in gm./c.c. g̅ is the average geometrical factor in cm. C0 is the initial concentration of the nuclide in the tissue in μc/gm. T is the effective half-life in days. These formulas give the radiation dose in rads. When considering doses of smaller magnitude, the use of the millirad is more convenient. A millirad is 1/1000 of a rad. E̅β and Iγ are characteristic of the particular radioactive isotope and may be obtained from various tables (10, 23); ρ for tissue is essentially 1.0.

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