Absorbed Fractions for Electron and Photon Emissions in the Developing Thyroid: Fetus to Five Years Old

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Abstract
The thyroid begins to accumulate iodine at about 10 weeks of gestation. At this time the mass of the gland is about 20 mg; it increases to 1300 mg at birth. The dosimetric methodology currently applied to radioiodines by the International Commission on Radiological Protection is largely that formulated for the adult worker; that is, electrons are considered to be non-penetrating radiations and hence deposit their kinetic energy at the site of emission. This conservative approach overestimates the dose to the thyroid in the young; the degree of overestimation depends on the size of the gland and the energies of the emitted radiations. In a similar manner, the energy of the secondary electrons liberated by photon interactions within the gland are assumed to be absorbed at the site of photon interaction; that is, kerma is equated to absorbed dose. The validity of this approach is questionable for small thyroid glands. The absorbed fraction in the thyroid for monoenergetic electron and photon emitters uniformly distributed in the gland has been calculated. For photon emissions, the calculations were performed using both the kerma approach and coupled photon-electron transport. The mass of the thyroid in these calculations ranged from 20 to 3450 mg. The results demonstrate the need for detailed radiation transport calculations for small thyroid glands; that is, current methodologies result in errors in excess of factor of two.

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