A TRACER-KINETIC APPROACH TO ESTIMATE RADIATION DOSIMETRY OF 0-15 WATER

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Previous dosimetry calculations have assumed that 0-15 water instantaneously equilibrates in the water space of the body (Annals ICRP 53:67, 1988). The initial distribution of 0-15 water depends on organ flow however, and it takes several minutes to reach equilibrium. Because 0-15 has a short half-life, the equilibrium assumption underestimates cumulated activity and radiation dose in high flow organs. We used a one-compartment model to calculate organ cumulated activity and mean residence times from organ blood flow and 0-15 water volume of distribution. Arterial time-activity curves were used from subjects undergoing rCBF measurements with bolus 0-15 water. Radiation dose estimates were obtained in collaboration with investigators at Oak Ridge Associated Universities. In contrast with previous estimates, high flow organs had substantially greater radiation doses, e.g., heart wall 8.2 mrad/mCi (vs. 2.1 in the literature), kidney 7.2 (2.1), lung 6.9 (2.0), thyroid 6.3 (1.9). This method is physiologically realistic and predicts organ residence times and dose estimates without requiring direct activity measurements. Verification of the method will be performed against organ time-activity curves from organs imaged for other purposes (e.g. brain, heart, lung), and the approach will be extended to obtain dose estimates in pediatric age groups.