In radionuclide therapy, activity kinetics in tissues determine the absorbed doses administered and thus efficacy and side effects of treatment. The objective of this work was to derive expressions for the parameters affecting the absorbed dose to a target tissue for first-order activity kinetics. The activity uptake results from contributions from the first-pass activity flow through the target tissue preceding systemic equilibration and uptake after distribution of the administered compound in the body. The absorbed dose from uptake after equilibration is the product of the mean energy deposited per decay in the target tissue, the time integral of the plasma activity concentration, the plasma volume flow per unit target tissue mass, the probability of activity removal during passage, and the mean lifetime of activity in the target tissue. Quantitative analysis of the determinants of absorbed dose exemplarily for radioiodine therapy indicates that the high uptake often observed in Graves’ disease must be associated with high tissue perfusion and removal probability and that administration of stable iodine increases mean lifetime. For therapies with long residence times of the active compound in the blood, such as radioiodine therapy, the contribution of the first-pass is small compared with uptake after equilibration. The relative first-pass contribution is higher for agents that are rapidly eliminated from the blood pool, such as radiolabelled somatostatin analogues, and may dominate after arterial application. Understanding the determining parameters in radionuclide therapy reveals dose-limiting factors and opens up opportunities to optimise and individualize therapy, potentially improving treatment success rates.
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