Determination of the radiation dose from administered apolipoprotein tracers in humans

Radioactive tracers are routinely used in investigation of the metabolism of apolipoprotein kinetics. Here, metabolic studies of apolipoprotein tracers labeled with radioiodine were analyzed to determine the absorbed radiation dose received by the subject. This analysis used compartmental modeling techniques to evaluate the radiation dose to various organs and the total body resulting from radioiodinated tracer injection. In this approach, we combined the published kinetic models of iodine and those of specific apolipoproteins. From the solution of the integrated compartmental models, residence times of the radiation in various source organs, in particular the thyroid, whole body, bladder, and red bone marrow, have been determined for the apolipoproteins apoA-I, apoA-II, very-low-density lipoprotein (VLDL)-apoB, and low-density lipoprotein (LDL)-apoB, each labeled with iodine 123, 133, 124, 131, 126, and 125. These tabulated values were used to calculate radiation doses to the different target organs. The thyroid is the organ that receives the largest dose of delivered radiation, and the importance of the duration of administration of iodine salts in blocking radiation to the thyroid is demonstrated. Optimal block times of 28 days for 131I and 42 days for 125I-labeled apolipoprotein tracers are proposed. When such a protocol is followed, the radiation dose to the thyroid and other organs is small by comparison to radiation doses allowed for workers whose occupation exposes them to radiation. The importance of frequent voiding to reduce the radiation dose to the bladder has also been demonstrated.