Biokinetics of iodide in man: Refinement of current ICRP dosimetry models

A compartmental model describing the distribution and retention of radioactive iodide in thyroid and other organs is presented. The model is developed from published ICRP models. It is designed primarily for radiation dosimetry of iodine radionuclides used in nuclear medicine, but may also be useful for occupational radiation protection. In the proposed model, the distribution of iodide to the thyroid is assumed to be more rapid than in earlier models. Uptakes in stomach wall and salivary glands are considered, and the absorbed doses to these organs calculated. The partitioning of iodide between stomach wall and content is also discussed. Recirculation of organic iodine is also taken into account. Age-dependent half-times for iodide in the thyroid, as well as for organically-bound iodine are presented. The proposed model is applicable for dose estimations with different uptakes in the thyroid as well as for the situation when the thyroid is blocked, completely or incompletely

A generic model for C-11 labelled radiopharmaceuticals for imaging receptors in the human brain

A large number of rachopharmaceuticals labelled with C-11 (half-time 0.340 h) are being developed for positron emission tomographic studies of different types of receptor in the human brain. For most of these agents, the available biokinetic data are insufficient to construct realistic compound-specific biokinetic models for calculating the internal radiation dose delivered to persons undergoing investigation. A generic model for brain receptor substances that predicts the internal dose with sufficient accuracy for general radiation protection purposes has, therefore, been developed. Biokinetic data for 13 C-11-radiopharmaceuticals used clinically for imaging different brain receptors indicate that, despite differences in chemical structure. their uptake and retention in the human brain and other tissues are broadly similar. The proposed model assumes instantaneous deposition of 5% of the injected radioactivity in the brain, with the remaining radioactivity being rapidly and uniformly distributed throughout all other tissues. Elimination from all tissues is assumed to occur with a half-time of 2 h. It is further assumed that 75% of the injected C-11 is excreted in the urine, and 25% via the gall bladder, with a half-time of 2 h. This model yields all effective dose of 4.5 X 10(-3) mSv MBq(-1), with doses of 3.2 X 10(-2), 1.7 X 10(-2), 8.7 X 10(-3), 5.2 X 10(-3), and 3.8 X 10(-3) mGy MBq(-1) to the urinary bladder, gall bladder, kidneys, brain and ovaries, respectively. These closes are well within the range of those reported using compound-specific models for the radiopharmaceutals studied