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

A large number of radiopharmaceuticals labelled with 11C (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 11C-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 11C is excreted in the urine, and 25% via the gall bladder, with a half-time of 2 h. This model yields an 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 doses are well within the range of those reported using compound-specific models for the radiopharmaceutals studied.