Dosimetry for radiopharmaceuticals

Developments in internal dosimetry for radiopharmaceuticals are summarised, with special reference to work carried out within the International Commission on Radiological Protection (ICRP). Differences and similarities with internal dosimetry for occupationally exposed workers and for members of the public are identified. What is unique for radiopharmaceuticals is their special biokinetics. The products are designed to get high uptake in certain organs and tissues. When a new compound is introduced there are few long-term retention data for humans available. Therefore efforts have continuously to be made to investigate the biokinetics and dosimetry of new products as well as older products, for which the dosimetry is uncertain, e.g. pure β-emitters. Serial, quantitative gamma camera images of patients will continue to be the base for biokinetic information together with analysis of urine samples. The observed time-activity curves are described using exponential functions with specified fractional activities and half-times. The physical calculations are based on the MIRD formalism. For more detailed dosimetry, CT, MR and ultrasound can be used to localise organs and to determine their volumes. Such measurements are also needed for the construction of realistic phantoms (mathematically describable phantoms, 'voxel' phantoms and anthropomorphic phantoms) which are the geometrical base for dose calculations. Variations in anatomy and biokinetics between individuals due to age, gender and disease have to be given greater consideration in the future. Information on the distribution of a radionuclide within organs and tissues is of importance for its therapeutic use as is the intracellular localisation of low energy electron emitters both in therapy and diagnosis.