Internal radionuclide dosimetry: diagnostic and therapeutic nuclear medicine, occupational and environmental exposures. Differences and similarities.

In diagnostic nuclear medicine, model-derived effective dose estimates have been considered adequate for risk estimates for various patient groups. Average anthropomorphic models (normally MIRD models) and representative biokinetic models are used, with the main uncertainty being due to limited information on the biokinetics of the substance in representative groups of patients. In nuclear medicine therapy it is necessary to make patient-specific absorbed dose estimates, especially to dose-limiting risk organs and to the tumor tissue. Together with information on the time-activity curve (which may differ for the low test activity and the high therapeutic activity) in different organs and tissues, there is a need for detailed anatomical information, normally collected through CT- and/or MR-imaging through the body volumes of interest. The wish to get the radionuclide localized in the tumor cells and preferentially in the cell nuclei makes it essential to consider the increased biological effect resulting from the nonuniform distribution of the absorbed energy in tumors as well as in dose-limiting organs such as bone marrow, liver, and kidneys. The situation in occupational and environmental internal dosimetry resembles that of diagnostic nuclear medicine. However, biokinetic models derived for the former purposes are often constructed for relatively long-lived isotopes, and cannot be used for the short-lived isotopes of the same element, which are used in diagnostic nuclear medicine. Similarities and differences in objectives and methods for dosimetry in the different areas are discussed.