Radiation exposure from long-lived beta emitters in clinical investigation

Potential hazards restrict the use of 14C in clinical investigation. Nevertheless, 14C, in readily metabolized compounds, continues to be administered and in relatively large doses. The compounds are converted to intermediates and products with longer half-lives than their half-lives. These longer biological half-lives have not always been taken into account, although calculations of radiation exposure can be markedly altered by their inclusion. The minimum dose of 14C needed to accomplish a study must also be considered, and benefits relative to risks in the administration must be assessed in terms of the kind and total number of subjects required. Similar considerations apply to the administration of 3H-labeled compounds. Furthermore, in assessing risks, the limited information available on the localization and concentration of the isotopes in the body should be taken into account. There are also concerns with regard to administrations in growth situations and to an individual on multiple occasions. Within those constraints, 14C and 3H can be given with relative safety.