SUMMARY A wide background of information has made it possible to predict, with some degree of confidence, the carcinogenic response of mice to bone-seeking radionuclides provided the test animal is a 70-day-old CF1 female and the test material is given in a single, intravenous injection. This paper describes five current experiments in various stages of completion in which some of these conditions were altered. A single injection of Sr⁹0 and of Ca45 is compared with 5 and 20 fractional injections, as well as with continuous ingestion. Continuous exposure to a radioactive diet from the time of conception is compared in female and male mice, and exposure of females from conception is compared with exposure starting at about 150 days of age. Fractional injection and continuous ingestion change the irradiation pattern that follows a single injection by providing uniform distribution of the dose in space and in time. The opposite approach is to deliver a dose that is limited in space and in time. Time limitation is being studied by use of Y⁰, which has a very short half-life, and by exposure of the hind legs alone to carcinogenic doses of X rays. Nat Cancer Inst Monogr 14: 243-270, 1964.

THE CARCINOGENICITY of radionuclides that may gain entrance into the body is related to certain of their physical and chemical properties. In producing bone cancer by radioactive materials that localize in the skeleton, alpha-emitters are more effective than beta-emitters, strong beta rays are more effective than weak beta rays, and isotopes with halflives measured in weeks, months, or years are more effective than isotopes with very short half-lives. Knowledge of these relationships and others associated with the daughter products of radioactive decay has made it possible to predict the carcinogenicity of untested bone-seeking isotopes under a set of standard conditions; these include the administration of the radionuclide in a single intravenous injection to 70-day-old CF1 female mice.