Young, adult, female C57BL/6J mice received a single injection of 55FeCl3 of high specific activity or equivalent amounts of cold iron. The animals were kept for duration of life to study late effects of 55Fe. The strain was selected because of known low tumor incidence and long life expectancy. The median survival times were 27, 117, and 439 days after treatment with 2.8, 1.4, and 0.7 mCi per mouse, respectively, and thus were inversely related to dose. Median survival in controls was more than 700 days. No tumors occurred in mice that survived less than 300 days. Six osteosarcomas developed in 14 mice that survived 55Fe treatment for more than 300 days. In addition, six other neoplasms were diagnosed: two leukemias, two thymic lymphomas, one hemangioendothelioma, and one reticulum cell neoplasm. Control mice had not reached their median survival time by Day 700 after treatment, and the only tumor noted was a reticulum cell neoplasm.

Radioiron was autoradiographically demonstrated in bone surfaces, bone marrow macrophages, and endosteal cells. Since osteosarcoma is believed to originate from the endosteum, it is conceivable that deposition within the target cell of 55Fe with its precisely located Auger electron radiation was instrumental in inducing neoplasms. Thus, identification of the cell at risk may be possible. Alternatively, but not exclusively, sufficient radiation for tumor induction may have been accumulated by the X-ray component of bone surface-seeking 55Fe.

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Research supported by the United States Energy Research and Development Administration and by NIH Grants HL 15685-03 and R01 15237-02. The research described in this paper involved animals maintained in animal care facilities fully accredited by the American Association for Accreditation of Laboratory Animal Care.

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