We have examined whether mammalian cells in vitro can be protected against the lethal effects of irradiation by Auger electrons emitted from DNA-incorporated125 I. Chinese hamster V79 lung fibroblasts were cultivated in the presence of <tex-math>$5\text{-}[{}^{125}{\rm I}]{\rm iodo}\text{-}2^{\prime}\text{-deoxyuridine}$</tex-math> (<tex-math>${}^{125}{\rm IdU}$</tex-math>) for 18 h and resuspended in ice-cold medium in the presence or absence of 10% dimethyl sulfoxide (DMSO). DNA-incorporated125 I activity was measured and the cells were plated for survival. A portion of the cell suspensions were also stored on ice to accumulate125 I decays for 6 to 48 h, after which the cells were plated to determine survival. Storage on ice up to 48 h without radioactivity reduced plating efficiency from 67 ± 4% (SEM) to 20 ± 1%. DMSO had a protective effect on colony formation, as the respective cloning efficiencies were 83 ± 3% and 72 ± 12% at 0 and 48 h. The survival curves for <tex-math>${}^{125}{\rm IdU}\text{-labeled}$</tex-math> cells are exponential with <tex-math>$D_{0}=36\pm 2$</tex-math> decays per cell in the absence of DMSO and 195 ± 20 decays per cell in the presence of DMSO. Thus the dose modification factor (DMF) at 37% survival for 10% DMSO is 5.4 ± 0.6 for DNA-incorporated125 I. In reference experiments, a DMF of 2.5 ± 0.8 was measured for cells irradiated with137 Cs γ rays. These results indicate that the radiotoxicity of Auger electrons from125 I decay in mammalian cells is caused mainly by an indirect mechanism(s).

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