A quantitative comparison of the radiotoxicity in V79 cells of the DNA-incorporated thymidine analogue, 5-iodo/bromo-2'-deoxyuridine labelled with 123I (123IUdR), 125I (125IUdR) and 77Br (77BrUdR) is presented. The total number of decays in the cell nucleus corresponding to a survival level of 37% (D37) is in the ratio of 2.2:1.0:3.0 for 123I, 125I and 77Br, respectively. The number of atoms that must be incorporated to produce a certain level of survival is radically different for the three nuclides because of their very different physical half-lives. However, the radiation dose to the nucleus (DN required to produce the D37 is approximately the same for all radionuclides (about 0.8 Gy) within the experimental uncertainty limits and, therefore, appears to be a good macroscopic parameter to describe radiotoxicity provided that the Auger electron emitters have identical intracellular localisation. The dose rate to the nucleus and its variation during the experiments is also about the same in the three cases. Dosimetric analysis at the microscopic level shows for the three isotopes that the number of decays needed to produce a certain level of survival is inversely proportional to the energy deposited per decay in 5 nm diameter volumes. This leads to considerations that preclude chemical identity, electron excitation energy, or nuclear recoil energy of the daughter atom as being important to the observed effects. In addition, it reveals that each of the two steps involved in the 125I decay is equally efficient in causing biological damage and that no saturation effects are present following the decay of 125IUdR in the DNA of V79 cells. Conclusions are reached regarding repair of sub-lethal damage caused by the Auger decay and the level of minimum energy required to be dissipated in 5 nm diameter sites in order to cause cell death (threshold for lethal radiation action).
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Kassis et al. (1990) studied this question.