Kinetics of 125IUdR uptake, retention, radiotoxicity, and microdosimetry in mammalian cells

Iodine-125, in the form of 5-(125I)-iododeoxyuridine, has been the subject of many investigations because of the extreme effectiveness of this Auger electron emitter in causing biological effects at the molecular and cellular levels. However, no study has been reported on the kinetics of the radiotoxicity and its relation to microdosimetric considerations. The authors examined the effects of varying the extracellular radioactive concentration, incubation period, and specific activity of 125IUdR on the incorporation of this radionuclide into the DNA of Chinese hamster V79 lung fibroblasts. The experimental results were related to the consequent alterations in the growth rate and survival and used to calculate the total number of decays occurring in the cell nucleus. The authors' data indicate that a) the mean lethal uptake at 37% survival (D37) is 0.035 rhoCicell; b) the total number of disintegrations is about 100cell; and c) using the Auger electron energy deposition patterns of 125I, the average energy deposited in the nucleus per 125I decay is estimated to be 11.2 keV. Since the nuclear volume of V79 cells is 268 μ3, the average D37 dose to the nucleus is about 67 rad; for the same cell line, the D37 for x-rays is 580 rad. Thus, 125I is about 8 times more effective than x-ray in causing the same biological end point