THE CYTOTOXICITY OF DECAYS OF TRITIUM AND I-125 INCORPORATED IN DNA OF MAMMALIAN-CELLS - IMPLICATIONS FOR THE LOW-LET DOSIMETRY OF INCORPORATED NUCLIDES

To quantify the toxicity of low-LET radiation from incorporated radionuclides, we have determined the toxicity of decays of [H-3]dThd pulse-incorporated into CHO cells in early S phase, with the cells frozen for decay accumulation at 30, 120 or 360 min after the pulse. D-37 values of 1500, 2000 and 2100 decays were found by colony formation assay, corresponding to average nuclear doses of 4.6 and 2.7 Gy at the 30- and 360-min times. As D-37 for external irradiation (Co-60, 2.2 Gy/min) under these conditions is approximately 18 Gy, these results confirm the inadequacy of the dosimetry used for external irradiation to predict the biological effects of the low-LET radiation from incorporated radionuclides. We also determined the toxicity of (125)IdU administered as above, and have confirmed the previously reported finding that D-37 falls dramatically from 165 decays at 30 min to 40 decays at 360 min. Using the data for tritium to estimate the effect of the dose of I-125 low-LET radiation, we conclude that even at 30 min, most of the toxicity of the I-125 decays is due to the high-LET portion of the I-125 electron spectrum, not the low-LET portion as reported previously. (C) 1995 by Radiation Research Society