Low dose-rate effects of cesium-137 and iodine-125 on cell survival, cell progression, and chromosomal alterations.

ABSTRACT

Chinese hamster ovary (CHO-AA8) cells in exponential growth were exposed to graded doses of radiation from iodine-125 (I-125) or cesium-137 (Cs-137) at various low dose rates, then monitored for cell number and clonogenic integrity. Cellular kinetics and the induction of sister chromatid exchanges and chromosomal aberrations were evaluated in cells after irradiation. Dose rates within the range 3-23 cGy/h had little effect on these fast growing cells but there was significant cell killing at dose rates of 33-70 cGy/h. The relative biological effectiveness (RBE) values for I-125 relative to Cs-137, calculated from the cell growth curves and the percentage of plating efficiencies of irradiated cells as a fraction of control, were 1.28 and 1.5., respectively, and did not vary over the dose-rate range from 3 to 70 cGy/h. The percentage of mitoses with chromosomal aberrations increased as a function of dose, but showed little if any change with dose rate. Cells passing through two cell cycles after irradiation also showed dose-dependent increases in the frequency of sister chromatid exchanges. That is, viable cells continue to show the effects of low dose-rate irradiation at the DNA level long after irradiation is concluded. Frequencies of cells in mitosis (mitotic index) and of cells incorporating bromodeoxyuridine (BrdU) (cycling S phase cells) indicated that low dose-rate I-125 irradiation produced a much more profound delay than low dose-rate of Cs-137 irradiation. This more pronounced inhibition may play a significant role in enhancing the effect of low dose rate I-125 in a clinical situation.