Studies with encapsulated 125I sources. II. Determination of the relative biological effectiveness using cultured...

Chinese hamster ovary (CHO) cells, in either exponential growth or unfed plateau phase were exposed to graded doses of radiation from Iodine-125 or from Cesium-137 at various dose-rates and the cells were assayed for reproductive integrity. From the patterns of cell survival obtained, the relative biological effectiveness (RBE) of the emission from 125I was determined relative to 137Cs. The RBE determined using cells in exponential growth was found to have a value of about 1.2, which was independent of the level of cell survival and did not vary over the dose-rate range from 7.5 to 53 cGy/hr. Using plateau phase cells the RBE has a constant value of about 1.3 between 13 to 46 cGy/hr, but is closer to 2.0 at lower dose-rates of 5 to 7 cGy/hr.