The relative biological effectiveness of photon radiation from encapsulated iodine-125, assessed in cells of human origin: I. Normal diploid fibroblasts

The relative biological effectiveness (RBE) of photon radiation from encapsulated Iodine-125 "seed" sources has not previously been investigated in human cells. The RBE of 125I photons relative to 137Cs gamma rays was examined in normal diploid human fibroblasts derived from lung and skin. The cells were irradiated in plateau phase using a specially designed incubator-irradiator which permitted simultaneous 125I and 137Cs irradiation. The cells were irradiated at various dose rates ranging from 7 to 70 cGy/hr. Dosimetry was performed using Monte Carlo computer calculations to simulate the 125I irradiations and the exposure-standardization measurements made by the U.S. National Bureau of Standards which are the basis for the specified strengths of 125I seeds. Simulation of the exposure standardization measurements revealed systematic errors due to the unrecognized presence of low-energy fluorescence X rays. The specified activity of the type of seeds used for this study (high-activity, no radiographic marker) was found to be too high by more than 10%. The RBE of 125I assessed with both lung fibroblast lines was found to be 1.2 and was 1.3 for the skin fibroblasts. The RBE did not change over the range of dose rates tested. In fact, for both 125I and 137Cs, the dose response curves did not change with dose rate over the range tested, implying full repair of sublethal damage at dose rates below 70 cGy/hr in these non-dividing cells.