The Dose Rate Dependence of the Relative Biological Effectiveness of 241 Am versus 226 Ra g Rays

Abstract

The survival of Chinese hamster cells exposed to 59.5 keV241 Am .gamma. rays was compared with that obtained after exposure to 226Ra .gamma. rays. The Fricke dosimeter in conjunction with the calculational techniques of transition-zone dosimetry was employed to determine the dose rates to the cells at the petri dish/growth medium interface. The dose rates to the cells ranged from 11 to 133 cGy/h. In all cases, cell survival versus dose was best described by a simple exponential function of dose. For both radiations, graphs of Do versus dose rate show complex but similar patterns of peaks and valleys. As the curve for 241Am is displaced toward lower dose rates compared with that for 226Ra, the relative biological effectiveness of 241Am vs 226Ra varies considerably with dose rate, ranging from 1.7 at 20 cGy/h to 1.1 at 40 cGy/h to 1.6 at 50 cGy/h. This phenomenon may be due to the LET-dependent accumulation of cells at the G2 + M interface in the cell cycle. The mean unrestricted track-average LET of 241Am (3.7 keV/.mu.m) is 12 times higher than that for 226Ra (0.31 keV/.mu.m) but only one-fifth that of carbon ions (18 kevV/.mu.m) for which G2 + M pile-up is observed. Application of the in vitro data derived from this study to the clinical situation, where the dose rate decreases rapidly with distance from the source, suggests that, dose for dose, 241Am will produce results little different from those obtained with 226Ra.