Variation of relative biological effectiveness with dose rate for different radiation qualities

Measurement of inhibition of root growth in seedlings of Vicia faba following exposure has been used to compare the effectiveness of X- and gamma-rays of different energies from 84 keV (from 170Tm) to several MeV (from 60Co and radium) and of exposure at various dose rates. The data are compared with earlier published results from the same laboratory for irradiation with low-energy beta particles from 3H Because of the reduced effectiveness of radiation delivered at low dose-rates, when compared to 220–250 kVp X-radiation at 50–65 rad/min, exposure at 1.5 rad/hr provides, as a safety factor, a dose ratio of 1.2 for 170Tm gamma rays, 2.2 for 3H beta particles and between 1.9 and 2.8 for radium gamma rays. However, if radium gamma radiation at low dose-rates is to be taken as the reference for exposure in view of long clinical experience with these sources in radiotherapy, at 1.5–1.75 rad/hr an RBE value of between 0.9 and 1.3 must be allowed for 3H beta particles and between 1.6 and 2.3 for low energy 170Tm gamma radiation. For exposure at 46–60 rad/hr the corresponding values are 1.0 and 1.5 for 3H and 170Tm respectively.