Schmid, E., Regulla, D., Kramer, H-M. and Harder, D. The Effect of 29 kV X Rays on the Dose Response of Chromosome Aberrations in Human Lymphocytes. Radiat. Res. 158, 771–777 (2002).

The induction of chromosome aberrations in human lymphocytes irradiated in vitro with X rays generated at a tube voltage of 29 kV was examined to assess the maximum low-dose RBE (RBE_M) relative to higher-energy X rays or ⁶⁰Co γ rays. Since blood was taken from the same male donor whose blood had been used for previous irradiation experiments using widely varying photon energies, the greatest possible accuracy was available for such an estimation of the RBE_M, avoiding the interindividual variations in sensitivity or differences in methodology usually associated with interlaboratory comparisons. The magnitude of the linear coefficient α of the linear-quadratic dose–effect relationship obtained for the production of dicentric chromosomes by 29 kV X rays (α = 0.0655 ± 0.0097 Gy⁻¹) confirms earlier observations of a strong increase in α with decreasing photon energy. Relating this value to previously published values of α for the dose–effect curves for dicentrics obtained in our own laboratory, RBE_M values of 1.6 ± 0.3 in comparison with weakly filtered 220 kV X rays, 3.0 ± 0.7 compared to heavily filtered 220 kV X rays, and 6.1 ± 2.5 compared to ⁶⁰Co γ rays have been obtained. These data emphasize that the choice of the reference radiation is of fundamental importance for the RBE_M obtained. A special survey of the RBE_M values obtained by different investigators in the narrow quality range from about 30 to 350 kV X rays indicates that the present RBE is in fairly good agreement with previously published findings for the induction of chromosome aberrations or micronuclei in human lymphocytes but differs from recently published findings for neoplastic transformation in a human hybrid cell line.