Schmid, E., Regulla, D., Guldbakke, S., Schlegel, D. and Bauchinger, M. The Effectiveness of Monoenergetic Neutrons at 565 keV in Producing Dicentric Chromosomes in Human Lymphocytes at Low Doses. The induction of dicentric chromosomes in human lymphocytes from one individual irradiated in vitro with monoenergetic neutrons at 565 keV was examined to provide additional data for an improved evaluation of neutrons with respect to radiation risk in radioprotection. The resulting linear dose–response relationship obtained (0.813 ± 0.052 dicentrics per cell per gray) over the dose range of 0.0213–0.167 Gy is consistent with published results obtained for irradiation with neutrons from different sources and with different spectra at energies lower than 1000 keV. Comparing this value to previously published “average” dose–response curves obtained by different laboratories for 60 Co γ rays and orthovoltage X rays resulted in maximum RBEs (RBE m ) of about 37 ± 8 and 16 ± 4, respectively. However, when our neutron data were matched to low-LET dose responses that were constructed several years earlier for lymphocytes from the same individual, higher values of RBE m resulted: 76.0 ± 29.5 for 60 Co γ rays and 54.2 ± 18.4 for 137 Cs γ rays; differentially filtered 220 kV X rays produced values of RBE m between 20.3 ± 2.0 or 37.0 ± 7.1. The results highlight the dependence of RBE m on the choice of low-LET reference radiation and raise the possibility that differential individual response to low-LET radiations may need to be examined more fully in this context.

The debate on the validity of the ratios of radiation-induced yields of chromosome aberrations, in particular the F value (dicentrics/ring chromosomes), as a chromosomal fingerprint of radiation quality is still in progress. From a recent analysis of their experimental data, Sasaki et al. (Radiat. Res. 150, 253-258, 1998) noted that despite a considerable variability in the data, the limiting F value at the lowest doses, or the F 0 value, obviously decreased with increasing LET, indicating that the LET could be a factor that determines the F value. We have reassessed here our own 13 cytogenetic data sets that cover a range of dose-averaged LET of 0.5 to 150 keV/μm in terms of this <tex-math>$F_{0}\text{-value}$</tex-math> approach, but we could not confirm such a dependence on LET at very low doses. The validity of the F value as a biomarker therefore remains questionable. For a final evaluation, scoring of a far greater number of cells at low doses would be necessary to reduce the large error ranges of F values.

A selection of published data on the ratio, F, of interchromosomal to intrachromosomal stable (reciprocal translocations and pericentric inversions) and unstable (dicentric chromosomes and centric rings) exchange aberrations in human lymphocytes has recently been presented as evidence for F values of about 15 for X and γ rays and about 6 for neutrons and α particles (D. J. Brenner and R. K. Sachs, Radiat. Res. 140, 134-142, 1994). On this basis it was proposed that low F values could serve as a chromosomal "fingerprint" of densely ionizing radiation. In the present commentary it is shown that some of the quoted data sets provide little support for this concept. It is further demonstrated that our own data, including a "head-to-head" experiment with γ rays and α particles, reveal no LET dependence, even in the comparison of F values from low-LET radiation with those from low doses of high-LET radiation. In this context it is pointed out that a change in F values cannot be expected at doses of high-LET radiation where the linear component of the dose-effect relationship for exchange aberrations prevails. Additional data for the effects of high- and low-LET radiation which have not been considered in the discussion so far confirm that support of the concept of F-ratio "fingerprinting" by experimental data is insufficient.

Human peripheral lymphocytes were irradiated with different doses of 3-MeV electrons. The dosimetry was performed according to Fricke. The frequency of different aberration types was determined and the dose-response relation was calculated. The data were fitted by least-squares regression analysis to different models. The dicentric, dicentric + centric ring, and different acentric data gave the best fit to the linear quadratic model. The RBE of 3-MeV electrons compared with 220-kV x-rays increases with increasing dose.