Is there reliable experimental evidence for a chromosomal "fingerprint" of exposure to densely ionizing radiation?

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 gamma rays and about 6 for neutrons and alpha 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 gamma rays and alpha 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.