The F value, the ratio of inter- to intrachromosomal interchanges, as a biomarker for densely ionizing radiation which was proposed by Brenner and Sachs (Radiat. Res. 140, 134-142, 1994) has been a matter of repeated discussion. We examined our experimental data on radiation-induced chromosome aberrations in human peripheral blood lymphocytes for the F value as measured by the ratio of dicentrics to centric rings. Because of the rarity of aberrations, the F value showed a considerable variability, with a large error range particularly in the low-dose range of low-LET radiation. However, the data showed a general trend that the F value tended to be lower for high-LET than low-LET radiations. The differential F value was more pronounced at low doses and diminished with increasing dose; the F values of all radiations tended to converge toward a similar value at high doses. The limiting F value at the lowest doses, or the F0 value, was dependent on LET for high-energy radiations that can produce an array of DNA double-strand breaks along the track of the charged particle. However, LET and dose dependence were not seen for the low-energy photons, where spatially uncorrelated random breaks were produced by independent photoabsorption events.

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