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 indivi dual irradiated in vitro with monoenergetic neutrons at 565 keV was examine d to provide additional data for an improved evaluation of neutrons with re spect to radiation risk in radioprotection, The resulting linear dose-respo nse relationship obtained (0.813 +/- 0.052 dicentrics per cell per gray) ov er the dose range of 0.0213-0.167 Gy is consistent with published results o btained for irradiation with neutrons from different sources and with diffe rent spectra at energies lower than 1000 keV, Comparing this value to previ ously published "average" dose-response curves obtained by different labora tories for Co-60 gamma rays and orthovoltage X rays resulted in maximum RBE s (RBEm) of about 37 +/- 8 and 16 +/- 4, respectively. However, when our ne utron data were matched to low-LET dose responses that were constructed sev eral years earlier for lymphocytes from the same individual, higher values of RBEm resulted: 76.0 +/- 29.5 for Co-60 gamma rays and 54.2 +/- 18.4 for Cs-137 gamma rays; differentially filtered 220 kV X rays produced values of RBEm between 20.3 +/- 2.0 or 37.0 +/- 7.1. The results highlight the depen dence of RBEm on the choice of low-LET reference radiation and raise the po ssibility that differential individual response to low-LET radiations may n eed to be examined more fully in this context. (C) 2000 by Radiation Resear ch Society.