Chromosome aberrations induced in human lymphocytes by 16.5 MeV protons.

In a track segment irradiation experiment with 16.5 MeV protons the induction of dicentrics was studied in human T-lymphocytes. The dose-response relationship was linear quadratic with estimated parameters alpha = (0.44 +/- 0.07) x 10(-1) Gy-1 and beta = (1.95 +/- 0.30) x 10(-2) Gy-2. With respect to X-rays a limiting RBE of 1.1-1.2 exists. The present findings are compared with data from other laboratories obtained with 4.9 and 8.7 MeV protons. It was found that the data do not fit theoretical predictions on a proportional relationship between alpha values and LET. Using the microdosimetric quantity yD (25 nm), the dose average of the lineal energy, for characterization of these proton radiations, at a relevant site diameter of 25 nm the ratio of yD = 1.15 is in accordance only with the observed alpha ratio of 1.0 +/- 0.23 for 8.7 and 16.5 MeV protons. In contrast to the prediction of a constant beta at low LET values, the quadratic coefficients increase with increasing LET between 3 and 8 keV/microns.