MICRONUCLEI INDUCED BY FAST-NEUTRONS VERSUS CO-60 GAMMA-RAYS IN HUMANPERIPHERAL-BLOOD LYMPHOCYTES

Here we compared the effectiveness of neutrons ([E] = 5.5 MeV) versus Co-60 gamma-rays in producing micronuclei (MN) in human lymphocytes. T o obtain dose-response data, blood samples of six donors were irradiat ed with doses ranging from 0.1 to 5 Gy for gamma-rays and 0.1-3 Gy for neutrons. A linear dependence of MN yield with dose was found for fas t neutrons while for gamma-rays a nonlinear dependence existed. For bo th radiation qualities no significant interindividual differences were found. Derived relative biological effectiveness Values decreased wit h increasing dose. The MN frequency distributions were overdispersed w ith respect to the Poisson distribution, with neutrons showing higher dispersion values than with gamma-rays. To compare the repair kinetics of both radiation qualities split-dose experiments were performed. A dose of 4 Gy gamma-rays (3 Gy neutrons) was delivered either as a sing le exposure or in two equal fractions separated by time intervals rang ing from 30 min to 10 h (30 min to 7 h for neutrons). The data showed for gamma-rays a significant decline (30% +/- 10%) in MN yield with in terfraction time due to repair of DNA damage. This repair is a continu ous process starting almost immediately after the first of the two dos es and lasting 3-5 h. For fast neutrons no decline was observed indica ting irreparable damage.