Comparison of Repair of DNA Double-strand Breaks Caused by Neutron or Gamma Radiation in Cultured Human Cells

Summary

The dose-response for the induction of initial double-strand breaks (dsb) in DNA of human epithelioid cells by JANUS 0·85 MeV fission-spectrum neutrons was parabolic as assayed by a calibrated neutral filter elution technique. The relative biological effectiveness (RBE) of these neutrons relative to ⁶⁰Co γ-rays was unity. The kinetics of repair after a 60 Gy γ-ray exposure were biphasic. About 65% of these dsb were rapidly repaired (T_1/2 of approximately 2 min), and the remainder were almost completely removed after 150 min at a slower rate (T_1/2 = 30 min). After the same dose of JANUS neutrons, the rapid repair component was markedly reduced (possibly not a significant repair component), and the bulk of the dsb were sealed more slowly (T_1/2 = 90 min). After 150 min, 25% remained unsealed. Even after a lower neutron dose (20 Gy), a proportion of the dsb were refractory to repair. Thus, unrepaired (or irreparable) dsb induced by high energy neutrons might explain the high RBE of neutrons for cell killing.