DRASTIC G2 ARREST IN MAMMALIAN-CELLS AFTER IRRADIATION WITH HEAVY-ION BEAMS

Unsynchronized exponentially growing V79 monolayer cultures were exposed to high-energy radiation heavy-ion beams (12C, 20Ne, 40Ar) at various positions in their unmodified Bragg ionization curves. High-LET particle beams proved to be much more effective in causing a G2 + M block, measured by the method of flow microfluorometry, than low-LET radiation. The number of cells arrested in G2 + M increased linearly with dose up to at least 75% of the maximum effect. In this domain, the RBE values were LET-dependent with a maximum (RBE50 = 8.3) at about 100 keV/μm. In contrast to γ and X rays, heavy ions with high LET did not affect cell traverse through the G1 and S phase. Though the RBE values for G2 + M block are much higher than the RBEs for survival, there was a linear relationship between accumulation of cells in G2 + M and cell killing.