Radiobiological characterization of the inactivating events produced in mammalian cells by helium and heavy ions

Abstract

Homogeneous populations of stationary phase and G1-phase Chinese hamster cells were irradiated in stirred suspensions placed in the plateau regions and the extended Bragg peaks of particle beams from the 184 in. cyclotron and Bevalac at Berkeley. Cell inactivation data were best-fitted to the linear quadratic expression. S S o = exp (− αD − βD 2) . The increase in RBE observed with increasing LET up to 100–150 keV/μ resulted predominantly from an increase in the single-hit mechanism (a). The double-hit mechanism (√β) increased by ∼ 10% over the same range of LET for these energetic particle beams. The ratio, RBE (spread peak)/RBE (plateau), is an important factor to maximize for therapeutic considerations. The ratio is greater than 1.0 and increases with particle Z-value up to carbon for beams whose Bragg peaks had been spread to 4 cm. Its value decreases for Ne and becomes less than 1.0 for Ar. Chemical radiosensitizers have been studied in combination with He and beavy-lon beams. OER's were measured for hamster cells irradiated in the plateaus and spread peaks and found to decrease with increasing LET. The sensitizers, metronidazole, Ro 7-0582, and Ro 7-0741, at 5 mM concentration were effective in reducing the OER's of the beams studied by ∼ 55%, ∼ 75% and ∼ 85%, respectively. Such drugs appear to have a potential benefit in radiotherapy planned with heavy ions.