Biophysical studies with high-energy argon ions. II. Determinations of the relative biological effectiveness, the oxygen enhancement ratio, and the cell cycle response

Chinese hamster cells cultured in vitro were used as a test system to investigate the radiobiological effects of 429 MeV/amu argon ions. Samples were irradiated at four positions in the depth dose profile. The first was located on the entrance plateau, the second on the ascending portion of the depth dose curve upstream from the Bragg peak, the third as close as possible to the Bragg peak, and the fourth on the ''tail'' beyond the peak. For a surviving fraction of 0.1, the relative biological effectiveness is about 2.4 for the plateau, ascending region, and the tail of the depth dose curve, and has a lower value of about 1.5 at the peak. The oxygen enhancement ratio is high, in excess of 2 in the incident plateau, and falls to about 1.3 close to the Bragg peak. Cells were irradiated with x rays or Bragg-peak argon ions at various times after synchronization with hydroxyurea. A 17-fold variation in survival through the cycle with x rays was reduced to a 2-fold variation with argon ions