Inactivation of Ehrlich ascites tumor cells by heavy ions

Exponentially growing and plateau-phase cultures of Ehrlich ascites tumor cells were irradiated with heavy ions (Z greater than or equal to 20) and assayed for loss of reproductive capacity either immediately or at delayed times after irradiation. The results indicated no modification of the exponential dose response due to conditions which usually favor the repair of potentially lethal damage at low ionization density. Postirradiation treatment of the cells with beta-arabinofuranosyladenine, a DNA synthesis inhibitor known to act on PLD repair, resulted in effects similar to those observed without this drug and confirmed the hypothesis that at such high values of ionization density only lethal, unmodifiable damage can be expressed. The inactivation cross-section values calculated from the slope of the measured survival curves showed no significant correlations with commonly used parameters of radiation quality such as LET or z2/beta 2. Instead, a functional dependence on the primary ion energy was indicated, being smaller by a factor of two at low energies (less than or equal to 2 MeV/amu) compared with values at energies above 4 MeV/amu, where agreement with the morphological nuclear cross section of the culture was found. This suggests that at higher specific ion energies energetic secondary electrons contribute to the induction of lethal damage, and that interaction of damaged sites between the primary track and the track ends of delta electrons may occur. The data are therefore also discussed in terms of the "penumbra model" which emphasizes the role of delta electrons in cell killing when radiations with very high ionization density are applied.