Possible role of inner-shell ionization phenomena in cell inactivation by heavy ions.

The existence of a correlation between experimental probabilities of cell inactivation by charged particles and calculated probabilities for K-vacancy production in heavy atoms (C, N, O, P) of the DNA of cell nuclei is established. Both phenomena display a similar dependence upon the linear energy transfer (LET) of incident particles. In particular for low LET values, K cross sections for various incident ions have nearly the same functional dependence on ion-LET and for higher LET-values, K cross sections display maxima which look like those of inactivation cross sections. These characteristics are well-understood features of the K-ionization phenomenon, in particular the maxima of probability occur for projectile velocities near orbital velocities of the ejected electrons. The meaning of the observed correlation is discussed in terms of deposited energy and in the light of existing experimental results on cell inactivation by X-ray absorption at K-threshold. We consider a mechanism which has already been evoked to explain these photo-absorption experiments and which assumes that a K ionization triggers a double-strand break by Coulomb explosion and energy dissipation of Auger electrons. However, it is seen that K cross sections are important for C, N and O atoms but negligible for P atoms. Thus, the lesion considered here affects other atoms than those involved in the K-photoabsorption experiments. The lesion efficiency with respect to subsequent double-strand breakage and repair processes is not yet known, however one may suspect a direct link between DNA blunt ends possibly induced by such K ionizations and cell inactivation.