Inactivation and mutation of cultured mammalian cells by aluminium characteristic ultrasoft X-rays. III. Implication for theory of dual radiation action.

Microdosimetric distributions for aluminium K characteristic ultrasoft X-rays and 4He ion tract intersections are calculated and used to analyse recent biological results obtained with these radiations. Results on inactivation and mutation-induction to thioguanine resistance of both V79 Chinese hamster cells and HF19 human diploid fibroblasts in vitro are analysed in terms of the Kellerer-Rossi "theory of dual radiation action". The small quantum energy of the aluminium X-ray photons and the very short length of the secondary electrons which they produce highlight the inadequacy of the model. It is shown that the model predicts r.b.e. values in conflict with those observed unless an additional variable is introduced, but that the introduction of such a variable creates mathematical inconsistencies. The experimental evidence is contrary to the conventional usage and basis of the model.