Inactivation and Mutation of Cultured Mammalian Cells by Aluminium Characteristic Ultrasoft X-rays

Summary

The induction of inactivation and mutation to thioguanine-resistance of two types of cultured mammalian cells, V79 Chinese hamster and HF19 human diploid, was studied after irradiation with aluminium K characteristic ultrasoft X-rays, helium ion track intersections of different LET, 42 MeV d-Be neutrons, and hard X- or γ-rays. The form of the dose—response curves was different for the two cell-types, and there was an overall difference in radiosensitivity, the human cells being the more sensitive to all radiations. However, for both inactivation and mutation-induction, the relative responses of both cell-types to these radiations was similar. Aluminium X-rays were considerably more effective than hard X- or γ-rays and were at least as effective as helium ions of 20–28 keV µm⁻¹, although aluminium X-rays produce tracks of very limited range (less than about 0·07 µm). Single track effects by aluminium X-rays cannot, therefore, extend beyond about 0·07 µm, and the subcellular sites involved in inactivation and mutation cannot be greater than this dimension or else the effectiveness of aluminium X-rays would be similar to that of low-LET radiations. This observation is in contradiction to models of radiation action which require relatively large sensitive sites; for example the ‘theory of dual radiation action’ requires a site diameter of about 0·4 µm to explain the shape of the dose—response curves for V79 hamster cells.