Is selective absorption of ultrasoft X-rays biologically important in mammalian cells?

Abstract

This paper tests whether photon absorption processes in particular atomic element(s) may be responsible for the observed high relative biological effectiveness (RBE) of ultrasoft X-rays. The effectiveness of titanium K characteristic X-rays (4.55 keV) is compared with previous observations for aluminium (1.5 keV) and carbon (0.28 keV) K ultrasoft X-rays. For a given absorbed dose, five times more Ti K than Al K photons are absorbed in phosphorus; since Al K X-rays are observed to be more effective in killing human and hamster cells it is concluded that absorption in phosphorus does not play a dominant lethal role. This is supported by the observation that the absolute number of Al K photons absorbed in phosphorus of DNA of human fibroblasts is less than 1 per lethal event. For no element is the relative number of absorbed photons of the three X-ray energies even approximately proportional to their observed RBEs. The effectiveness of ultrasoft X-rays is apparently not due to selective absorption but rather to the secondary electrons; consequently the mechanism of action should be common to the large numbers of low energy secondary electrons produced by most other ionising radiations, including gamma-rays.