Is selective absorption of ultrasoft x-rays biologically important in mammalian cells?
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.
D000535:Aluminum; D000818:Animals; D002244:Carbon; D002470:Cell Survival / Q000528:radiation effects*; D002478:Cells, Cultured; D006224:Cricetinae; D004247:DNA / Q000528:radiation effects; D006801:Humans; D012062:Relative Biological Effectiveness; D014025:Titanium; D014965:X-Rays
D002244:Carbon; D004247:DNA; D000535:Aluminum; D014025:Titanium