The unified approach of radiation on biological matter: Action of sparsely ionizing radiation on mammalian cells

Using a wide range of published data from irradiation experiments by sparsely ionizing radiation over mammalian cells, a tangible relationship between the cellular damage; effect cross-section and radiation quality represented the spacing between ionizing events over the path of generated primary electrons; the mean free path for linear primary ionization. The maximum damage by sparsely ionizing radiation can only be established by ultra-soft x-rays from C (k) where the mean free path along the track of the cell nucleus is about 2 nm. At higher mean free path, other ultra-soft and soft x-rays become potentially harmful only with the contribution of water radicals. A simple semi-theoretical model is proposed to define absolute biological effectiveness based on effect inactivation cross section which is interrelated to the mean free path for linear primary ionization. For sparsely ionizing radiation, the model shows a feasible saturation region for the effect cross-section 5 µm2 for mean free path less than 1.8 nm. The model explains the mechanisms leading to cell death via DNA strand scissions. The effect of sparsely ionizing radiation contributes only to a fraction of (1/20) of the maximum reachable damage.