On the mechanism of the biological effect of ionizing radiation

The mechanisms of the biological effects of ionizing radiation (IR) and ultrasound (US) were considered. The current views on the nature of "toxicity" of IR, which is usually assigned to the formation of radicals in living tissues and to the straight-line collision of an ionizing particle with the DNA molecule, were analyzed. It was established that the amount of radicals formed in biological tissues in conditions of ultrasonically induced cavitation can be as large as that for IR; however, the biological effect of US is much softer as compared to IR. It was shown that the contribution of the indirect mechanism to the total biological effect of IR can be estimated by comparing US and IR in their chemical action; the contribution of the indirect mechanism to the biological effect of IR was found to be negligibly small. An alternative mechanism was proposed to explain the biological effect of IR. In accordance with the proposed model, IR with a high linear energy transfer (LET) value breaks through cell walls and biological membranes and causes damage to them, such that the cell can lose its regenerative capacity. Moreover, high-energy heavy ionizing particles perforate cytoplasm to form channels. Ionizing radiation with a low LET value (gamma- and X-rays) causes multiple damages to biological membranes. Ionizing particles can also cause damages to membranes of mitochondria thus affecting the mechanism of cellular respiration, which will cause neoplastic diseases. The straight-line collision of an ionizing particle with a DNA molecule was found to be 5-7 orders of magnitude less probable as compared to the collision with a wall or membrane. It was shown that multiple perforations of cell walls and damages to membranes are characteristic only of ionizing particles, which have sufficiently long tracks, and do not occur upon exposure to ultrasonic waves, microwaves, UV radiation, magnetic fields, etc.