An ion beam biology puzzle – Why is the low-energy ion beam fluence to induce cell mutation orders of magnitude greater than the cell lethal dose?

Abstract

© 2019 Elsevier B.V. Experimental observation has shown that in the use of low-energy ion beams for living biological material modification, the ion beam fluence required to induce cell mutation is orders of magnitude greater than the cell lethal dose. This seems contradictory to a simplistic perception that DNA modification should be proportional to radiation dose, and thus that high dose irradiation should cause a high degree of damage to DNA, leading to cell death, while the more limited DNA damage produced by relatively low dose irradiation should enable cell mutation. Here we suggest an answer to this puzzle from the perspectives of both physics and biology. Key points include the physical differences between high-energy ionizing irradiation and low-energy ion beam irradiation, and the cell's non-linear response to exogenous physical actions. Low-energy ion beam processes are dominated by nuclear interactions while the effects of high-energy ionizing irradiation are dominated by electronic interactions, in addition to the energy differences leading to different modification depths depending on what kind of radiation is applied. The response of living cells to ion beam bombardment is different from that of non-biological solid material, the former having a special suicide mechanism to protect the cell from being modified or mutated while the latter responds monotonically. Only when the ion beam fluence/dose increases to a level such that adjacent atoms in DNA double strands are displaced, do irreversible double strand breaks (DSBs)dominate the DNA damage so that mutation can occur. We can estimate this fluence/dose level, which turns out to be in good agreement with experimentally determined values.