Biological Effectiveness of Ionizing Radiation: Acute vs. Protracted Exposures

Abstract

This letter refers to the current discussion around re-evaluation of the dose and dose rate effectiveness factor (DDREF) equal to 2, presently recommended by the International Commission on Radiological Protection. The topics of the threshold, hormesis and DDREF are interrelated with the linear no-threshold theory (LNT). The LNT does not take into account that DNA damage and repair are permanent processes in dynamic equilibrium. Given the evolutionary prerequisite of best fitness, it would be reasonable to assume that living organisms have been adapted by the natural selection to the background levels of ionizing radiation. Accordingly, there must be an optimal exposure level, as it is for many environmental factors. Several studies cited in the literature in support of the LNT and lowering of the DDREF down to 1 are discussed here. In the author's opinion, the dose-effect relationships with non-neoplastic diseases found in certain exposed populations call in question dose-effect relationships with cancer. Self-selection and other biases in epidemiological studies are discussed. The dose-response relationships should be clarified in largescale experiments involving different animal species. In conclusion, the LNT and under-estimation of DDREF tend to exaggerate radiationrelated health risks at low radiation doses and dose rates. Arguments against Linear No-Threshold Theory (LNT)Radiation-related cancer risk estimates have been primarily based on the data from atomic bomb survivors. To adjust the risk estimates at acute exposures to low dose and continuous (low dose rate) exposures, a dose and dose rate effectiveness factor (DDREF) is used [1]. This letter refers to the discussion around re-evaluation of the DDREF value equal to 2, currently recommended by the International Commission on Radiological Protection (ICRP) [2]. The topics of the threshold, hormesis and DDREF are interrelated with the linear no-threshold theory (LNT). Hormesis and LNT are considered controversial by many scientists; discussion is in [3][4][5][6][7][8]. The LNT is corroborated by the following arguments: the more tracks go through a cell nucleus, the more DNA damage would result and the higher the risk of malignant transformation would be. "Decreasing the number of damaged cells by a factor of 10 would be expected to decrease the biological response by the same factor of 10" [9]. This concept does not take into account that DNA damage and repair are permanent processes in dynamic equilibrium. Given the evolutionary prerequisite of best fitness, it would be reasonable to assume that living organisms have been adapted by the natural selection to background levels of ionizing radiation [10]. Accordingly, there must be an optimal exposure level, as it is for many environmental factors: visible and ultraviolet light, different chemical elements and compounds [11], as well as the products from radiolysis of water [12]. Evolutionary adaptation to a changing environmental factor would lag behind its current value and correspond to some ...

Thyroid cancer after chernobyl: re-evaluation needed

“…Correlations between individual doses and Ret/PTC among atomic bomb survivors ( 48 ) could have been caused by a bias similar to that discussed here as well as by higher doses. Note that for the low-LET (linear energy transfer) radiation, acute exposures are generally more efficient than the same doses protracted over a long time; an overview of literature was provided previously ( 49 ). The enhanced frequency of Ret/PTC was reported in papillary carcinomas from patients who had undergone radiotherapy in childhood.…”