Molecular biology, epidemiology, and the demise of the linear no-threshold (LNT) hypothesis.

The prime concern of radiation protection policy since 1959 has been protecting DNA from damage. The 1995 NCRP Report 121 on collective dose states that since no human data provides direct support for the linear no threshold hypothesis (LNT), and some studies provide quantitative data that, with statistical significance, contradict LNT, ultimately, confidence in LNT is based on the biophysical concept that the passage of a single charged particle could cause damage to DNA that would result in cancer. Current understanding of the basic molecular biologic mechanisms involved and recent data are examined before presenting several statistically significant epidemiologic studies that contradict the LNT hypothesis. Over eons of time a complex biosystem evolved to control the DNA alterations (oxidative adducts) produced by about 10(10) free radicals/cell/d derived from 2-3% of all metabolized oxygen. Antioxidant prevention, enzymatic repair of DNA damage, and removal of persistent DNA alterations by apoptosis, differentiation, necrosis, and the immune system, sequentially reduce DNA damage from about 10(6) DNA alterations/cell/d to about 1 mutation/cell/d. These mutations accumulate in stem cells during a lifetime with progressive DNA damage-control impairment associated with aging and malignant growth. A comparatively negligible number of mutations, an average of about 10(-7) mutations/cell/d, is produced by low LET radiation background of 0.1 cGy/y. The remarkable efficiency of this biosystem is increased by the adaptive responses to low-dose ionizing radiation. Each of the sequential functions that prevent, repair, and remove DNA damage are adaptively stimulated by low-dose ionizing radiation in contrast to their impairment by high-dose radiation. The biologic effect of radiation is not determined by the number of mutations it creates, but by its effect on the biosystem that controls the relentless enormous burden of oxidative DNA damage. At low doses, radiation stimulates this biosystem with consequent significant decrease of metabolic mutations. Low-dose stimulation of the immune system may not only prevent cancer by increasing removal of premalignant or malignant cells with persistent DNA damage, but used in human radioimmunotherapy may also completely remove malignant tumors with metastases. The reduction of gene mutations in response to low-dose radiation provides a biological explanation of the statistically significant observations of mortality and cancer mortality risk decrements, and contradicts the biophysical concept of the basic mechanisms upon which, ultimately, the NCRPs confidence in the LNT hypothesis is based.