Are We Approaching the End of the LNT Model Era?

Abstract

The linear no-threshold (LNT) model for radiation-induced cancer was adopted by national and international advisory bodies in the 1950s, and has guided radiation protection policies worldwide since then. The resulting strict regulations have increased the compliance costs for the various uses of radiation including nuclear medicine. The concerns regarding low levels of radiation due to the absence of threshold has also resulted in adverse consequences. The LNT model was justified based on the concepts that low levels of radiation increase mutations, and increased mutations imply increased cancers. These concepts may not be valid. Low-dose radiation boosts defenses such as antioxidants and DNA repair enzymes. The boosted defenses would reduce the endogenous DNA damage that would have occurred in the subsequent period, and so the result would be reduced DNA damage and mutations. Whereas mutations are necessary for causing cancer, they are not sufficient since the immune system eliminates cancer cells or keeps them under control. The immune system plays an extremely important role in preventing cancer, as indicated by the substantially increased cancer risk in immune-suppressed patients. Hence, since low-dose radiation enhances the immune system, it would reduce cancers, resulting in a phenomenon known as radiation hormesis. There is considerable evidence for radiation hormesis and against the LNT model, including studies of atomic bomb survivors, background radiation, environmental radiation, cancer patients, medical radiation, and occupational exposures. Though the Commentary No. 27 published by National Council on Radiation Protection and Measurements (NCRP) concluded that recent epidemiologic studies broadly support the LNT model, a critical examination of the studies has shown that they do not support the LNT model. Another deficiency of the Commentary is that it did not consider the vast available evidence for radiation hormesis. Other advisory body reports that have supported the LNT model have similar deficiencies. Advisory bodies are urged to evaluate the evidence supporting both the sides critically and arrive at an objective conclusion regarding the validity of the LNT model. Considering the strength of the evidence against the LNT model and the weakness of the evidence for it, the present analysis indicates they would be compelled to reject the LNT model. Hence, we may be approaching the end of the LNT model era.