Product Abstract

The Electric Power Research Institute (EPRI) has performed a systematic review of recently published, peer-reviewed scientific studies in the fields of epidemiology and radiobiology that discuss health risks associated with exposure to low levels of ionizing radiation. As a result of this study, the EPRI team concludes that there is a need to re-evaluate the magnitude of dose and dose-rate effectiveness factors (DDREF), including the significant body of radiobiology data that suggests non-linear risks at low and very low doses, implying that health effects may be significantly less at low dose rates than estimated, based on currently used risk models.

Background

Four organizations recently published important reports that address health issues associated with low-dose radiation effects, which form the baseline for our knowledge on radiation health effects. The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), the International Commission on Radiological Protection (ICRP) Report 99, and the National Academy of Sciences Biological Effects of Ionizing Radiation (BEIR) VII acknowledged that there were some biological arguments against the linear no-threshold (LNT) hypothesis, but concluded that insufficient proof existed to modify the risk assessments currently based on LNT. In contrast, the French Academy report placed emphasis on cellular defense mechanisms that could eliminate damaged cells and concluded that LNT was no longer plausible.

Objective

• To evaluate the literature which was not included in the four earlier reports, with special emphasis on new information obtained since the publication of these reports.
• To determine how this new literature impacts our understanding of the health effects of low doses of radiation.

Approach

EPRI formed a scientific review committee to perform a systematic assessment of recently published studies in the epidemiology and radiobiology literature that are related to health risks associated with exposure to low levels of ionizing radiation. The committee reviewed over 200 publications as part of this effort. They evaluated the most current knowledge on the mechanisms of action associated with these cellular/molecular responses and how these responses may influence the shape of the dose-response relationships in the low-dose region.

Results

The EPRI research team has identified specific scientific advances and research needs in radiobiology, radiation epidemiology, and communication issues. The atomic bomb (A-bomb) study remains one of the most important studies about excess cancer (and non-cancer) risk. There is increasing evidence for non-linearity in the A-bomb data on solid tumors. Research progress in radiobiology has made significant contributions to our understanding of low-dose effects. Data from molecular, cellular, tissue, and whole animal studies, as well as high background human studies suggest the need for re-evaluation of DDREF. Researchers may characterize bystander effects, adaptive responses, and gene expression changes as binary or biphasic processes that suggest non-linearity in the low-dose region.

Research into low-dose radiation biology, dose reconstruction, and epidemiology should continue and should utilize systems biology approaches to develop models that define the shape of the dose-response relationships in the low-dose region. Methods to incorporate radiobiology findings into the epidemiological risk models are required. Understanding the significance of reductions or increases in DNA repair mechanisms on cancer risk will have important implications on the study of health effects of radiation exposure.

A re-evaluation of DDREF, based on the latest work in radiobiology and epidemiology, may result in reduced risk estimates, and have regulatory implications. This is especially important in low dose-rate exposures to high total doses. In addition, a reduction in risk estimates might well result in a re-evaluation of the basis for personnel dose limits, and more consistent, risk-based clean-up criteria for decommissioning activities.

EPRI Perspective

Researchers have traditionally estimated radiation health effects from high-dose studies, such as the A-bomb studies. The key question is to determine whether these results are directly applicable to the nuclear power plant situation, where doses are much lower, and dose rates are several times less. Major advances are currently occurring in improving the understanding of radiobiology.

EPRI distributed the initial results of this work to attendees of the April 2008 meeting of the National Council on Radiation Protection (NCRP). EPRI also presented the results at the July 2008 meeting of the Health Physics Society and at the August 2008 meeting of the Nuclear Energy Institute Health Physics Forum with the objective of encouraging further work in this area and supporting input to the development of future regulations.