The linear nonthreshold (LNT) model as used in radiation protection: an NCRP update

Purpose: The linear nonthreshold (LNT) model has been used in radiation protection for over 40 years and has been hotly debated. It relies heavily on human epidemiology, with support from radiobiology. The scientific underpinnings include NCRP Report No. 136 ('Evaluation of the Linear-Nonthreshold Dose-Response Model for Ionizing Radiation'), UNSCEAR 2000, ICRP Publication 99 (2004) and the National Academies BEIR VII Report (2006). NCRP Scientific Committee 1-25 is reviewing recent epidemiologic studies focusing on dose-response models, including threshold, and the relevance to radiation protection. Methods and materials: Recent studies after the BEIR VII Report are being critically reviewed and include atomic-bomb survivors, Mayak workers, atomic veterans, populations on the Techa River, U.S. radiological technologists, the U.S. Million Person Study, international workers (INWORKS), Chernobyl cleanup workers, children given computerized tomography scans, and tuberculosis-fluoroscopy patients. Methodologic limitations, dose uncertainties and statistical approaches (and modeling assumptions) are being systematically evaluated. Results: The review of studies continues and will be published as an NCRP commentary in 2017. Most studies reviewed to date are consistent with a straight-line dose response but there are a few exceptions. In the past, the scientific consensus process has worked in providing practical and prudent guidance. So pragmatic judgment is anticipated. The evaluations are ongoing and the extensive NCRP review process has just begun, so no decisions or recommendations are in stone. Conclusions: The march of science requires a constant assessment of emerging evidence to provide an optimum, though not necessarily perfect, approach to radiation protection. Alternatives to the LNT model may be forthcoming, e.g. an approach that couples the best epidemiology with biologically based models of carcinogenesis, focusing on chronic (not acute) exposure circumstances. Currently for the practical purposes of radiation protection, the LNT hypothesis reigns supreme as the best of the rest, but new epidemiology and radiobiology might change these conclusions. Stay tuned!