Nuclear worker data examined in new low-dose radiation health effects study

The need for low-dose data: Estimates of the health effects of radiation primarily have been based on the Life Span Study of Japanese atomic bomb survivors (begun in 1950), extrapolated—under the linear no threshold (LNT) model—to the much lower dose and lower dose rate radiation that may be encountered by workers in the nuclear industry and in sectors such as health care.

In the absence of reliable data on the health effects of low-dose exposures, the validity of the LNT model and its application in regulatory decision-making has been repeatedly and vigorously questioned, while the benefits of nuclear applications—including clean energy and medicine—have only grown more apparent. According to the new study, in the United States “the average person’s annual effective dose doubled between 1985 and 2006 and has remained elevated since, primarily owing to increases in exposure to ionizing radiation from medical imaging procedures.”

Support for ongoing research to obtain data on low-dose radiation health effects that could inform regulatory decision-making has been highlighted as an American Nuclear Society Grand Challenge and is emphasized in ANS Position Statement #41, Risks of Exposure to Low-Level Ionizing Radiation (updated in 2020). A June 2022 report issued by the National Academies of Sciences, Engineering, and Medicine (NASEM) recommended a coordinated research program led by the Department of Energy and the National Institutes of Health to investigate causal links and better define the impacts of radiation doses, dose rates, types of radiation, and exposure duration.

The results: Data from INWORKS have been published previously. “Here, we report on a major update of analyses of associations between radiation dose and mortality due to solid cancers in INWORKS, with follow-up extended by 10 or more years in each country,” state the authors of the new study.

The researchers attempted to estimate the risk of death from solid cancers based on workers’ exposure to radiation 10 years previously and estimated that the risk increased by 52 percent for every gray (Gy) of radiation that workers had absorbed. Based on their results, the authors suggest that risk estimates based on acute exposures to an extremely high dose of radiation, such as those received by Japanese atomic bomb survivors, may underestimate the cancer risks from exposure to much lower doses of ionizing radiation delivered over a prolonged period in the workplace (specifically, in this study, that exposure is from external photon sources.)

The data showed a higher risk of death from solid cancers in analyses restricted to the low dose range (0-100 mGy) and to workers hired in the more recent years of operations, when recording techniques were more complete.

“This major update to INWORKS provides a direct estimate of the association between protracted low dose exposure to ionizing radiation and solid cancer mortality based on some of the world’s most informative cohorts of radiation workers. The summary estimate of excess relative rate solid cancer mortality per Gy is larger than estimates currently informing radiation protection, and some evidence suggests a steeper slope for the dose-response association in the low dose range than over the full dose range,” the researchers concluded in part, adding that “Our study does not find evidence of reduced risk per unit dose for solid cancer among workers typically exposed to radiation at low dose rates.”

Ongoing need for research: Shaheen Dewji, an ANS member and assistant professor in the Nuclear and Radiological Engineering and Medical Physics Programs at the Georgia Institute of Technology, is a member of the NASEM Nuclear and Radiation Studies Board. She told Nuclear News that the INWORKS results “are essentially not ‘out of the ballpark’ from what has been developed previously but provide a more focused dose-response model for low-dose radiation exposures, specifically from a controlled cohort study, specifically exposed to protracted low-dose radiation.”

Dewji, who was not involved in the new study, added that it “only focuses on one aspect of radiation exposure, which is solid cancer mortality,” and that “further research is also needed in the very-low-dose and low-dose-rate ranges (around <10 mGy) which become more comparable to background exposures we receive on a population basis.”

Amir Bahadori, an ANS member and associate professor in the Department of Mechanical and Nuclear Engineering at Kansas State University, who like Dewji was not involved in the study, said, “The key for this and other cohort studies such as the Million Person Study is that the population members were mostly exposed occupationally at relatively low dose rates over years, making results more applicable to present-day occupational exposure situations than those from studies of the atomic bomb survivors. The results are broadly compatible with existing knowledge on radiation risks, considering the uncertainties associated with state-of-the-art analyses. The authors do a good job of highlighting study limitations, which in my opinion include the use of cumulative dose as the exposure metric and grouping all solid cancers as a single outcome.”

Dewji added, “While the INWORKS study is a key milestone in large-scale radiation epidemiology cohort studies, there is still much to investigate about the impacts of low-dose and low-dose-rate radiation, including from a plurality of radiation sources (external [neutron] and internalized radiation sources), confounding variables such as the effects of smoking or frequent medical procedures, measurement uncertainty and bias, and delineation of population-specific risk-response for cancer and non-cancer etiologies.

“Ongoing large-scale studies such as INWORKS further exemplify the need for ongoing research traversing radiation biology, dosimetry, and epidemiology to more holistically assess the impacts of exposure to low-dose radiation in our everyday lives. Such efforts are also critical to inform the radiological protection system.

“In the case of low-dose research, where there is still much to be learned,” Dewji continued, “we should be reminded of the wisdom of one of our most preeminent scientific pioneers in the study of radiation, Marie Curie: ‘Nothing in life is to be feared, it is only to be understood. Now is the time to understand more, so that we may fear less.’”

Questioning historical data: The new study may help researchers understand the impact that inaccurate early measurements of worker exposures might have had on low-dose radiation research and research protection. Personal dosimeter technology and monitoring practices have improved as the nuclear industry has matured. As a result, the study states, “Concerns have been expressed that errors in radiation dose estimates for workers employed in the early years of the industry’s operations could lead to biased estimates of radiation dose-cancer mortality associations.”

To address those concerns, the researchers analyzed a subset of the data including only workers hired “in more recent periods,” and found that “our overall results were not driven solely by information contributed by workers employed in the earliest years of the industry. To the contrary, after exclusion of workers hired in the earliest years of operations our estimate of the excess relative rate per Gy for solid cancer was larger than the estimate derived from analysis of the full cohort. . . . Among contemporary workers with presumably higher quality dosimetry information, the linear estimate of the radiation dose–solid cancer mortality association was larger than the overall summary estimate of association.”

The researchers did note an “important exception” in studies of higher dose exposures: A study of workers employed in plutonium production at the Soviet Union’s Mayak facilities, which reported a cancer rate “three to four times lower than the our [sic] INWORKS summary estimate and the summary estimate derived from the Life Span Study of the Japanese atomic bomb survivors.” The researchers note, “Given its size and the high magnitude of doses, the Mayak study exerted substantial influence on meta-analytic estimates of the excess relative rate for solid cancer per Gy that included higher dose studies. The reasons for differences between the Mayak study and INWORKS are unclear, but in the early years of operation at the Mayak facilities many workers were highly exposed with substantial uncertainty about their internal and external radiation doses.”

INWORKS data: According to a description published by the U.K. Health Security Agency, INWORKS is “an international epidemiological study on workers in the nuclear sector launched in 2011 and coordinated by the International Agency for Research on Cancer (IARC), which combines data from nuclear workers in the U.K., U.S., and France for pooled analysis. It seeks to gain greater knowledge relating to the risks of cancer and noncancerous diseases linked to chronic exposure to low doses of ionizing radiation at low dose rates.”

The INWORKS data pool includes 309,932 nuclear industry workers (including 40,455 women) for whom individual ionizing radiation monitoring data were collected in France, the United Kingdom, and the United States. Data were collected for some of the workers as early as 1944; the latest updates were added to the INWORKS data pool in 2016. During the monitoring period (1944–2016), 103,553 workers died, and 28,089 of these deaths were attributed to solid cancers, described in the study as including most cancers other than leukemia. Information about cause of death was obtained from death certificates.

The U.S. cohort included 101,363 workers employed at the DOE’s Hanford Site, Savannah River Site, Oak Ridge National Laboratory, and Idaho National Laboratory, and from the Portsmouth Naval Shipyard.