NRC Committee

WASHINGTON, DC—Even small doses of low-level ionizing radiation have the potential to cause an increased risk of adverse health risks including cancer, according to a new report from the National Research Council (NRC), a component of the National Academy of Sciences here. The report, the seventh in a series from the NRC written to advise the federal government on the health effects of ionizing radiation, for the first time includes estimates of cancer incidence (including non-fatal cancers) as well as cancer mortality. Previous reports in the series focused on cancer mortality.Figure: Richard R. Monson, MD, ScD, noted that ionizing radiation has sufficient energy to irreparably change the structure of molecules, including DNA, in cells, leading to the multistep changes that increase the risk of cancer.The committee that prepared this seventh report defined low doses as exposure from nearly zero to about 100 millisievert (mSv) of ionizing radiation. “Ionizing radiation has sufficient energy to change the structure of molecules, including DNA, within cells,” said the Chairman of the NRC committee that wrote the comprehensive 712-page report, Richard R. Monson, MD, ScD, Associate Dean for Professional Education and Professor of Epidemiology at Harvard School of Public Health. He noted that once changed, some cells cannot repair themselves, laying the groundwork for multistep changes that increase the risk of cancer. The committee focused on low-dose, low linear energy transfer (LET) ionizing radiation (such as x-rays and gamma rays) that has enough energy to break biomolecular bonds and damage DNA. Speaking at an NRC news briefing here, Dr. Monson said the committee's review of available scientific evidence—which has mushroomed since the NRC's 1990 report on the health risks of low levels of ionizing radiation (see box)—strongly supports the linear-no-threshold (LNT) risk model, which states that there is no threshold of exposure below which low levels of ionizing radiation can be demonstrated to be either harmless or beneficial. Health Risks Rise as Overall Lifetime Exposure to Low Levels of Ionizing Radiation Rises Dr. Monson said the LNT model shows that the health risks, especially the development of solid cancers in organs, rise as the overall lifetime exposure to low levels of ionizing radiation rises. But those risks are still low. The committee calculated that in a lifetime, about 42 of 100 people will be diagnosed with cancer from causes unrelated to radiation. About one cancer in 100 people (solid cancer or leukemia) could result from a single exposure to 100 mSv of low-dose ionizing radiation, the committee calculated. About one in 1,000 people would be expected to develop cancer from exposure to 10 mSv. Don't Shun Medically Necessary X-Rays Dr. Monson cautioned in a question-and-answer period that the committee did not want the public to shun medically necessary x-rays out of fear of increasing the risk of cancer. “We can reassure people that medical radiation as currently used is done for good reason,” he said. When a physician recommends medical radiation, people should not worry about exposure. Model Not Universally Accepted Dr. Monson noted in the news briefing that the LNT model of low levels of ionizing radiation and health risk is not universally accepted, and has two competing hypotheses. One dissenting camp supports the hypothesis that low doses of radiation are more harmful than the LNT model would suggest. The other dissenting camp accepts the hypothesis that the health risks are smaller than predicted by the LNT model, are nonexistent, or that low doses of radiation may even be beneficial. But Dr. Monson said that after reviewing the available evidence, the committee believes that the LNT risk model is “the most reasonable, according to our own judgment.” Asked by OT whether those who accept one of the two hypotheses other than the LNT model might find their minds changed by this highly detailed and lengthy report, Dr. Monson said, “This report stands on its own…some minds will be changed, others will not.” He said that some readers will find it persuasive, while those who choose to stick to a hypothesis other than the LNT risk model will find the report “uninformative.” The proposed initial composition of the NRC committee engendered controversy; some voiced concern that it was not balanced, and contained people who had concluded that radiation was less damaging than current regulations assume. Asked about this controversy, Dr. Monson said that some of the people who were proposed as committee members were not selected for potential conflict-of-interest reasons under rules of the Federal Advisory Committee Act. Asked if the committee that wrote the report is balanced, Dr. Monson said, “very much so.” In addition to members from the United States, the committee that produced the report includes members from Canada, the United Kingdom, France, the Netherlands, and Germany. The NRC committee concluded that about 82% of an individual's exposure to low levels of ionizing radiation comes from background radiation—radon gas and its decay products, cosmic rays that travel through the universe, terrestrial radiation from rock and soil, and radioisotopes present in food and drinking water. In the United States, people are exposed to about 3 mSv of natural background radiation yearly. Man-made radiation accounts for about 18% of exposure, and of this amount, medical x-rays and nuclear medicine account for about 79% of that manmade exposure (the dose from a chest x-ray is about 0.1 mSv), while elements in products used by consumers (tobacco, tap water, and building materials) make up about 16% of exposure to low-level ionizing radiation. Occupational exposure, fallout, and the use of nuclear fuel constitute about 5% of the man-made exposure nationwide. New Epidemiologic Information In addition to new epidemiologic information used to bolster the cancer risk estimates in this report, a wealth of new information has emerged from the biological sciences on DNA damage responses, gene and chromosomal mutations, and multistage cancer development, said committee member William C. Dewey, PhD, Emeritus Professor of Radiation Oncology at the University of California, San Francisco. Findings from the new biology such as information on DNA repair “are quite spectacular. “We looked at many, many studies,” he noted. “The biology would indicate that the linear-no-threshold response is correct.” Asked during the briefing if exposure to low levels of ionizing radiation over a lifetime could play at least a partial role in the rise of cancer incidence and mortality with aging, committee member Ethel C. Gilbert, PhD, a biostatistician in the Radiation Epidemiology Branch of the National Cancer Institute, said no. “Most of the cancers we see are not related to background radiation,” she said. Warning About Whole-Body CT Scans While Dr. Monson stressed that the committee did not want the public to shun medically necessary radiation, he strongly discouraged consumers from self-referral for whole-body computed tomography scanning as a way to screen for early signs of disease, including cancer, in the absence of symptoms. Centers that advertise such self-referred scanning—often through direct mailings—continue to spring up across the country. “I do not believe people should make medical decisions without the advice of a physician,” Dr. Monson said. His advice to anyone thinking of having a whole-body scan is to consult a physician first before electing to be screened on his or her own. Indeed, Dr. Monson noted that CT examinations result in higher doses of radiation to organs than conventional single-film x-rays. That is because during CT, scanners rotate the body in order to take a series of cross-sectional x-rays that produce a three-dimensional picture. In its report, the committee used data from a 2004 study that calculated that a single full-body CT scan results in a mean effective radiation dose of 12 mSv. In comparison, a typical mammogram has an effective dose of 0.13 mSv—almost 100 times less. According to data from this 2004 study cited in the report, a 45-year-old adult who plans to undergo 30 annual full-body CT scans in a lifetime would potentially acquire an estimated lifetime cancer mortality risk of 1.9% (almost one in 50). Sponsors The new NRC report was sponsored by the US Departments of Defense, Energy, and Homeland Security; the US Nuclear Regulatory Commission; and the US Environmental Protection Agency. New Data on Cancer Risk Models The seventh report in the National Research Council's series on low levels of ionizing radiation and human health included this new scientific information: ▪ For atomic bomb survivors, new cancer incidence data from the Hiroshima and Nagasaki tumor registries have become available (13,000 cases of solid cancers), and the number of solid cancer deaths available for analysis has nearly doubled (10,000 deaths). ▪ The availability of cancer incidence data on atomic bomb survivors has made it possible to develop estimates for 11 specific cancer sites. ▪ A newly implemented dosimetry system for atomic bomb survivors has provided a more accurate basis for evaluating the dependence of risk on dose. ▪ There are new data from studies of people exposed to radiation for medical reasons; these data have been incorporated in estimating the risks of female breast cancer and thyroid cancer. ▪ New data from studies of nuclear workers exposed at low doses of radiation were evaluated and found to be compatible with risk estimates in the new NRC report. ▪ Estimates of both cancer incidence and cancer mortality are included, not just cancer mortality data as in previous reports.

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