Diagnostics & Imaging Week Washington Editor
WASHINGTON — Low doses of radiation such as X-rays can lead to cancer, according to a new report from the National Academy of Science (Washington).
Though the news may come as no surprise, there is a "preponderance of evidence" that shows that even low doses of ionizing radiation — such as gamma rays and X-rays used in modern diagnostic medical equipment — can pose "some adverse health effects," according to the report. It is the seventh in a series of reports on the biological effects of ionizing radiation.
The focus of the research was low-dose, low-linear energy transfer (LET) ionizing radiation that is energetic enough to break biomolecular bonds. In living organisms, the radiation can cause DNA damage that eventually leads to cancers, the study says.
However, the report also said that more research would be needed to determine whether low doses of radiation cause other health problems, such as heart disease and stroke, which are now seen with high doses of low-LET radiation.
The study committee defined low doses from nearly zero to about 100 millisievert (mSv), which are units that measure radiation energy deposited in living tissue. The radiation dose from a chest X-ray is about 0.1 mSv. In the U.S., people are exposed on average to about 3 mSv of natural "background" radiation annually, according to the study.
"The scientific research base shows that there is no threshold of exposure below which low levels of ionizing radiation can be demonstrated to be harmless or beneficial," said Richard Monson, chairman of the research committee and associate dean for professional education and professor of epidemiology at Harvard School of Public Health (Boston).
Monson said the health risks, particularly the development of solid cancers in organs, rise proportionally with exposure.
"At low doses of radiation, the risk of inducing solid cancers is very small," he said. "As the overall lifetime exposure increases, so does the risk."
People are exposed to natural background ionizing radiation from the universe, the ground, and basic activities such as eating, drinking, and breathing. These sources account for about 82% of human exposure.
Nationwide, man-made radiation comprises 18% of human exposure, according to researchers at the National Academies. In this overall category, medical X-rays and nuclear medicine account for about 79%, the report says. Elements in consumer products, such as tobacco, tap water and building materials, account for another 16%.
Occupational exposure, fallout and the use of nuclear fuel constitute roughly 5% of the man-made component.
Factors that could increase exposure include greater use of radiation for medical purposes, working around radioactive materials and smoking. Living at low altitudes, where there is less cosmic radiation, and living and working on the upper floors of buildings, where there is less radon gas — a primary source of natural ionizing radiation — are factors that could decrease exposure, the report said.
In general, the report findings support previously reported risk estimates for solid cancer and leukemia. The availability of new and more extensive data has strengthened confidence in these estimates, researchers said.
The committee conducting the research said its "thorough review" of available biological and biophysical data supports a "linear, no-threshold [LNT] risk model," which means that the smallest dose of low-level ionizing radiation has the potential to cause an increase in health risks.
In the past, some researchers have argued that the LNT model exaggerates adverse health effects, while others have said that it underestimates the harm.
The risk models used in the study predict that about one out of 100 people would likely develop solid cancer or leukemia from an exposure of 0.1 Sv (100 mSv). About 42 additional people in the same group would be expected to develop solid cancer or leukemia from other causes. Roughly half of these cancers would result in death.
Researchers cautioned that the estimates come with some uncertainty because of limitations in the data used to develop risk models.
Survivors of atomic bombings in Hiroshima and Nagasaki, Japan, were the primary sources of data for estimating risks of most solid cancers and leukemia from exposure to ionizing radiation.
The committee's review included an examination of updated cancer-incidence data from tumor registries of the survivors, and of research data on solid cancer deaths.
The committee combined this information with data on people who had been medically exposed to radiation to estimate risks of breast cancer in women and thyroid cancer. Data from additional medical studies and from studies of people exposed to radiation through their occupations also were evaluated and found to be compatible with the committee's statistical models. Follow-up studies should continue for the indefinite future, the report says.
The report also urges follow-up studies of people who receive computed tomography (CT) scans, especially children. Studies of infants who are exposed to diagnostic radiation because catheters have been placed in their hearts, as well as infants who receive multiple X-rays to monitor pulmonary development, also were suggested.
CT scans often result in higher doses of radiation than typically experienced with conventional X-rays.