Cumulative exposure to x-rays and gamma rays causes cancer

A new National Academy of Science report predicts that one of 100 people exposed to the equivalent of 10 CT scans (100 mSv) will eventually develop solid cancer or leukemia, leading to death in half of the cases. Forty-two of the same 100 would develop cancer from other causes.

In the report, released June 29, the National Academy's National Research Council announced support for the linear, no-threshold (LNT) risk model. The LNT risk model says the smallest dose of low-level ionizing radiation has the potential to cause an increase in health risks in human.

The Biological Effects of Ionizing Radiation-VII (BEIR-VII) report is the seventh in a series on radiation issues published by the NAS since 1970. The last study to specifically examine x-rays and gamma rays used in medical imaging was BEIR-V in 1990. It also supported an LNT risk model, according to Dr. Richard R. Monson, chair of the National Academy committee that prepared the BEIR-VII report.

The National Research Council calls for the establishment of a threshold for safe human exposure to ionizing radiation.

The NAS news announcing the report noted that some researchers have argued that the LNT model exaggerates adverse health effects, while others have said that it underestimates harm. The new report concludes that the preponderance of evidence supports the LNT model.

Dr. Henry Royal, a professor of radiology at Washington University in St. Louis, criticized the report's conclusions. They are misleading because they fail to take into account the small risks inherent in any human behavior, he said. Royal is a member of the National Council on Radiation Protection and United Nations' Scientific Committee on the Effects of Atomic Radiation.

"Although there are risks associated with many everyday activities, we consider these activities safe. So to say that because radiation is associated with risk that this makes it unsafe at any dose is just the wrong concept," he said.

Although the NAS report took more than six years to prepare, it does not depart from previously accepted assumptions about radiation exposure and cancer risk, Royal said. The International Commission on Radiation Protection has published data estimating that a 10-rad exposure carries a one in 1000 lifetime risk of cancer. That is the same as saying a 10 mSv exposure produces a one in 100 risk.

"There is nothing new here," he said.

Monson agreed that the report does not endorse a major policy shift. Rather, radiobiological data that have accumulated over the past 15 years, including updated Hiroshima and Nagasaki atomic bomb survivor information, strengthened the council's previous conclusions, he said.

The report focused on low-dose, low-linear energy transfer ionizing radiation that is energetic enough to break biomolecular bonds. In living organisms, such radiation can cause DNA damage, leading to cancer. The report recommended more research to determine if low exposure levels contributed to other health problems, such as heart disease and stroke. These conditions are more common among people who have absorbed higher doses than the general population, it said.

The report also recognized the relevance of uncertainties associated with estimating health risks from very low level radiation exposure, as from a chest x-ray, according to Monson.

"We are basically saying that the model that is most consistent with the data is the linear, no-threshold model, but at low levels, there is uncertainty, so this is not an absolute statement," he said.

Dr. Bruce McClennan, president of the American Roentgen Ray Society, noted in a written response that radiologists are trained to follow the principle of ALARA (as low as reasonably achievable) as a guiding principle when exposing patients and staff to ionizing radiation.

"The risk of any medical procedure deserves our careful attention," McClennan said in the release. "Diagnostic radiology tests should only be performed when medically necessary. Screening tests, such as mammography, should only be performed when the benefits outweigh the risk."

Patients should not hesitate to undergo x-ray examinations or radiation treatment on the advice of their physicians, Monson said.

The NAS study defined low dose as ranging from nearly zero to about 100 mSv. The radiation dose from a chest x-ray is about 0.1 mSv. U.S. residents are typically exposed to about 3 mSv annually from natural background radiation.

Coincidentally, the FDA's Center for Drug Evaluation and Research reviewed its standards for safe exposure limits in human drug trials at the 2005 Society of Nuclear Medicine in June. According to standards set by its Radioactive Drug Research Committee (RDRC), an effective dose of 0.2 mSv from a standard chest x-ray translates into a one in a million lifetime risk of developing cancer.

At the upper range, CT and PET each involves exposures of about 10 mSv, said Orhan Suleman, Ph.D., senior science policy advisor at the FDA's drug evaluation center. A patient receiving a PET/CT scan is exposed to about 20 mSv, which carries a one in 1000 lifetime cancer risk, according to the RDRC.

For more information from the Diagnostic Imaging archives:

Top five features for future CT target clinical developments

Pediatric heart scan delivers high radiation dose

ACR protests listing of x-rays as human carcinogen

Infant radiation dose poses threat to cognitive growth

Radiation: Radiology reexamines pediatric scanning