However, not everyone agreed. "Trying to go down to lower doses is really almost impossible. Though I don't think we should give up the game," said Dr. David Brenner, a professor of radiation biophysics and the director of the Center for Radiological Research at the Columbia University Medical Center in New York. He suggested that advances in genetic testing could perhaps tease out vulnerable subpopulations or groups whose risk even from lower doses might be more easily quantified.

He also challenged the definition of the LNT model as a "conservative" estimate, saying it was better thought of as the "consensus" estimate. "Certain radiation biology [studies] point to lower doses being higher [risk]," he said. "It's an intermediate-risk estimate, not a conservative one."


He also said the epidemiological data were actually fairly robust. "We have a population of 30,000 A-bomb survivors who did get pretty comparable organ doses," he said.

Policy concerns

Despite the general LNT model skepticism, most panelists agreed that the guiding policy for clinicians should be ALARA, or "as low as reasonably achievable," for radiation dosage.

But they thought the big issue was how the medical radiation-cancer risk was presented to the public, with Dr. James Thrall, president of the American College of Radiology, saying the LNT model was taken to "absurd extents" by some researchers, being used to calculate cancers and lifetime risk to a patient.

For instance, in 2009, a study published in the Annals of Internal Medicine estimated that 29,000 future cancers could be attributed to CT scans performed in 2007 alone.

"The public is now being confronted with what appears to be scientific certainty on this issue," Thrall said.

McCollough also noted there were international differences in presenting the issue. In the United States, the seventh Biological Effects of Ionizing Radiation report, commissioned by several government agencies and released in 2005, argued for the LNT model, and found that "every exposure to radiation produces a corresponding increase in cancer risk."

McCollough said that in compiling the BIER report, any findings showing a hormetic effect were tossed out. However, a report by the French Academy of Sciences on CT risks, released in the same year, took into account hormetic findings. The French scientists concluded that LNT was "not based on biological concepts of our current knowledge."

"They don't endorse the linear no-threshold model. Both are experts of experts," McCollough said.

More Industry Headlines