"CT is generally considered to have a very favorable risk-to-benefit profile among symptomatic patients," Smith-Bindman and her colleagues wrote. "However, the threshold for using CT has declined so that it is no longer used only in very sick patients, but also in those with mild, self-limited illness who are otherwise healthy. In these patients, the value of CT needs to be balanced against this small but real risk of carcinogenesis resulting from its use."
In the study by Berrington de González and colleagues, the risk of future cancers from CT scans in 2007 was estimated using risk models based on the National Research Council's "Biological Effects of Ionizing Radiation" report.
Of the 29,000 projected cancers -- or about 2 percent of all cancer diagnoses -- most were expected to be caused by scans of the abdomen and pelvis (14,000), chest (4,100), and head (4,000). About 2,700 would be caused by CT coronary angiography.
The most commonly caused cancers would be those of the lung (6,200) and colon (3,500), as well as leukemia (2,800).
Acknowledging the estimates involved several assumptions, the researchers wrote that "further work is needed to investigate the balance of the risks and benefits from CT scan use and to assess the potential for dose or exposure reduction."
But in an interview, Dr. James Thrall, radiologist-in-chief at Massachusetts General Hospital and chair of the American College of Radiology's board of chancellors, said the projections in the Berrington de González study are likely overestimates, he cautioned, which could scare patients and physicians away from using necessary CT scans.
"Radiation should never be given unnecessarily," said Thrall, who noted that he, too, believes there are too many CT scans performed each year.
"If a scan is necessary, it should be done with the lowest dose possible," he said, "and before a scan is done the reasons for doing it should be challenged and subject to appropriateness review."
He also attempted to put the 29,000 projected cancers into context. Because about a quarter of the population will develop cancer with or without radiation exposure, about 15 million people in the analysis would develop cancer anyway.
"In that context, 29,000 is a very small number when you put it up against the immediate benefits to the patients from the scans they receive," he said.