The risk of cancer associated with popular CT scans appears to be greater than previously believed, according to two new studies published Monday in the Archives of Internal Medicine.

The findings support caution against the overuse of CT scans and other medical technologies that use radiation. The studies also bolstered the rationale behind controversial new breast-cancer screening guidelines, which pushed back the recommended age for annual mammograms to 50 from 40. Mammograms also use radiation, but in smaller doses.

The CT — short for computerized tomography scan — can detect injuries and tumors. Its use has tripled in the U.S. since the early 1990s to more than 70 million in 2007. Though it has long been known that radiation increases a person's chance of getting cancer, the exact risk of these scans wasn't clear.

One of the studies, which examined more than 1,000 adult patients at four hospitals, projected that the dose of radiation received in a single heart scan at age 40 would later result in cancer in 1 in 270 women and 1 in 600 men.

Risks were lower for those who received a head CT scan: 1 in 8,100 women and 1 in 11,080 men would likely develop cancer from the radiation, the study said.

Doses of radiation from the scans varied wildly, according to the study, even within the same procedure at the same hospital.

Some patients got only one-tenth the radiation that others got, according to Rebecca Smith-Bindman, the first author on the study and a professor of radiology and biomedical imaging and epidemiology and biostatistics at the University of California San Francisco.

The findings raise questions about why radiation doses differ, and whether the variation is acceptable. "These are doses we should be concerned about," said Dr. Smith-Bindman. "They don't have to be this high."

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