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

Based on the findings, reported in the December 14 to 28 Archives of Internal Medicine, the study authors, joined by Archives editor Dr Rita F. Redberg, are calling on clinicians to limit radiation exposure to patients.

The findings support caution against the overuse of CT scans and other medical technologies that use radiation. The studies also boosted the grounds behind 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.

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.

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.

Smith-Bindman's team collected data from 1,119 patients who received 11 common types of CT scans. For each type of CT scan, the dose of radiation varied widely within and across hospitals.

The cancer risk was greatest for young patients, this study found.

In a separate analysis, Amy Berrington de Gonzalez, an investigator in the Radiation Epidemiology Branch of the National Cancer Institute's Division of Cancer Epidemiology and Genetics, projected future cancer risk from current CT use according to age, sex and scan type.