Diagnostic X-rays and increasingly popular CT scans are linked to a slight increase in cancer risk, according to a new study

The tests are commonly used to diagnose disease or broken bones and show real medical benefits.

Researchers have previously reported a slightly increased risk of cancer associated with X-rays.

An X-ray to diagnose osteoporosis

The study in Friday's issue of the medical journal The Lancet is the first detailed attempt to estimate the number of cancer cases from the equipment's ionizing radiation.

Berrington de Gonzalez of Cancer Research U.K., and Sarah Darby of Oxford University looked at data from 14 industrialized countries.

By analysing the number and type of X-rays performed and the doses of radiation, the pair estimated the number of cancer cases linked to the tests.

The research indicates the cancer risk ranges from 0.6 to 3.2 per cent, depending on the frequency of tests.

In Canada, they estimate the radiation cancer risk is 1.1 per cent, leading to nearly 800 cases of cancer every year.

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In Britain, where X-rays are used least frequently, the authors estimated the cumulative cancer risk for those under 75 accounted for about 700 of the nation's 124,000 annual cancer diagnoses. The U.S. risk is 0.9 per cent, which translates to 5,695 cases per year.

"Our calculations depended on a number of assumptions, and so are inevitably subject to considerable uncertainty," the researchers wrote.

"The possibility that we have overestimated the risks cannot be ruled out, but it seems unlikely that we have underestimated them substantially."

Epidemiologist Richard Gallagher of the BC Cancer Agency agrees the study probably overestimates the number of cases of cancer triggered by diagnostic X-rays.

Nonetheless, Gallagher applauded the authors for raising awareness of how some cancers are caused by our attempt to treat, screen for and diagnose other medical problems.

The authors themselves stressed X-rays and CT or CAT scans are an extremely important tool for doctors and their patients.

Radiology experts say the bottom line is physicians should be careful to only order X-rays when necessary.

In a commentary on the study, Dr. Peter Herzog and Dr. Christina Rieger of Ludwig-Maximilians University in Munich, Germany, noted the researchers did not assess the benefits of the tests.

They say as lower-radiation equipment is developed, the low risks will be further reduced.