CHICAGO (Reuters) – Radiation from CT scans done in 2007 will cause 29,000 cancers and kill nearly 15,000 Americans, researchers said this week.

The findings, published in the Archives of Internal Medicine, add to mounting evidence that Americans are overexposed to radiation from diagnostic tests, especially from a specialised kind of X-ray called a computed tomography, or CT, scan.

"What we learned is there is a significant amount of radiation with these CT scans, more than what we thought, and there is a significant number of cancers," said Dr. Rita Redberg, editor of the Archives of Internal Medicine, where the studies were published.

"It's estimated that just from the CT scans done in one year, just in 2007, there will be 15,000 excess deaths," Redberg said in a telephone interview.

"We're doing millions of CT scans every year and the numbers are increasing. That is a lot of excess deaths."

CT scans give doctors a view inside the body, often eliminating the need for exploratory surgery.

But CT scans involve much higher radiation dose than conventional X-rays. A chest CT scan exposes the patient to more than 100 times the radiation dose of a chest X-ray.

About 70 million CT scans were done on Americans in 2007, up from three million in 1980. Amy Berrington de Gonzalez of the National Cancer Institute and colleagues developed a computer model to estimate the impact of so many scans.

They estimated the scans done in 2007 will cause 29,000 cancers.

A third of the projected cancers will occur in people who were ages 35 to 54 when they got their CT, two-thirds will occur in women and 15 percent will arise from scans done in children or teens.

The researchers estimated there will be an extra 2,000 excess breast cancers just from CT scans done in 2007.

Redberg, who wrote a commentary on the studies, said US doctors' enthusiasm for the tests has led to an explosion in their use that is putting patients at risk.

"While certainly some of the scans are incredibly important and life saving, it is also certain that some of them were not necessary," Redberg said.

In a separate study, Dr. Rebecca Smith-Bindman of the University of California, San Francisco, and colleagues analysed data from 1,119 patients undergoing the 11 most common types of diagnostic CT scans at four institutions in 2008.

They found radiation dosage varied widely between different types of CT studies, from a median or midpoint of two millisieverts for a routine head CT scan to 31 millisieverts for a scan of the abdomen and pelvis, which often involves taking multiple images of the same organ.

By comparison, the average American is exposed to about 3 millisieverts of radiation a year from ground radon or flying in an airplane – a level not considered a risk to health.

The researchers said efforts need to be taken to minimise CT radiation exposure, including reducing the number of unnecessary tests, cutting the dose per study, and standardising the doses across facilities.

Imaging equipment makers such as GE Healthcare, Siemens, Philips and Toshiba Medical Systems are working to develop low-dose CT scanners.