The high doses of radiation patients receive from CT scans may cause thousands of additional cancer cases each year, according to a startling new U.S. study.

The research, published in Tuesday's edition of the Archives of Internal Medicine, suggests that about 2 per cent of all cancers in the United States are caused by exposure to radiation during computed tomography scans. CT scans are performed less frequently in Canada, but may still be responsible for a significant number of extra cancer cases.

CT scans are used to diagnose various conditions, including heart blockages, colon cancer, brain tumours and pneumonia. The new research shows that the risk of developing cancer varies considerably based on the part of the body that is scanned.

Amy Berrington de Gonzalez of the radiation epidemiology branch of the U.S. National Cancer Institute in Bethesda, Md., and her team found 14,000 additional U.S. cancer cases a year related to scans of the abdomen and pelvis, 4,100 from chest scans, 4,000 from head scans and 2,700 from CT angiography. Two-thirds of the cancers were found in women.

A second study, led by Rebecca Smith-Bindman of the department of radiology and biomedical imaging at the University of California, San Francisco, helps explain these differences.

She found radiation doses vary a lot among types of CT scans: A patient undergoing a head scan will be exposed to as little as two millisieverts of radiation, while one undergoing an abdominal-pelvic scan will be exposed to 31 millisieverts.

“While CT scans can provide great medical benefits, there is concern about potential future cancer risks because they involve much higher radiation doses than conventional diagnostic X-rays,” Dr. Smith-Bindman said.

For instance, a chest CT scan exposes a patient to more than 100 times the radiation dose of a chest X-ray.

The second study also shows that future cancer risk varies tremendously based on the type of test that is performed, as well as the patient's age and gender.

For example, one in 270 women and one in 600 men who undergo a CT coronary angiography (a heart scan) at age 40 will develop cancer as a result.

But those same patients have a far lower risk of they undergo a CT scan of the head to detect an aneurysm: one in 8,100 women and one in 11,080 men will subsequently develop cancer as a result.

Dr. Smith-Bindman stressed that many CT scans are valuable but expressed concern that the tests are done with increasing frequency and with little thought to balancing the risks and benefits.

About 70 million scans were performed in the United States in 2007, up from three million in 1980.

In Canada, there were about 3.4 million CT scans performed in 2007, according to the Canadian Institute for Health Information. Proportionally, that is about half the number performed in the United States.

But, based on the new research, that would work out to about 1,500 additional cancers a year being caused by the radiation exposure.

There is also evidence that, in Canada, CT scans are being used inappropriately. In a study published earlier this year by the Institute for Clinical Evaluative Sciences, John You found that CT scans of the brain were commonly ordered for headaches, but fewer than 2 per cent of those scans revealed a treatable abnormality.

The Canadian Association of Radiologists, for its part, estimates that up to one-third of CT scans are inappropriate.

There are various forms of medical imaging that allow physicians to see inside the body.

Ultrasounds and magnetic resonance imaging (MRI) scans do not involve radiation, but provide limited detail.

CT scans and positron emission tomography (PET) scans provide far more clear images but expose patients to radiation. (So do X-rays, but to a lesser degree.)

Exposure to ionizing radiation can lead to mutations in DNA that can eventually lead to cancer.

Concerns about the negative effects of CT scans have been growing in recent years.

Last month, the National Research Council Canada announced plans to track radiation exposure of patients across the country and create a national radiation dose and exposure registry.

CT scan radiation may cause cancers
André Picard Globe and Mail Dec, 2009