US researchers found that repeated exposure to ionizing radiation from medical imaging such as x-rays, fluoroscopy, computed tomography (CT) and nuclear medicine scans can accumulate over time to substantial cancer-causing doses, and recommend that doctors and patients always weigh up the benefits of imaging against the risks.

The study was the work of lead author Dr Reza Fazel and colleagues and appears in the 27 August issue of the New England Journal of Medicine, NEJM. Fazel, a cardiologist in the Department of Medicine at Emory University School of Medicine in Atlanta, Georgia, told the media that:

"We know that the use of medical imaging procedures in the US has increased rapidly over the past three decades, resulting in higher average radiation doses for individuals."

For the study, Fazel and colleagues examined claim data covering nearly one million adults aged 18 to 65 living in five regions across the US and who were insured with UnitedHealthcare. From the claims they then estimated the overall rates of radiation exposure for each patient over a three year period covering 2005 to 2007.

Different types of imaging procedures expose patients to different doses of ionizing radiation. Each patient's claim record shows which types of procedure they claimed for, thus for each patient the researchers were able to estimate the total accumulated dose over the period they examined.

The unit of exposure they used was the millisievert (mSv). Previous research has already shown that exposure to radiation, especially to doses higher than 50 to 100 mSv, is linked to higher risk of developing solid cancers and leukemia later in life.

Fazel and colleagues used the following categories of exposure:
  • Low: under 3 mSv per year (this is about the same as what you would accumulate every year from the background radiation in the US without undergoing imaging procedures).

  • Moderate: up to 20 mSv per year (this is the 5 year annual average limit for people who work with radiation equipment).

  • High: up 50 mSv per year (this is the annual limit for people working with radiation equipment).

  • Very high: more than 50 mSv per year.
They found that nearly 7 out of every 10 adults in the study had been exposed to radiation from at least one imaging procedure during the three years of study. However, most of these were low-dose x-rays, such as in a chest x-ray.

They also found that radiation exposure from medical imaging procedures was higher in women, went up with advancing age, and 80 per cent of it occurred in outpatient settings.

The procedure that exposed patients to the most radiation was myocardial perfusion imaging, followed by abdominal, pelvic, and chest CT scans. Myocardial perfusion imaging is a "stress test" where a small amount of radioactive material is introduced to your bloodstream so a nuclear imaging device can observe your heart muscle as you rest or walk slowly and then fast.

On average, Fazel and colleagues found that effective dose of radiation from all procedures was 2.4 mSv, which is about 20 per cent less than background exposure that the average adult receives in the US from natural sources.

However, there were some instances of higher exposure, although relatively uncommon. In about 2 per cent of the patients they found doses above 20 mSv per year, and doses of over 50 mSv per year in about 0.2 per cent of the patients.

Generalizing these results to the population at large would suggest millions of adults in the US are being exposed to levels of radiation through medical imaging that exceeds the limit set for people who work routinely with radiation, and thousands of people are exposed to levels even higher than this.

Co-author of the study, Dr Brahmajee Nallamothu, a cardiologist from the University of Michigan said:

"While the risk to any individual for a single test may be small, the overall risk to the population becomes a concern if one considers the large number of these procedures being performed each year."

An important finding of the study was that it confirmed the results of an earlier study from the National Council for Radiation Protection released earlier this year.

Co-author Dr Harlan Krumholz, a cardiologist from Yale University, said:

"People need to be sure that there is value in the testing because it costs both in terms of dollars and radiation exposure."

"Patients need to ask if the imaging center is accredited, the imaging personnel are credentialed, and the protocols used are weight-based and indication-based to ensure that they receive high-quality imaging," added co-author Dr Kimberly Applegate, an Emory radiologist and radiation safety expert.

"Exposure to Low-Dose Ionizing Radiation from Medical Imaging Procedures."
Fazel, Reza, Krumholz, Harlan M., Wang, Yongfei, Ross, Joseph S., Chen, Jersey, Ting, Henry H., Shah, Nilay D., Nasir, Khurram, Einstein, Andrew J., Nallamothu, Brahmajee K.
N Engl J Med Volume 361, Number 9, pages 849-857, August 27, 2009.

Source: Emory Woodruff Health Sciences Center.

Written by: Catharine Paddock, PhD