News | August 27, 2009

Exposure to Low-Dose Radiation May Result in a High Cumulative Effect

August 27, 2009 - Imaging procedures are an important source of exposure to ionizing radiation in the United States and can result in high cumulative effective doses of radiation, according to a study published in the New England Journal of Medicine (Volume 361:849-857, Number 9, August 27, 2009.)

The study, led by Dr Reza Fazel , Emory University, Emory University School of Medicine, and colleagues from Mayo Clinic - Rochester, Minn., the Johns Hopkins Ciccarone Preventive Cardiology Center and others, was conducted in response to growing use of imaging procedures in the United States and concerns about exposure to low-dose ionizing radiation in the general population.

Investigators identified 952,420 nonelderly adults (between 18 and 64 years of age) in five healthcare markets across the United States between January 1, 2005, and December 31, 2007. Using utilization data, doctors were able to estimate cumulative effective doses of radiation from imaging procedures and to calculate population-based rates of exposure, with annual effective doses defined as low (?3 mSv), moderate (>3 to 20 mSv), high (>20 to 50 mSv), or very high (>50 mSv).

Of the total enrollees, 655,613 (68.8 percent) underwent at least one imaging procedure associated with radiation exposure. Researchers found that, in general, cumulative effective doses of radiation from imaging procedures increased with advancing age and were higher in women than in men. Computed tomographic and nuclear imaging accounted for 75.4 percent of the cumulative effective dose, with 81.8 percent of the total administered in outpatient settings.

The unit of exposure 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 U.S. 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:

Doctors noted that 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.

For more information: www.nejm.org

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