A recent issue of the New England Journal of Medicine contained an expert review on the health risks of radiation exposure from computed tomography (CT) (1). The article was authored by David J. Brenner and Eric J. Hall, from the Center for Radiological Research in New York.  Brenner, in particular, is a leader in the field of research regarding the health effects of ionizing radiation from environmental, occupational and medical sources. Among his numerous publications are several research papers on radiation exposure associated with CT, as well as two books on radiation and health for the lay person.

In this latest paper, Brenner and Hall report data documenting an almost exponential increase in the number of CT examinations performed annually in the US, from 3 million in 1980 to an estimated 62 million in 2006. They note that the growth areas for CT are screening in asymptomatic adults and diagnosis in children, in whom currently 6%-11% of all CT examinations are performed.  

Referring to one of their previous works (2), the authors summarize their calculations of typical organ doses during single CT scans which illustrate the dramatic increase in dose with decreasing age in adolescents and children.  Considering that an average CT examination involves 2 or 3 scans, the authors estimate that typical organ doses are 30-90 mSv. These values, while low in themselves, are similar to the exposures of a large cohort of survivors of the atomic bombs deployed in World War II and of a 15-country cohort of nuclear industry workers. Studies of these two cohorts, the first of which has been followed for more than 50 years, have demonstrated significant increases in the risk of cancer, which were quantitatively similar in the two cohorts despite the different types of exposure.  

On this basis, Brenner and Hall conclude that there is direct epidemiological evidence that irradiation during a typical CT examination poses an increased risk of cancer. Again referring to their earlier research [2], they show how the lifetime risk of death from specific cancers is highly dependent on the age at which the CT examination was performed. Although the individual risk is small (<0.01% risk of death from head CT and <0.07% from abdominal CT when performed at age 20 years), the authors caution that the widespread and increasing use of CT may make this risk a major public health problem in the future.  In fact, they estimate that the risk of cancer associated with the current use of CT in the US approaches 2%.  

In light of their findings, the authors are critical of the lack of large epidemiological studies on the cancer risks posed by CT. This should change, as both the US National Cancer Institute and the UK Department of Health have recently funded research on the health effects of CT-associated radiation in children (for example, see http://www.ncl.ac.uk/plerg/Research/grants.htm). And while noting the favorable risk-benefit ratio of most diagnostic CT examinations, the authors argue that – at least in the US – CT is often used unnecessarily.  In particular, they criticize the use of CT to diagnose appendicitis in children, its use in defensive medicine, and unnecessary repeat CT examinations due to poor communication among healthcare professionals.  Finally, they recommend greater awareness of radiation risks among physicians who request CT examinations, the selective use of CT when its great diagnostic potential is required, and the increased use of modern scanner technology to control radiation exposure. Overall, the paper is well argued and provides many points of discussion regarding the US situation. Considering the different approaches to healthcare in individual European countries, studies that assess  European physicians' knowledge of radiation risks, that characterize current patterns of CT use in Europe and that propose optimized uses of CT are welcomed.  

References

1. Brenner DJ, Hall EJ (2007) Computed tomography – an increasing source of radiation exposure. N Engl J Med 357:2277-2284
2. Brenner DJ, Elliston CD, Hall EJ, Berdon WE (2001) Estimated risks of radiation-induced fatal cancer from pediatric CT. AJR Am J Roentgenol 176:289-296