Diagnostic x-rays are the leading man-made source of radiation exposure, accounting for about 14 percent of all radiation exposures worldwide. Although the risk of inducing cancer is believed to be small, the large number of patients exposed to x-rays means that a significant number of cancers in the population could be attributable to these diagnostic tests. Berrington de González and Darby estimated the cumulative risk of inducing cancers at different sites associated with common radiologic investigations.

They used data from 15 countries to develop statistical models relating cumulative x-ray dose to nine common cancers. Data used in the study included surveys of the frequency and types of x-ray studies undertaken in the countries studied, cancer statistics, and studies of the organ-specific radiation dose associated with the specific tests.

The highest organ-specific radiation dose estimated was 44 mGy delivered to the thyroid during computed tomographic (CT) examinations of the cervical spine. CT of the chest was associated with more than 20 mGy to the breast, lung, and esophagus. Similarly, CT of the pelvis exposed the bladder to 23 mGy, and imaging of the abdomen was associated with 22 mGy to the stomach. Coronary procedures such as angiography and catheterization were associated with high radiation exposures to the lungs (more than 37 mGy), thyroid (25 mGy), and esophagus (13.79 mGy). The researchers calculated average age- and sex-specific radiation doses for each major organ in each of the participating countries. These data were correlated with nine common cancer types to calculate site-specific radiation-induced cancer risks.

The researchers estimate that diagnostic radiology causes 5,695 cancers in Americans younger than 75 every year. These attributable cancers are predominately colon, bladder, and breast cancer, and leukemia. The risk is strongly age related, beginning to rise at about age 40 and still increasing at 70 years of age. CT was associated with the largest number of induced cancers, followed by barium enemas, and x-rays of the hip and pelvis. Diagnostic x-rays could cause about 1 percent of U.S. cancers, approximately double the attributed rate in 1981.

editor’s note: The tables in this study make sobering reading, especially those relating radiation exposure from CT studies. The bladder and colon appear to be particularly sensitive to the oncogenic effects of radiation, especially in older patients. Although the risks to patients are small, this study emphasizes the need for prudence in ordering any radiologic study and attention to potential long-term risks, as well as short-term diagnostic gains.—a.d.w.