Growth in cancer patients’ radiation exposure exceeds that of general population

Cancer patients saw a four to five times greater increase in their average annual exposure to imaging-related ionizing radiation than the general population since 1994, according to a retrospective study of more than one million privately insured people in the U.S. 

Published research in general has established a sixfold increase in the average exposure to ionizing radiation for people in the U.S. since the 1980s. More than 85% of that growth was attributed to medical imaging, especially multislice CT and nuclear medicine applications. Pediatric radiologist Dr. David Brenner estimated in his 2007 analysis in the New England Journal of Medicine (NEJM 2007;357:2277-2284) that the explosion of CT then in use in the U.S. would expose patients to enough ionizing radiation to induce 1.5% to 2% of future cancers.

These and other revelations have led to numerous efforts to modify equipment performance and medical imaging practices to reduce patient exposure and cancer risks.

Now Dr. Rebecca Smith-Bindman, a radiologist/epidemiologist at the University of California, San Francisco, has uncovered data suggesting the risk for future induced cancers may be particularly acute for current cancer patients. By examining annual exposures from diagnostic imaging involving ionizing radiation, Smith-Bindman has established that from 1994 to 2008, the average annual rate for radiation exposure for privately insured cancer patients increased four to five times faster than rates for beneficiaries who did not have cancer.

Smith-Bindman’s retrospective analysis was based on the medical imaging experience of 1,081,923 members of a large integrated health system involving eight HMOs across the country, according to Dr. Ingrid Burger, a radiology resident at UCSF. She presented the results on Wednesday at the 2010 RSNA meeting.

The UCSF research group found that the mean annual effective dose for cancer patients rose from 8 mSv in 1994 to 19.5 mSv in 2008. During the same 14-year period, the annual effective dose for non-cancer patients increased from 4 mSv to 5 mSv, Burger said. Median results were much higher than the mean findings, suggesting that a subset of patients received particularly high doses that drove up the means.

As expected, the percentage of cancer patients exposed to particularly high doses of radiation jumped markedly faster than that for non-cancer patients, Burger said. In 1994, 3.2% of cancer patients received more than 50 mSv. By 2008, 11.3% of cancer patients in the integrated system were exposed to an effective dose of more than 50 mSv, which compares with less than 2% of non-cancer beneficiaries. From 1994 to 2008, the percentage of cancer patients receiving more than 100 mSv rose from 0.4% to 3.2%.

The researchers noted that cancer patients continued to undergo imaging with elevated radiation doses long after the treatment for their cancers had ended. Cancer patients received a mean annual effective dose of 37.8 mSv in the first five years after diagnosis. Mean annual exposure decreased to 30.7 mSv for the monitoring phase (years six to 10 after diagnosis). About 10% of cancer patients in the treatment period and 7% during the monitoring period received doses of more than 100 mSv annually.

Few clear trends emerged from evaluation of the relationship between radiation exposure and the cancer patient’s age. Researchers found that the annual effective dose tended to rise with age for patients who were older than 25, but no correlations between age and exposure could be made for patients younger than 25.

“Those radiation exposures are all over the map,” Burger said. “They don’t follow any trend.”

Overall, the study underscores the need to track patients’ radiation exposure histories over time to evaluate the intensity of long-term imaging surveillance after cancer treatment, Burger said.