Despite the fact that radiation doses have been increasing along with a growing reliance on CT scans for diagnosis and therapy, the secondary cancer risk from multi-detector CT (MDCT) scans is low among older adults, the group subjected to the most frequent scanning, according to a study published in the April issue of the Journal of the American College of Radiology.

The number of CT exams performed in the U.S. has risen from 2.8 million in 1981 to more than 62 million in 2006, according to the study background information. In the era of MDCT, it has been hypothesized that 1.5 percent to 2 percent of all cancers may be caused by CT-induced radiation.

“Seniors harbor most diseases that typically rely on CT scanning for diagnosis and management and therefore receive the highest exposure to CT imaging,” Aabed Meer, a student in the department of radiology at Stanford University Medical Center in Stanford, Calif., and lead author of the study, said in a statement. “What our study found is that the risk of developing a secondary cancer from these CT scans may potentially be lower than previously thought.”

The researchers conducted a retrospective cohort study using Medicare claims over an eight-year period. Data from over 10 million claims were divided into two four-year cohorts—1998 to 2001 and 2002 to 2005—and the number and type of CT scans received were analyzed to determine the level of radiation to which patients were exposed. Doses of 50 mSv to 100 mSv were considered low-risk, while doses in excess of 100 mSv were considered high risk. The number of radiation-induced cancers was estimated by using the National Research Council’s Biological Effects of Ionizing Radiation VII models.

Results showed that CT scans of the head were the most common exam in both cohorts, but abdominal imaging delivered the greatest proportion of radiation with 43 percent in the first cohort and 40 percent in the second cohort. The 1998 to 2001 cohort saw 42 percent of Medicare patients undergo CT scans, with 2.2 percent and 0.5 percent receiving radiation doses in the low and high ranges, respectively. Fifty percent of Medicare patients in the 2002 to 2005 cohort received CT scans, with 4.2 percent and 1.2 percent receiving low and high doses, respectively.

Of the cancers in the earlier cohort, 0.03 percent were estimated to be related to ionizing radiation, while that latter cohort had an estimated 0.04 percent of CT-induced cancers.

“It is worth pointing out that for many of the patients in our cohorts who received high radiation doses, the imaging they received may have been in response to the presence of serious medical conditions, such that the receipt of imaging could have produced important benefits for these patients,” wrote the authors. “For such a population, the surveillance and diagnostic benefits of a CT examination may outweigh the risk of radiation from the examinations.”

They added that radiation dose isn’t necessarily the most important consideration when making decisions about patient management, but at the same time, unnecessary imaging is never warranted and steps to reduce dose should be taken.

 “Although modern CT imaging is a vital part of medical care, its utilization must include a conscious acknowledgment of radiation exposure and an awareness of the pursuant carcinogenic risk. As medical care continues its dependence on advanced technologies such as MDCT, we emphasize the importance of continuing research that encourages the exploration of lower radiation dose technologies,” said Meer.