CHICAGO--The risk of developing radiation-induced cancer from CT may be lower than previously thought, according to a study presented this morning at the annual meeting of the Radiological Society of North America (RSNA). “We were surprised [by the results]. We expected the numbers to be higher,” shared Pat A. Basu, MD, MBA, faculty radiologist at Stanford University in Palo Alto, Calif., and a White House Fellow.

Specifically, the study calculated a 0.02 percent increase in excess cancers among a 1998 to 2001 cohort and a 0.04 increase among a 2002 to 2005 cohort. "Our findings indicate a significantly lower risk of developing cancer from CT than previous estimates of 1.5 percent to 2.0 percent of the population," said co-author Scott W. Atlas, MD, chief of neuroradiology at Stanford University Medical Center in California.

In one of the first studies to track CT utilization in a large population, the researchers conducted a retrospective study using Medicare claims from 1998 through 2005 to analyze the distribution of CT scans, determine the ionizing radiation exposure associated with the exams and estimate the associated cancer risk in a population of older adults.

The data were studied in two groups, including 5,267,230 records from 1998 through 2001 and 5,555,345 records from 2002 through 2005. For each group, the researchers analyzed the number and types of CT scans that each patient received to determine the percentage of patients exposed to low radiation doses of 50 mSv to 100 mSv and high radiation doses, in excess of 100 mSv. They then used standard Biological Effects of Ionizing Radiation (BEIR) VII cancer risk models to estimate the number of cancers induced.

CT scans of the head were the most common exams, representing 25 percent of the first group and 30 percent of the second group. However, abdominal CT delivered the greatest proportion of radiation, accounting for approximately 40 percent of the total radiation exposure in each group. Imaging of the pelvis and chest represented the second and third largest sources of radiation.

From 1998 to 2001, 42 percent of patients underwent CT scans. From 2002 to 2005, 49 percent of patients underwent CT scans. The percentage of patients exposed to radiation doses in both the low and high ranges approximately doubled from the first group to the second group. The researchers found this to be consistent with the increasing use of high-speed CT in patient diagnosis and management.

Cancer incidences related to ionizing radiation from CT were estimated to be 0.02 percent and 0.04 percent of the two groups, respectively.

Future directions, radiologists’ roles
The authors elucidated the need for continuing research and communication. Radiation-induced cancers take 10 to 20 years to develop, shared Basu. The ideal study is an extremely large, long-term study that tracks one million patients forward for 20 years, he claimed.

Basu suggested that physicians and patients need to weigh the risks and benefits of CT. Radiologists can assist by sharing data about the radiation ranges for typical studies with ordering physicians. “Put it in understandable terms such as an x-ray or airplane flight. And be open to discussing the reason for the study and its advantages and disadvantages,” he advised.

“We also need a way to individually track patient’s radiation dose risks,” he noted.