Dec. 14, 2009 -- Radiation doses from CT scans are often high and vary widely, and excessively high doses may contribute substantially to future cancers, a study shows.
CT scans are noninvasive medical tests that combine special X-ray equipment and computers to produce detailed cross sectional images of the body. The number of CT scans performed has exploded over the last three decades, growing from about 3 million yearly in 1980 to about 70 million in 2007.
The new study is published in the Archives of Internal Medicine. Study researcher Rebecca Smith-Bindman, MD, of the University of California San Francisco, says the idea for the research began "when I was looking at some individual scans; I was surprised at how high the radiation dose was. I thought it was time to start looking."
The new research comes in the wake of the discovery earlier this year that more than 200 stroke patients at Cedars-Sinai Medical Center in Los Angeles had received more than eight times the necessary radiation dose when undergoing CT scans. That, in turn, prompted the FDA to encourage CT facility personnel to review their protocols and be sure the values displayed on the control panel jibe with doses normally associated with the scan being performed.
In Smith-Bindman's study, researchers evaluated radiation doses given to 1,119 patients getting CT scans and found that ''the differences in radiation exposure were dramatic," she says. "The doses are on one hand higher than they should be, but also the variation [for the same procedure] is much higher than it should be."
The message from her research, Smith-Bindman says, is for doctors and patients not to panic but to become more aware of the issues. She says the findings also point to the need for more oversight of the scans.
Radiation From CT Scans
Smith-Bindman and her team evaluated CT scan patients who were getting care at four San Francisco area facilities in 2008. They calculated the radiation dose involved with each scan.
The doses varied widely between the different types of scans. The median doses (half higher, half lower) ranged from 2 millisieverts (the measures of radiation used in CT scans) for a routine head CT scan to 31 millisieverts for a multiple-phase abdomen and pelvic scan.
The dose ranges were high. For example, for a head CT scan, while the median dose was 2, the range was 0.3 to 6. "'That is a huge range," she says.
Most dramatic, she says, was the dose and the dose range for a multiphase abdomen and pelvic series. While the median dose was 31, the range was from 6 to 90.
Then the researchers estimated the lifetime cancer risk linked to the CT scan. They estimated that one in 270 women and one in 600 men who got a CT coronary angiogram at age 40 would develop cancer from that scan. They also estimated that one in 8,100 women and one in 11,080 men who had a routine head CT scan at age 40 would develop cancer.
CT Scans and Cancer
In another report published in the Archives of Internal Medicine, a team of researchers led by Amy Berrington de Gonzalez, DPhil, an investigator at the National Cancer Institute, also estimated the risk of cancer attributable to CT scans.
After looking at data from previous reports of radiation-linked cancer risk, insurance claims and nationwide surveys, they concluded that 29,000 future cancers could be related to the 70 million CT scans performed in the U.S. in 2007.
This includes an estimated 14,000 cases resulting from scans of the abdomen and pelvis; 4,100 from chest scans; 4,000 from head scans; and 2,700 from CT angiograms. One-third of these projected cancer cases would occur following scans performed on people ages 35 to 54. Two-thirds of the cancers would be in women, according to a news release.
The high number of cancers attributed to scans of the abdomen and pelvis is not surprising, according to Berrington de Gonzalez, since they are so commonly done. "One-third of the 70 million [scans] were abdominal and pelvic."
The new research will hopefully raise awareness among doctors and consumers, says Rita Redberg, MD, professor of medicine at the University of California San Francisco and editor of the Archives of Internal Medicine, who wrote an editorial to accompany the reports.
''I don't think people routinely question, 'Do I need this CT scan at this time?'" she tells WebMD.
And they should, she says. If a doctor orders a CT scan, she advises: "Ask your doctor, 'How is this CT scan going to contribute to my medical care?' [and] 'How will it change how you are going to treat me and how will that help me?'"
The new findings about radiation doses and ranges should be balanced with consideration of the benefits of the technique, says Donald Frush, MD, chairman of the American College of Radiology Commission on Pediatrics and professor of radiology and pediatrics at Duke Medical Center, Durham, N.C. ''CT is a very helpful technique," he says. "It's one of the greatest medical advances."
But he, too, says consumers should specifically ask if the CT scan is necessary or if another imaging technique that doesn't require radiation, such as ultrasound, could be used.
Part of the problem, Smith-Bindman says, is a lack of consensus on what the radiation dose should be for different CT scans. In some cases, radiologists set the parameters, she says; other times manufacturers do. It's done in a variety of ways, she says.
The FDA has done survey studies, says Smith-Bindman, finding that ''doses need to be between X and Y." More oversight from the FDA would help, she says.