CT Scan Radiation

Radiation from computerized tomography scans may cause cancer decades later, according to a study that estimated about 29,000 future malignancies would occur in the U.S. because of CT scans done in 2007.

Most of the cancers are predicted to strike women, who receive more CT scans than men, and about one-third of the projected malignancies may occur from scans performed in people ages 35 to 54, said research published today in the Archives of Internal Medicine. The cancer forecast was based on an estimate that about 72 million CT scans were done in 2007.

The number of CT tests in the U.S. has risen three-fold since 1993, according to the study. More research is required to determine the lowest dose of radiation needed for clear pictures from CT scans to help reduce radiation exposure, said lead study author Amy Berrington de Gonzalez.

“We know that there are great medical benefits to CT scans, but they also involve small risks of cancer because of radiation exposure,” said Berrington de Gonzalez, a researcher at the National Cancer Institute in Bethesda, Maryland, in a Dec. 11 telephone interview. “For an individual, the risks are small. So if the scan is clinically justified, then the benefits should outweigh the risks.”

CT scans produce detailed images of the body that provide more details than a traditional X-ray. CT scanners are made by Fairfield, Connecticut-based General Electric Co., Toshiba Corp. based in Tokyo, Munich-based Siemens AG and Royal Philips Electronics NV based in Amsterdam.

Medicare Data

Researchers in the study looked at data from Medicare, the U.S. government health program for those older than 65 or disabled, plus a survey and an insurance database with information on the types of CT scans and the age and gender of patients taking the tests.

Berrington de Gonzalez said the overall risk for any individual is small and depends on the type of scan given and a person’s age. A 70-year-old who has a CT scan of the head would have a 1 in 10,000 chance of developing cancer from the test, while a baby who had a chest CT scan would have a 1 in 200 chance, she said.

The researchers found that about 30 percent of scans in the study were performed in adults ages 35 to 54, 13 percent in those 18 to 34 and 7 percent in children younger than 18.

Lung Cancer

The authors predicted that lung cancer will be the most common radiation-related cancer followed by colon cancer and leukemia. Of the 29,000 people who may get cancer from CT scans done in 2007, about 50 percent will die, the researchers estimated. If CT scan use remains at its current level or higher, eventually 29,000 cancers every year could be related to past CT scan use. That number is equal to about 2 percent of the 1.4 million cancers diagnosed each year in the U.S., they said.

A second study in the journal found that radiation doses from CT scans vary greatly and are higher than previously thought. The researchers reviewed CT procedures performed on 1,119 patients in the San Francisco Bay area over five months.

They found a 13-fold variation between the highest and lowest radiation dose for each type of CT procedure. Patients’ exposure to radiation needs to be reduced by standardizing and limiting the radiation associated with each scan, they said. The number of CT scans should be reduced, they said, citing previous reports that 30 percent or more may be unnecessary.

The U.S. Food and Drug Administration issued interim regulations Dec. 7 requiring closer monitoring of CT scans after more than 250 cases of exposure to excess radiation were reported since October.

CT Heart Scans

The researchers, led by Rebecca Smith-Bindman at the University of California, San Francisco, also estimated cancer risk. They project that 1 in 270 women who undergo a CT scan of the heart’s blood vessels at age 40 will develop cancer from the procedure compared with 1 in 600 men.

In certain groups of patients for certain kinds of scans, the risk is as high as 1 in 80, said Smith-Bindman, a professor of radiology at UC San Francisco, in a statement. The risk of developing cancer declined “substantially” with advancing age, the authors said.

“It is imperative, particularly given these results, that we start collecting radiation dose data at the individual patient level,” Smith-Bindman said in a statement. “Our results point toward the need to start collecting data on what actually happens in clinical practice and then to establish the appropriate standards.”