Ever since physicians started regularly ordering CT (computed tomography) scans four decades ago, researchers have worried that the medical imaging procedure could increase a patient's risk of developing cancer. CT scanners bombard the human body with x-ray beams, which can damage DNA and create mutations that spur cells to grow into tumors.
Doctors have always assumed, however, that the benefits outweigh the risks. The x-rays, which rotate around the head, chest or another body part, help to create a three-dimensional image that is much more detailed than pictures from a standard x-ray machine. But a single CT scan subjects the human body to between 150 and 1,100 times the radiation of a conventional x-ray, or around a year's worth of exposure to radiation from both natural and artificial sources in the environment.
A handful of studies published in the past decade have rekindled concerns. Researchers at the National Cancer Institute estimate that 29,000 future cancer cases could be attributed to the 72 million CT scans performed in the country in 2007. That increase is equivalent to about 2 percent of the total 1.7 million cancers diagnosed nationwide every year. A 2009 study of medical centers in the San Francisco Bay Area also calculated an elevated risk: one extra case of cancer for every 400 to 2,000 routine chest CT exams.
The reliability of such predictions depends, of course, on how scientists measure the underlying link between radiation and cancer in the first place. In fact, most estimates of the excess cancer risk from CT scans over the past several decades rely largely on a potentially misleading data set: cancer rates among the long-term survivors of the atomic bomb blasts in World War II.
“There are major concerns with taking the atomic bomb survivor data and trying to understand what the risk might be to people exposed to CT scans,” says David Richardson, an associate professor of epidemiology at the University of North Carolina Gillings School of Global Public Health who has done research on the atomic bomb survivors.
About 25,000 atomic bomb survivors were exposed to relatively low doses of radiation comparable to between one and three CT scans. The number of cancer cases that developed over the rest of their lives is not, however, large enough to provide the necessary statistical power to reliably predict the cancer risk associated with CT scans in the general population today. Given these difficulties, as well as renewed concerns about radiation levels and the lack of mandatory standards for safe CT exposure (in contrast to such procedures as mammography), a dozen groups of investigators around the world have decided to reevaluate the risk of CT radiation based on more complete evidence.
A growing number of clinicians and medical associations are not waiting for definitive results about health risks and have already begun figuring out how to reduce radiation levels. Two radiologists at Massachusetts General Hospital, for example, think that they can decrease the x-ray dosage of at least one common type of CT scan by 75 percent without significantly reducing image quality. Likewise, a few medical associations are trying to limit superfluous imaging and prevent clinicians from using too much radiation when CT scanning is necessary.
For obvious ethical reasons, researchers cannot irradiate people solely to estimate the cancer risk of CT. So scientists turned to data about survivors of the atomic bombs dropped on Hiroshima and Nagasaki in August 1945. Between 150,000 and 200,000 people died during the detonations and in the months following them. Most individuals within one kilometer of the bombings perished from acute radiation poisoning, falling debris or fires that erupted in the immediate aftermath of the attack. Some people within 2.5 kilometers of ground zero lived for years after exposure to varying levels of gamma rays, from a high end of more than three sieverts (Sv)—which can burn skin and cause hair loss—to a low end of five millisieverts (mSv), which is in the middle of the typical range for CT scans today (2 to 10 mSv). A sievert is an international unit for measuring the effects of different kinds of radiation on living tissue: 1 Sv of gamma rays causes the same amount of tissue damage as 1 Sv of x-rays.