Like X-rays and PET scans, CT scans use ionizing radiation, which can damage DNA and cause cancer. Two other imaging technologies, MRI scans and ultrasound, do not use radiation. CTs are used for a plethora of reasons, among them finding kidney stones, evaluating chest pain and detecting tumors or other abnormalities.
Widely hailed as one of the most important medical advances of the past century, CT scans were developed in the 1970s. Their use in the United States grew from 3 million in 1980 to more than 85 million in 2011. Although CT scans are an essential diagnostic tool, the Food and Drug Administration reports that an estimated 30 to 50 percent of imaging tests are believed to be medically unnecessary.
“We have this view that we only really use imaging when it’s really necessary,” which is no longer the case, said Rebecca Smith-Bindman, a professor of radiology, epidemiology and obstetrics and gynecology at the University of California, San Francisco. “The truth is, now it’s ubiquitous. And many of these tests don’t need to be done.”
Like X-rays, CT scans represent a potentially significant health problem, one that experts say may not show up for years: cancer caused by radiation. In most cases, it is impossible to definitively attribute cancer to radiation exposure that occurred years or even decades earlier. And overall, the risk from a single scan is small: The National Cancer Institute (NCI) estimates that the additional risk of developing a fatal cancer from a scan is 1 in 2,000, while the lifetime risk of dying of cancer is 1 in 5.
The question of risk remains a matter of fierce debate among radiologists: Some say that the amount of radiation used in diagnostic studies is safe and that the benefits far outweigh the small chance that a person will develop cancer. But other experts, including Smith-Bindman, say that while patients should never avoid scans that are medically necessary, excessive radiation doses and indiscriminate use of imaging pose a clear and demonstrable danger.
Studies published in 2007 and 2009 by teams from Columbia University and the NCI predicted that up to 2 percent of future cancers about 29,000 cases and 15,000 deaths annually might be caused by CT scans. A 2011 report by the Institute of Medicine found that the two environmental factors most strongly associated with breast cancer were radiation exposure and the use of post-menopausal hormones.
While a single scan would rarely be concerning, many Americans undergo multiple tests. A 2009 study by researchers at Brigham and Women’s Hospital in Boston found that among 31,000 patients who had a diagnostic CT scan in 2007, 33 percent had more than five during their lifetime, 5 percent received 22 or more, and 1 percent underwent more than 38 scans.