Since the 1970s, computed tomography (CT) has become an increasingly important diagnostic tool whose use has expanded in recent years to replace such conventional procedures as X-rays and ultrasound. But greater dependence on this imaging technique comes at a price: increased exposure to radiation that could increase a patient’s risk of cancer.

In the keynote address at the 43rd National Council on Radiation Protection and Measurements meeting in April, James A. Brink, M.D., professor and chair of diagnostic radiology, emphasized that physicians need to be more aware of the risk of CT and other imaging devices that use radiation. The number of CT exams administered each year in the United States has increased exponentially, growing from 3 million in 1981 to 63 million in 2005. CT scans commonly give the patient a dose of 10 to 25 millisieverts (mSv), compared to 0.10 mSv for a chest X-ray. But even those low-dose exposures typical of CT scans can increase the risk of cancer, according to the Radiation Effects Research Foundation, formerly the Atomic Bomb Casualty Commission, which studies the long-term effects of radiation from the uranium fission bombs detonated at Hiroshima and Nagasaki. That means that patients who undergo numerous CT scans, which is not uncommon, may be in the medium-dose exposure range of the atomic bomb. And smaller doses have a cumulative effect. “That’s important, because if you do several low-dose CT scans, they add up to a larger dose that has the same risk,” said Brink.

In the United States, medical necessity and the judgment of physicians determine whether CT scans will be approved, but radiation exposure is not part of the equation, according to Brink. In Europe, however, each country regulates exposure from CT and other imaging modalities that use radiation. “I think we’re at least a decade behind Europe in terms of attention and regulation regarding medical radiation,” Brink said.

Although Brink is quick to point out that CT has an important place in diagnosis, he also notes that there are a number of ways to control the radiation exposure it entails. “CT should be avoided when MRI or ultrasound is of comparable diagnostic utility,” he said, citing one patient who had 18 CT scans over six years for flank pain, a symptom that usually indicates kidney problems. “He probably only needed the first one and didn’t need the next 17.” In addition, physicians should avoid repetitive exams, tailoring them to the individual patient and the individual application. Manufacturers provide tools to alter the technical parameters of scans, so that a patient having a CT for kidney stones, for example, won’t receive as much radiation as a patient undergoing a scan for metastatic colon cancer.

Since physicians can’t take steps to avoid or reduce exposure if they aren’t aware that the problem exists, education is perhaps the most important component of controlling exposure. In a 2004 study at Yale, Brink and colleagues found that only 9 percent of emergency physicians who had ordered CT scans for pelvic pain over a two-week period believed there is an increased cancer risk from CT. When asked how many chest X-rays are equivalent to one CT, 44 percent of radiologists thought it was equal to between one and 10 chest X-rays—the correct answer is between 100 and 250. “Even radiologists didn’t appreciate the difference,” said Brink. “It shows that education is lacking across the board.”