Computed Tomography (CT) scans are an increasingly used X-ray-based tool for providing a three-dimensional view of a particular organ or tissue. The value of CT scanning to diagnose injury, cancer and other health problems is undisputed. But are these scans being used too frequently, in some cases unnecessarily? What are the health consequences of having too many CT scans over the course of a person's life?

In a Nov. 29, 2007 article in The New England Journal of Medicine, David J. Brenner, Ph.D., and Eric J. Hall, Ph.D., from the Center for Radiological Research at Columbia University Medical Center, argue that the potential carcinogenic effects from using CT scans may be underestimated or overlooked. This is of particular concern, because perhaps one-third of all CT scans performed in the United States may not be medically necessary, the radiation researchers say.

It is estimated that more than 62 million CT scans per year are currently given in the United States, compared to three million 1980. Because CT scans result in a far larger radiation exposure compared with conventional plain-film X-ray, this has resulted in a marked increase in the average personal radiation exposure in the United States, which has about doubled since 1980, largely because of the increased CT usage.

It used to be widely believed that all radiological examinations were essentially harmless, because of the small amounts of radiation involved, but Drs. Brenner and Hall show that this is unlikely to be true for CT scans. In particular, Japanese atomic bomb survivors who were about two miles away from the explosions, actually received radiation doses quite similar to those from a CT scan.

Sixty years of study of these survivors have provided direct evidence that there will be an increased individual cancer risk, though small, for those who have this same dose of radiation from CT scans. Although the individual risk is small, the large number of CT scans currently being given may result in a future public health problem. In particular, Drs. Brenner and Hall suggest that, in a few decades, about 1½ to 2 percent of all cancers in the United States may be due to the radiation from CT scans being done now.

Defensive Medicine Leads to Overuse

Drs. Brenner and Hall suggest that the rapid increase in CT usage represents a potential public health problem in the United States that should be proactively addressed. This is particularly important for children, who are more sensitive than adults to radiation exposure. The issue arises, for example, when CT scans are requested in the context of so-called "defensive" medicine, or when scans are repeated as a patient passes through different parts of the medical system.

Compounding the issue, surveys suggest that the majority of radiologists and emergency-room physicians may not appreciate that CT scans are likely to increase the lifetime risk of cancer. Ultimately, the health care system, the doctor, and the patient (who can perhaps best track of the number of CT scans performed when dealing with multiple doctors) may have to share the burden of monitoring the appropriate dosage and number of scans.

Drs. Brenner and Hall suggest three strategies for proactively addressing the potential increased radiation risks associated with CT scans:

1. Reduce the CT-related radiation dose in individual patients.
2. Replace CT use, when appropriate, with other options that have no radiation risk, such as ultrasound or magnetic resonance imaging (MRI).
3. Decrease the total number of CT scans prescribed.

Drs. Brenner and Hall suggest in their paper's conclusion that these strategies could potentially keep 20 million adults and, crucially, more than one million children annually from being irradiated unnecessarily. They stress, however, that in the majority of individual cases, the benefits associated with a correct diagnosis through CT will far outweigh the individual risk.