No published studies have directly attributed cancer to CT scanning, and it is important to recognize how difficult it would be to perform such a study. The lifetime risk of fatal cancer in the general population is approximately 1 in 5. To perform a study to detect an increase from 0.2000 (the 1-in-5 risk in the general population) to 0.2002 (the 1-in-5 risk seen in the general population plus a 1-in-5000 potential risk from a CT scan) would require hundreds of thousands to millions of subjects and extremely careful matching of the subjects in the study to ensure an accurate result. Until such a study is completed and verified by the scientific community, estimates of risk must be based on other forms of ionizing-radiation exposure, and some assumptions must be made to apply these risks to the risks from diagnostic imaging. The most widely used source of risk estimates comes from data on atomic bomb survivors.
CT scanners and other diagnostic imaging equipment use low-dose radiation, which is defined as a dose of less than approximately 100 mSv. There are numerous studies of populations receiving high doses of radiation above 500 mSv that have demonstrated an increased risk of cancer. These studies, reviewed in the 2005 report of the Biological Effects of Ionizing Radiation (BEIR) Committee of the National Academy of Sciences,7 provided widely accepted evidence that, at higher exposures, the risk of cancer increases linearly with increasing dose until extensive cell killing takes place at very high exposures. The relationship between radiation exposure and cancer risk from low-dose radiation is less clear.
Because of the diversity of opinion and the many different studies that have been performed, a broad range of estimates of the risk of ionizing radiation from diagnostic imaging can be supported by selecting specific publications from the peer-reviewed literature. It is impossible to provide a complete review of this literature here, and without a complete review, any summary could be biased. To our knowledge, there are no reviews that are considered to be authoritative.
Statements that are based on expert panel reviews of available information are additional sources of estimates of the risks of low-level radiation. The BEIR Committee of the National Academy of Sciences recently released their seventh statement in 2005. The committee concluded that “the risk of cancer proceeds in a linear fashion at lower doses without a threshold and that the smallest dose has the potential to cause a small increased risk to humans.”7 The United Nations Subcommittee on Atomic Radiation 2000 report stated that “an increase in the risk of tumor induction proportionate to the radiation dose is consistent with developing knowledge and that it remains, accordingly, the most scientifically defensible approximation of low dose response.”8 The International Commission on Radiation Protection recommendations (2005) stated that “the weight of evidence on fundamental cellular processes supports the view that in the low dose range up to a few tens of mSv, it is scientifically reasonable to assume that in general and for practical purposes cancer risk will rise in direct proportion to absorbed dose in organs and tissues.”9
In the absence of definitive evidence of the effects of low-level radiation, these consensus statements provide useful guidance. They suggest that it is reasonable to act on the assumption that the low-level radiation used in diagnostic imaging may have a small risk of causing cancer. If one assumes that radiation from a CT examination may cause cancer, it is reasonable that the medical community seek ways to decrease radiation exposure. Two ways to achieve this reduction are to use radiation doses that are as low as reasonably achievable (ALARA), which means that no more radiation should be used than is required to achieve the necessary diagnostic information, and to perform these studies only when they are necessary.
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