CT Scans

Balancing Health Risks and Medical Benefits

Charles W. Schmidt

Disclosures

Environ Health Perspect. 2012;120(3):a118-a121. 

In This Article

Calculating Risks

CT scanners were developed during the 1960s by England's EMI Central Research Laboratories. Interestingly, EMI's parent company owned a record label whose superstar act—the Beatles—generated funding that helped propel CT scanners into routine medical use.[4] The technology started becoming widespread during the 1980s, and uses rose dramatically when faster scanning speeds made it possible to image large sections of anatomy in seconds.

An estimated 75 million CT exams were performed in the United States in 2009.[5] Various estimates suggest that anywhere from 5% to 30% of these exams—each costing hundreds to thousands of dollars—may be medically unnecessary. Why would a clinician request an unnecessary scan? In a 2010 perspective article Rebecca Smith-Bindman, professor in residence at the University of California, San Francisco, School of Medicine, explained, "[T]here are few evidence-based guidelines regarding their appropriate use, and institutional use varies widely, reflecting physicians' preferences and manufacturers' promotion of these capabilities, rather than scientific evidence of improved clinical outcomes. . . . Ironically, technical improvements have led to increases in the identification of incidental (and almost certainly irrelevant) findings that result in follow-up CT for surveillance."[5]

CT scanners emit X rays. Different tissue types absorb X rays in varying amounts, and the resulting contrasts provide detailed images of anatomy and disease. Absorbed radiation can break chemical bonds in tissues, liberating charged ions (hence the term "ionizing radiation") that can damage DNA and produce cancer should cells be unable to repair themselves. Nonionizing radiation—lower-energy radiofrequency waves such as those emitted by microwave ovens and cell phones—doesn't break chemical bonds.

Scientists can't state conclusively that CT scans cause cancer until ongoing prospective studies of that link generate results. In the meantime, they estimate cancer outcomes using dose–response models derived from other radiation-exposed groups, such as atomic-bomb survivors and patients treated with radiation.

The dominant risk assessment model appears in a 2006 report from the National Research Council's Biological Effects of Ionizing Radiation (BEIR) subcommittee.[6] The BEIR VII model postulates there is no safe level of ionizing radiation exposure; carcinogenic effects are assumed to follow a linear dose response, meaning even the smallest exposure carries some level of cancer risk. The BEIR VII model generates so-called lifetime attributable risk (LAR) factors, which estimate the likelihood of cancer in hypothetical individuals as a function of dose. Multiplying the LAR by the number of people exposed to a given dose yields an estimate of expected cancers from that exposure in the population.

Berrington de González relied on BEIR VII to derive her estimate of 29,000 additional cancers resulting from CT scans performed in 2007.[2] Likewise, Smith-Bindman used it to estimate that 1 cancer might appear for every 270 middle-aged women who undergo CT coronary angiography, a high-dose diagnostic procedure that scans heart vessels repeatedly after injection of a contrast dye.[7] Young people face especially high risks, Smith-Bindman says, in part because they live long enough for cancer to develop after a carcinogenic exposure. Therefore, she estimates that women aged 20 who undergo the same coronary procedure have twice the risk as middle-aged women.[7]

On the flip side of the risk spectrum, Smith-Bindman also estimates that 1 cancer could appear for every 11,080 men who get a routine head CT scan.[7] Head scans involve less risk in part because they dose a single organ—the brain—unlike coronary and abdominal scans that dose multiple organs, including the breasts and lungs.

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