No Uniform Cancer Risk from CT Heart Scans

NEW YORK, July 17 -- The radiation dose from computed tomography (CT) coronary angiography carries a "nonnegligible" cancer risk that varies widely with age, gender, and scan protocol, researchers said.

Lifetime cancer risk from a standard cardiac scan was 4.8-fold higher among women than men at age 20 and 2.4-fold higher for women than men at age 80, found Andrew J. Einstein, M.D., Ph.D., of Columbia University Medical Center here, and colleagues.

Lifetime attributable risk dropped from 1 in 143 to 1 in 219 for a 20-year-old woman with use of a dose reduction strategy and from 1 in 3,261 to 1 in 5,017 for an 80-year-old man, they reported in the July 18 issue of the Journal of the American Medical Association.

"The general perception is that a cancer risk is associated with CT coronary angiography, although few quantitative data are available," they wrote.

The FDA has stated that a 10-mSv CT cardiac study may be associated with approximately a 1 in 2,000 risk of fatal cancer, but it was unclear how risk varied among patient groups.

Thus, their findings may help physicians choose among noninvasive tests for coronary artery disease for individual patients, Dr. Einstein and colleagues said.

"The results of this study suggest that CT coronary angiography should be used particularly cautiously in the evaluation of young individuals," they wrote, "especially women, for whom alternative diagnostic modalities that do not involve the use of ionizing radiation should be considered, such as stress electrocardiography, echocardiography, or magnetic resonance imaging."

The researchers used the National Academies' Biological Effects of Ionizing Radiation 7th Report published in 2006 to estimate cancer risk estimates for radiation exposure from CT coronary angiography. This report was generated from epidemiological models and data from atomic bomb survivor studies and medical and occupational radiation studies.

The researchers also conducted computer simulations to model 64-slice CT photon transport in hypothetical patients.

They found that risk decreased with age and was lower for men than for women at every age, such that the risk was the same for a 20-year-old man and 70-year-old woman.

For women versus men, the lifetime attributable risk for a standard scan was:

• 1 in 143 (0.70%) versus 1 in 686 (0.15%), at age 20.
• 1 in 284 (0.35%) versus 1 in 1,007 (0.099%) at age 40.
• 1 in 466 (0.22%) versus 1 in 1,241 (0.081%) at age 60.
• 1 in 1,338 (0.075%) versus 1 in 3,261 (0.044%) at age 80.

The differences by age were likely a factor of the long lag times between exposure and development of cancer and decreasing organ sensitivity to radiation with age, the researchers said.

One of the major contributors to the difference between men and women was the risk of breast cancer, they noted. Breast and lung cancers, both organs in the field of radiation, together accounted for 80% to 85% of women's attributable cancer risk at every age.

The other major contributor was the "strikingly" higher radiosensitivity seen for women, the researchers said. For the same 100-mSv dose of radiation to the lung at age 20, the attributable lung cancer risk was 346 cases per 100,000 for women but only 149 per 100,000 for men, according to the National Academies report.

Among the scan protocols considered, the researchers found that ECTCM (a strategy that reduces radiation during part of the heartbeat) reduced cancer risk. A dose reduction of 35% reduced estimated risk by the same amount.

The cancer risk with ECTCM was 1 in 219 for a 20-year-old woman (down from 1 in 143), 1 in 715 for a 60-year-old woman, 1 in 1,911 for a 60-year-old man, and 1 in 5,017 for an 80-year-old man.

A protocol to scan both the heart and aorta, on the other hand, increased the risk 43% to 46% in men and 24% to 28% in women. The lifetime attributable cancer risk with this protocol was as high as 1 in 114 for women age 20. The protocol "is commonly performed for patients with coronary artery bypass grafts and in the 'triple rule-out' of coronary artery disease, aortic dissection, and pulmonary embolism," the authors noted.

The investigators suggested that dose reduction strategies, such as ECTCM, should be used whenever possible along with adequate beta-blockade. "Particular attention should be given to the avoidance of unnecessary repeat studies," they said.

They concluded that their risk estimates could be used to assess the risk-benefit tradeoff for CT coronary angiography for patients, although the estimates were extrapolated without epidemiological data from patients actually undergoing scans.

"If CT coronary angiography is considered as an alternative to invasive coronary angiography, the risks and benefits of each test require consideration," they wrote, while noting that gold invasive angiography carries a 1.7% risk of major complications.

"The careful selection of patients for CT and the careful optimization of scan protocol in patients referred for testing can help to minimize cancer risk," they added.