Long-Term Cancer Risks From Cardiac Imaging Radiation Remain Unknown

Marlene Busko

April 30, 2012

April 30, 2012 (Montreal, Quebec City) — Although radiation doses from cardiac computed tomography (CT) imaging tests have dropped dramatically, the increased cancer risk from even low doses is unknown, so physicians need to carefully weigh benefits vs risks when ordering these tests, according to Dr Brett Heilbron (St Paul's Hospital, Vancouver, BC) [1].

"The radiation-dose discussion is going to be with us for a long time," he added, speaking at the Society of Cardiovascular Computed Tomography (SCCT) Cardiac CT Imaging Day.

"I think that we're unlikely to have more data on the risks of low-dose radiation ionization in the near future, so we're beholden to minimize unnecessary tests and, when we do a test, to use the minimum dose that’s necessary to achieve diagnostic image quality," he told heartwire .

Concerns about the effects of radiation dose followed the exponential growth in the use of radiation for cardiac imaging, Heilbron said. During the 1980s, on average, a person living in the US was exposed to 3.6 mSv of radiation a year, with 15% coming from medical sources. In 2006, on average, Americans were exposed to 6.2 mSv of radiation a year, with 47.8% coming from medical imaging tests (including 18.9% from cardiac imaging).

Meanwhile, the National Academy of Science's Biological Effects of Ionizing Radiation VII (BEIRVII) report concluded that "scientific evidence is consistent with the hypothesis that there is a linear dose-response relationship between exposure to ionizing radiation and the development of radiation-induced solid cancers in humans" [2].

However, it's very hard to estimate the actual long-term cancer risk from very low doses of radiation. "You'd need to do a study involving about a million people and follow them for 20 years, which will never be done," Heilbron said.

The good news is that radiation doses from coronary CT angiography have steadily declined--from 20 mSv eight years ago to doses below 2 mSv that are attainable with new technology and dose-limiting strategies. Heilbron coauthored a real-world, single-center study that reported a mean effective radiation dose from cardiac CT of 3.7 mSv, which was much less than the mean dose from other modalities [3].

Average Effective Radiation Dose for Cardiac Imaging Modalities

Cardiac imaging modality Average effective dose (mSv)
Cardiac CT angiography 3.7
Myocardial perfusion imaging  
With attenuation correction 16.8
Without attenuation correction 12.1
Diagnostic catheter coronary angiography 11.4

Dose-reduction strategies include using prospective gating (which provides "the biggest bang for the buck" according to Heilbron), limiting the z axis, reducing tube voltage and amperage, and using iterative reconstruction to reduce image noise. The group published several studies of dose-reducing strategies. One report showed that using 80 kVp vs 100 kVP was associated with a nearly 50% reduction in radiation dose with no significant difference in interpretability and image quality.

Need to Image the Right Patients

Dr Jonathon Leipsic (St Paul's Hospital), who was involved in many of the coronary CT angiography studies at his center, said the main message is, "No dose is acceptable if it's not appropriate, but in the appropriate setting, cardiac CT is a safe test, and there's been an exponential reduction in dose over historical norms."

Dr Benjamin JW Chow (University of Ottawa Heart Institute, ON) echoed these comments: "We need to select patients carefully to minimize the radiation risk associated with the test, and we have to make sure we select the patients who will derive the greatest benefit from imaging." Patients who would not benefit include those where test results would not alter treatment strategies. Patients who may benefit include those where the test detects significant, treatable, obstructive disease that would not otherwise be found or where the patient has a negative CT angiography scan and is spared from an invasive angiogram with its associated risks.

Heilbron has received research funding from GE Healthcare.

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