Before CT ousts angiography, radiation risk must be understood
A debate at the recent 2008 VEITH Symposium in New York City pitting catheter angiography against coronary CT angiography concluded that there is still a role for conventional angiography, specifically in patients with clear-cut evidence of coronary disease and unstable syndromes. The caveat concerning coronary CTA, however, is the radiation risk, according to participant Glenn A. Hirsch, MD.
Hirsch, an assistant professor in the division of cardiology at Johns Hopkins University School of Medicine in Baltimore, said that the quoted radiation and/or cancer risk from CT is generally from one scan and does not take into consideration that people often undergo multiple scans during their lifetime. “It’s the cumulative scan risk that is the problem,” he said.
In the last 10 years, emergency department visits have gone up, while the actual number of departments has declined, creating high time-pressures for many ED physicians. Coupled with the inclination to practice defensive medicine and the results are a dramatic rise in the number of CT scans. “Ten percent of people presenting to an emergency department today get a CT scan,” Hirsch said.
Those presenting with chest pain often undergo a CT scan to rule out pulmonary embolism. One study found that 33 percent of these patients return to the ER within five years and have another negative CT scan of the chest. More importantly, about 5 percent of them came back within five years and had six or more CT scans, including younger people for various reasons such as kidney stones and abdominal pain. “The truth is we are irradiating people at an earlier age and it continues throughout their lifetime,” Hirsch said.
Tracking cumulative scan radiation exposure
A national health record would at least allow physicians to track the number of CT scans that patients receive. Hirsch cited an abstract that examined CT exposure at two trauma centers. Within those two centers, the researchers found that 10 percent of the patients were exposed to more than 100 mSv in the preceding five years, and 6 percent were exposed to more than 400 mSv. “It’s generally agreed that a cumulative dose exposure of more than 100 mSv causes excess cancer,” he said. “From a population perspective, even if the risk is 0.01 percent, that’s thousands of excess cancers and deaths you could be causing with a single scan.”
The effective dose for diagnostic coronary angiography is about 3 to 5 mSv. Newer CT scanners offer the ability to perform CTA with an effective dose of 5 mSv, “but the average dose is more like 17 mSv,” Hirsch said. A nuclear SPECT perfusion study is approximately 15 mSv.
Hirsch said studies have demonstrated that many ER physicians, radiologists, cardiologists and patients are unclear on some level about radiation risk from CT scans. Compounding the problem is the modest use of dose modulation tools, which can reduce dose by up to 50 percent—when used. One international study of 44 sites, for example, showed that only 82 percent of them used the dose modulation tools, he said.
“There is a disconnect between what’s possible and what we are actually doing that has to be reconciled,” Hirsch said.
As the temporal resolution of CT improves and as the dose enters the 1 to 2 mSv range, Hirsch has no doubt that CTA will replace diagnostic catheter angiography. But for now, he said CT scanning is being overused without physicians and patients alike truly understanding the radiation risk involved.
|Cancer risk for cardiac CT overstated|
|Researchers from the Medical University of South Carolina presented a poster at the 2008 American Heart Association meeting that concluded the risk of radiation-induced cancer from cardiac CT is substantially lower than previously reported for general populations.
Walter Huda, PhD, et al said that previous estimates of cancer risk from cardiac CT were 1 in 114 (based on data from atomic bomb survivors), while the new estimate is 1 in 1000 (based on “a real-life clinical patient population”).
Researchers adjusted the risk in accordance with patient sex, age and weight, the latter being “an often neglected factor influencing radiation risk.”
Patients are typically older, heavier males, which lowers the risk compared with younger females. Appropriate