National Cancer Institute NCI Cancer Bulletin: A Trusted Source for Cancer Research News
March 10, 2009 • Volume 6 / Number 5

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Spotlight

Tracking Heart CT Scans and Radiation Dose

A cardiac computed tomography angiogram. (Copyright © 2009 Massachusetts Medical Society. All rights reserved.) A cardiac computed tomography angiogram. (Copyright © 2009 Massachusetts Medical Society. All rights reserved.)

On a nearly daily basis, most people are exposed to small amounts of so-called background radiation from naturally and not-so-naturally occurring sources. For many people, though, medical procedures are a significant source of radiation exposure, and this has raised concerns among some physicians about whether procedures intended to help patients may eventually cause long-term harm by increasing their cancer risk.

One increasingly common source is computed tomography, or CT, which delivers a dose of radiation nearly 600 times that of a typical x-ray. According to one analysis, approximately 50 percent of the collective medical radiation exposure in 2006 was attributable to CT scans. For that reason, CT has come under intense scrutiny.

A new study by German researchers, dubbed PROTECTION-1, provides a clearer picture of the real-world use of CT scans to perform the increasingly popular diagnostic cardiovascular procedure CT angiography. The procedure is a noninvasive alternative to standard coronary angiography, which also requires radiation, but typically a lower dose. Both procedures are used to investigate cardiac symptoms such as chest pain and identify underlying problems like dangerous arterial plaque buildup.

In PROTECTION-1, which involved 50 facilities and nearly 2,000 patients, there was a sixfold difference between the facility that reported the highest median radiation dose with CT angiography and the lowest median dose. Published in the February 4 Journal of the American Medical Association (JAMA), the study also offered some encouraging news: Many facilities are taking steps to reduce the radiation doses delivered during CT angiography procedures. One tactic, called electrocardiographically controlled tube current modulation (ECTCM), for example, was used in nearly three-quarters of procedures, the researchers found, reducing radiation doses by approximately 25 percent.

Quantifying Risk

One attempt to help quantify the risk associated with CT angiography came in a July 2007 study, co-authored by Dr. Einstein, that used a computer model to estimate the cancer risk of CT angiography in different patient populations with and without dose-reducing strategies. Younger patients, women in particular, were found to be at the highest cancer risk, while dose-reducing strategies were found to significantly decrease cancer risk.

Even so, in a related commentary in JAMA, Dr. Andrew J. Einstein, from the departments of radiology and cardiology at Columbia University, called the dose differential seen in the observational study “striking.” The study, he continued, demonstrates that cardiac CT angiography “is still a potentially high-dose procedure, and like all procedures involving the use of ionizing radiation, a patient-specific benefit-risk analysis should always be performed to justify the imaging study.”

Risk versus Benefit: A Difficult Measure

Perhaps the chief concern with CT scans is their ballooning use in susceptible populations, especially children and young adults, following a car accident, for example, or to investigate reports of persistent headaches or stomach pain. Because of the growth in its popularity, CT angiography is increasingly under the microscope.

Two days prior to the PROTECTION-1 publication, the American Heart Association issued a science advisory that recommended against the routine use of CT angiography in “asymptomatic patients at low risk of ischemic heart disease,” arguing that any benefit does not outweigh the small cancer risk associated with the procedure.

This recommendation is appropriate for asymptomatic, low-risk patients, Dr. John McB. Hodgson, chair of the cardiology department at Geisinger Health System in Pennsylvania noted, adding that the decision to perform a CT scan has to be individualized.

“Scanning an asymptomatic 60-year-old, where the likelihood [of underlying cardiovascular disease] is higher, might make sense in some situations,” Dr. Hodgson said. “You have to contrast that with scanning a healthy 20-year-old. You would certainly have to have much greater justification in younger people who are asymptomatic.”

It can be very difficult to quantify the risks or benefits of some procedures with precision, explained Dr. Amy Berrington de Gonzalez, from NCI’s Division of Cancer Epidemiology and Genetics. “If the exam is clinically justifiable, then the benefits should outweigh the risks,” she said. In the case of CT angiography, she noted, there have been reports of physicians using the procedure as a screening test. “And in those circumstances it is less certain that the benefits will exceed the risks.”

Teach Them and They Will Lower

CT angiography has grown in popularity, explained Dr. Hodgson, due in large part to the advent of 64-slice CT scanners, which can scan the heart more quickly, making the procedure more accurate and easier to conduct. In fact, according to a recent market research report, the number of 64-slice CT scanners in U.S. cardiology practices has more than doubled over the last 2 years.

Coupled with CT angiography’s expanded use, Dr. Hodgson stressed, are newer, more effective dose-reduction strategies and a concerted effort by cardiac and radiation medicine organizations and the device manufacturers to educate medical personnel about the importance of avoiding unnecessary cancer risks by consistently implementing these strategies.

“Along with the [CT] manufacturers, we are doing whatever we can to reduce the radiation dose patients receive,” Dr. Hodgson said. “Are we going to get rid of all of it? No. But remember, we’re trying to diagnose the number one cause of death in the country. You have to weigh the risks and benefits.”

The available data indicate that education and quality improvement programs can have a significant impact on the radiation doses used in CT angiography procedures. In Michigan, for example, a quality improvement program led by the Advanced Cardiovascular Imaging Consortium showed that in less than a year, the average radiation dose used at the 15 participating centers decreased by nearly 50 percent without diminishing the quality of the resultant images.

—Carmen Phillips