Coronary Calcium Scans Can Raise Cancer Risks
MONDAY July 13, 2009 -- When weighing whether a coronary calcium scan is worth the risk, a new study suggests that arriving at an answer won't be clear-cut or easy.
A team of researchers from the U.S. National Cancer Institute and Columbia University found that the average range of radiation exposure from having such a screening test every five years would cause 42 additional cases of cancer among 100,000 men and 62 additional cases among 100,000 women. However, given the wide range of radiation doses seen in the study, the increase could be as low as 14 cases and as high as 200 cases among 100,000 men, and as low as 21 cases or as high as 300 cases among 100,000 women.
This is an issue of growing importance on the American medical scene, said Dr. Andrew J. Einstein, director of cardiac computed tomography research at Columbia University, and a member of a team that reports its findings in the July 13 issue of the Archives of Internal Medicine.
"There has been great interest recently in computed tomography, owing to the fact that the number of CAT scans has grown tremendously in the United States," Einstein said. "The National Council on Radiation Protection & Measurement estimates that 70 million are done per year."
The study in which Epstein took part looked at a form of computed tomography that scans for calcium deposits in heart arteries. CAC scanning, as it is called, is one of the lesser-done forms of computed tomography, but a private organization, Screening for Heart Attack, Prevention and Education, has proposed that it be done annually on 50 million Americans, and a new Texas law mandates health insurance coverage of the procedure.
The new study looked at what a dose of radiation in a single CAC scan would be, and found an enormous variation. There is no single protocol -- set of rules -- for such a scan, which can be done on a variety of equipment, Einstein said. "This was first proposed in 1990, and CT scanner technology has changed, so it is not clear what the protocol might be," he noted.
The study found roughly a 14-fold difference in radiation dosage among the various CAC scan protocols. Eliminate two or three "outlying" readings, and the difference is still threefold, Einstein said.
But those estimates are suspect, said Dr. Thomas G. Gerber, an associate professor of medicine and radiology at the Mayo Clinic, and co-author of an accompanying editorial, because it is based on extrapolation of the damage done to people exposed to high doses of atomic bomb radiation at Hiroshima and Nagasaki.
"At the very low doses used in medical imaging, there is a huge controversy about whether there is an increased risk of anything," Gerber said. "Estimates of increased risk are based on a linear no-threshold hypothesis. There is even a theory that chronic exposure to low doses of radiation might be beneficial."
There is equal cloudiness on the benefit side of the equation, said Gerber. "I am not a strong proponent of screening," he added.
"The risk of cardiac events increases if calcium is present in the arteries," Gerber said. "But you can't pick up blockages before they are 70 percent or more. There is some debate about whether the risk predicted by coronary calcium screening is incremental [adds to] the risk predicted by conventional risk factors. It stands to reason that it might be, but that is not proven."
The U.S. Preventive Services Task Force recommends against using CT scans in screening programs, and the American Heart Association says they should be used for "selected individuals" at intermediate risk.
So what do physicians do about CAC scanning in the real world?
"I like using it for patients at intermediate risk of coronary disease, when I do not know how aggressive therapy should be," Einstein said. "For such patients, it is a fantastic test."
"In my practice I use it for patients with no symptoms but an unfavorable risk factor profile," Gerber said. "If there are risk factors but they are adamant about not changing their lifestyle or taking coronary medication, I think it sometimes helps patients realize their coronary atherosclerosis [hardening of the arteries] has begun."
A definitive study of the risk-benefit ratio of CAC scanning is unlikely, Einstein said. The people in question are not at high risk of heart disease, and "the rarer an event is, the larger the sample size that is needed," he said. "A randomized controlled trial would require hundreds of thousands or millions of patients, with adequate follow-up."
For more on coronary calcium scans, go to the U.S. National Heart, Lung, and Blood Institute.
Posted: July 2009