CHICAGO — Repeated myocardial perfusion imaging is common and associated with high cumulative radiation doses that are well within the range believed to increase cancer risk, according to a large, single-center study.
This retrospective study of 1,097 consecutive patients who underwent MPI at Columbia University Medical Center, New York, in the first 100 days of 2006 showed that 39% of them received more than one MPI during the study period running from 1988 through June 2008; 18% had at least three MPIs, and 5% had five or more, Dr. Andrew J. Einstein reported at the annual scientific sessions of the American Heart Association.
Among patients with multiple MPIs, the median time between the imaging studies was just under 2 years. However, 56% of patients with multiple MPIs had two within 2 years, and 28% had two within 1 year.
Patients with more than one MPI had a median 121-mSv cumulative estimated effective radiation dose from all medical sources. That’s more than in Japanese atomic bomb survivors, as documented in the landmark Life Span Study. By comparison, 1 year’s background radiation exposure is about 3 mSv, noted Dr. Einstein, a cardiologist at Columbia.
The radiation burden accruing from CT scans has drawn much attention in recent years, but in fact MPI entails the highest radiation exposure of all imaging procedures. Moreover, MPI is booming in popularity: The volume in the United States rose from fewer than 3 million of the imaging procedures in 1990 to 9.3 million in 2002.
In the Columbia University series, men, whites, and patients with health insurance had significantly greater likelihood of undergoing multiple MPIs, compared with women, nonwhites, and the uninsured. They also received higher cumulative radiation doses over the 20-year study period. But whether this increased utilization resulted in improved cardiovascular outcomes requires further study.
The great majority of MPIs ordered in the Columbia study were medically justified as an aid to therapeutic decision making, given that more than 80% of initial MPIs and 90% of repeat procedures were performed in patients with known cardiac disease or symptoms consistent with it. But in ordering these imaging studies, physicians often don’t consider that patients with heart disease undergo numerous additional procedures involving radiation exposure, including cardiac catheterizations. Indeed, the 1,097 patients in this study had a median of 15 procedures involving radiation exposure, including 4 high-dose procedures, Dr. Einstein noted.
Alternative tests without radiation exposure include stress MRI, stress echocardiography, and exercise ECG. Lower radiation exposure alternatives to MPI for use in ruling out cardiac causes of atypical symptoms are CT angiography and percutaneous angiography, he said.
In addition to utilizing tests other than MPI when appropriate, another means of reducing cumulative radiation doses is to avoid the dual-isotope MPI imaging protocol, which typically entails more than twice as great a radiation dose than does technetium-99m MPI, the cardiologist added.
Although the high cumulative radiation doses documented in the Columbia study are "certainly a matter of concern and an important target for improvement," in Dr. Einstein’s view it is worth bearing in mind that solid tumors generally don’t develop until at least 5-10 years following radiation exposure. Patients undergoing MPI are typically older than the general population, and they have a shorter-than-average life expectancy for their age because of their cardiac disease. So the risk:benefit ratio of radiation exposure from MPIs isn’t the same as a similar exposure would be in healthy young adults.
The study was published simultaneously with his presentation at the American Heart Association meeting (JAMA 2010;304:2137-44).
Dr. Einstein’s study was supported by the National Institutes of Health and university research grants. He declared having served as a consultant to the International Atomic Energy Agency as well as GE Healthcare.