AHA: SPECT MPI targeted for cumulative rad exposure
New York researchers found that very high cumulative estimated effective radiation doses for many patients undergoing SPECT myocardial perfusion imaging (MPI) and other ionizing radiation sources, but especially high cumulative doses for patients undergoing repeat MPI testing, according to a study in the Nov. 17 issue of Journal of the American Medical Association. The study is being released early online because it is being presented at the American Heart Association's (AHA) annual meeting this week in Chicago.

Andrew J. Einstein, MD, PhD, of Columbia University Medical Center and New York-Presbyterian Hospital, and colleagues noted the increased attention paid to CT scanning regarding radiation exposure. But a study by Fazel et al in 2009 revealed the MPI was the "single test with the highest radiation burden, accounting for 22 percent of cumulative effective dose from medical sources."

The investigators commenced on the current study because few data are available to characterize the total radiation burden received over an extended period by patients undergoing MPI.

They mined two EHR systems at Columbia and identified 1,097 patients who underwent MPI during the first 100 days of 2006 (Jan. 1 through April 10) at Columbia University Medical Center. The researchers evaluated all preceding medical imaging procedures involving ionizing radiation undergone beginning October 1988, and all subsequent procedures through June 2008, at the center.

Patients underwent a median of 15 procedures involving radiation exposure, of which four were high-dose procedures, defined as an effective dose of at least 3 mSv. They found that 18.2 percent had at least three MPI exams and 4.9 percent had at least five MPI exams.

The study revealed that the median cumulative estimated effective dose from MPI alone was 28.9 mSv, and for all medical testing, was 64 mSv. They found that 6.5 percent of patients received cumulative doses of more than 100 mSv due to MPI alone, and 31.4 percent received more than 100 mSv from all medical sources, including 10.9 percent who received cumulative dose of more than 200 mSv.

More than 100 mSv is "a level at which there is little controversy over the potential for increased cancer risks," they wrote.

The primary reason for testing was chest pain, breathing difficulty, or both in 66.8 percent of examinations. More than 80 percent of initial and 90 percent of repeat MPI exams were performed in patients with known cardiac disease or symptoms consistent with it. Of 1,097 patients undergoing index MPI, 38.6 percent underwent additional MPI studies. For this group of patients undergoing multiple MPIs, the median cumulative estimated effective dose was 121 mSv.

"Although the high cumulative doses observed are certainly a matter of concern and an important target for improvement, these doses should not be viewed in isolation but rather within the clinical context where radiation risk for a specific patient is balanced against potential benefits," according to the study.

Einstein et al wrote that patients receiving SPECT MPI exams are generally older than the general population and have known or increased risk for cardiac disease. Additionally, they found that "the clear majority of MPI examinations were performed for reasons presently regarded as appropriate and with the potential to effect therapeutic management."

Nevertheless, they concluded, "Efforts are needed to decrease this high cumulative dose and its potential attendant risks."

The researchers noted that their nuclear cardiology laboratory has moved away from the dual-isotope protocol in an effort to decrease effective dose of an individual MPI exam. They had found that two-thirds of MPI exams in the 20-year period studied were performed using a dual-isotope protocol,

In addition, they said that PET MPI and low-dose protocols using newer camera and image reconstruction technologies offer the potential to decrease dose even further.