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Radiation From Imaging a Growing Concern


 

Medical imaging exposes a significant portion of patients to various doses of ionizing radiation, and in some cases, to substantial doses, potentially increasing the associated risk of cancer, according to findings of a retrospective cohort study.

The results are based on an analysis of 952,420 nonelderly adults who were enrolled in United Healthcare's database between Jan. 1, 2005 and Dec. 31, 2007, and living in Arizona, Dallas, Orlando, South Florida, and Wisconsin.

Roughly 70% of the study population underwent at least one imaging exam during the 3-year study period, “resulting in mean effective doses that almost doubled what would be expected from natural sources alone,” wrote Dr. Reza Fazel of Emory University, Atlanta, and her coinvestigators.

While most patients received less than 3 millisievert (mSv) per year—which was considered low exposure—there was a sizable minority of patients who received moderate, high, or very high radiation doses, they wrote.

CPT codes for imaging procedures involving radiation were used to identify claims from hospitals, outpatient facilities, and physicians' offices. They excluded procedures in which radiation was specifically delivered for therapeutic purposes, such as high-dose radiation for cancer. Procedures were categorized by technique: plain radiography, CT, fluoroscopy (including angiography), and nuclear imaging. They also categorized the procedures by area of focus: chest (including cardiac imaging), abdomen, pelvis, arm or leg, head and neck (including brain), multiple areas (including whole-body scanning), and unspecified.

To account for the possibility of procedure overlap—for example, coronary stent placement and catheterization of the left heart performed at the same time—subjects were limited to one procedure per day that involved the same type of technique and the same anatomical area, selecting the highest dose.

Estimates of typical effective doses from published literature were used to approximate radiation exposure for each imaging procedure. The effective dose is an inexact measure of the overall detrimental biologic effect from radiation exposure.

Patients were stratified by gender and age: 18–34, 35–39, 40–44, 45–49, 50–54, 55–59, and 60–64; 52% were women. The researchers calculated effective doses for the population overall and for each age-based and sex-based group and categorized them by dose: low (no more than 3 mSv/yr, the background level of radiation from natural sources in the United States); moderate (3–20 mSv/yr, the upper annual limit for occupational exposure for at-risk workers, averaged over 5 years); high (20–50 mSv/yr, the upper annual limit for occupational exposure for at-risk workers in any given year); and very high (greater than 50 mSv/yr).

A total of 3,442,111 imaging procedures associated with 655,613 patients were identified in the 3-year period. The average number of procedures per person per year was 1.2 and median number was 0.7/person per year. The mean effective dose was 2.4 mSv/person per year with a median effective dose of 0.1 mSv/year.

The proportion of patients undergoing at least one procedure during the study period increased with age—from 50% in those aged 18–34 years to 86% in those aged 60–64 years. A total of 79% of women underwent at least one procedure during the study period, compared with 60% for men (N. Engl. J. Med. 2009;361:849–57).

Moderate doses occurred at an annual rate of 199 per 1,000 patients. High and very high doses occurred at annual rates of 19 and 2 per 1,000 patients, respectively. “Each of these rates rose with advancing age,” noted Dr. Fazel.

“Generalization of our findings to the United States suggests that these procedures lead to cumulative effective doses that exceed 20 mSv per year in approximately 4 million Americans,” the researchers wrote.

Myocardial perfusion imaging accounted for almost a quarter of the total effective dose (22%). CT of the abdomen, pelvis, and chest accounted for 38% of the total effective dose.

“CT and nuclear imaging accounted for 21% of the total number of procedures and 71.4% of the total effective dose,” the researchers reported. By anatomical site, chest procedures accounted for 45% of the total effective dose. Lastly, the bulk of the total effective dose—82%—was delivered in outpatient settings, primarily physicians' offices.

The findings are concerning, particularly for patients who undergo several imaging tests in a short time, Dr. Michael S. Lauer wrote in an accompanying editorial (N. Engl. J. Med. 2009;361;841–3).

“Though the danger may be small, it is cumulative and hence of particular relevance to the small but substantial minority of patients, who … undergo clusters of tests.”

Despite the cumulative risk associated with radiation exposure, it's generally not something that is discussed with patients undergoing an imaging procedure, noted Dr. Lauer, who is director of the prevention and population sciences division of the National Heart, Lung and Blood Institute in Bethesda, Md. “The issue of radiation exposure is unlikely to come up because each procedure is considered in isolation [and] the risks posed by each procedure are low and seemingly unmeasurable….

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