Study refuels debate over whole-body CT screening

A study has revived the controversy over full-body CT screening's benefits versus its associated risks. According to Columbia University researchers, asymptomatic patients undergoing full-body CT scans face an increasing risk of dying from cancer, due to radiation exposure. Some screening supporters, however, question the study's methodology, contending that valid evidence to support its claims does not exist.

"Some of my colleagues feel that CT scanning is not just a major weapon in the diagnostic armamentarium but that it's a weapon of mass destruction," said Dr. Michael N. Brant-Zawadzki, medical director of radiology at Hoag Memorial Hospital in Newport Beach, CA. "It's a disservice to an extremely useful, Nobel Prize-winning technology, which, nonetheless, should be used responsibly."

The Columbia investigators estimated the radiation-related mortality risks associated with full-body CT screening based on Japanese atomic bomb survivor data, which have had 60 years to mature. They found that the cumulative dose from a full-body CT scan can be almost as high as the dose absorbed by survivors of the atomic bombs the U.S. dropped on Hiroshima and Nagasaki (Radiology 2004;232:735-738).

If a 45-year-old person had annual full-body CT screening exams until turning 75, that person's estimated odds of radiation-induced cancer mortality would be about one in 50, or approximately two percentage points above the normal average, said principal investigator David J. Brenner, Ph.D., a professor of radiation oncology and public health at Columbia. Single full-body CT scan doses can be modest, though not negligible, the study found. Atomic bomb survivors received doses averaging 20 mSv. The effective dose for one full-body CT scan is approximately 12 mSv.

Calculating cancer risks to the public based on theoretical analyses that conflict with hard data on millions of patients-many of whom received radiation doses above what this study projects-is wrong, said Dr. Harvey Eisenberg, medical director of HealthView Center for Preventive Medicine in Newport Beach, CA.

Specialists agree that significant studies that scrutinize whole-body screening are missing. But an extensive literature does exist on screening for vascular disease such as aortic abdominal aneurysms and coronary plaque, as well as on breast, lung, and colon cancer screening. Most studies affirm the benefits of screening for early signs of disease, Eisenberg said.

Other radiologists consider the extrapolation of data to formulate projections not only questionable but also one-sided. A similar line of argument with existing supporting data could easily be used to present, for instance, opposing views suggesting that low-dose radiation can prevent cancer, Brant-Zawadzki said. A follow-up study of 7852 villagers exposed to radiation after an explosion at the Mayak nuclear weapons complex in the former Soviet Union found that this group had a much lower cancer mortality rate than unexposed villagers (Kostyuchenko et al, Sci Total Environ 1994;142:119-125).

A variety of reports provide data on one side or the other. The bottom line is that there is no direct evidence that carcinogenesis occurs at the low-dose levels of CT body scans, nor is there evidence that these scans do not provide benefit, Brant-Zawadzki said.

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ESTIMATED ORGAN DOSES FOR TYPICAL FULL-BODY CT EXAM

Organ dose (mGy)

Thyroid 24.7

Bone surface 15.7

Esophagus 16.2

Lung 15.5

Stomach 14.4

Liver 14

Bladder 13.9

Breast (female) 12.3

Gonads (female) 12.2

Colon 11.6

Red bone marrow 9.9

Skin 7.5

Gonads (male) 2.6

Source: D. Brenner