Dr. Thomas Dehn, chief medical officer for National Imaging Associates Inc., which manages health plans for private insurers, reviewed more than 800,000 imaging claims over a four-year period and found 11,535 patients who had received more than 50 millisieverts, mostly from CT scans.
Of those patients, 107 had each received more than 200 millisieverts, including one who got 992 millisieverts.
"The patients should be asking the physicians, 'Should I be getting this exam?' " Dehn said.
The number of multiple scans is growing. A study by the Government Accountability Office on the rising cost of medical imaging found that patients who got a CT scan averaged 2.5 scans in 2006, up from 2.1 in 2000.
Scanner manufacturers have responded to rising concerns about radiation by improving their machines, allowing operators to select the lowest dosage necessary to get a useful image.
Still, CT scans have become the primary driver of the nation's rising radiation exposure.
Between 1980 and 2006, the dose per person from medical testing more than quintupled from 0.55 millisieverts to an estimated 3 millisieverts a year.
Medical tests are now the biggest source of radiation exposure, recently surpassing background radiation, according to the National Council on Radiation Protection & Measurements.
Of particular concern is the rising use of CT scans for children and pregnant women. Children -- who account for 11% of CT scans -- face significantly higher risks than adults because they are more sensitive to radiation and have more years ahead for a cancer to develop.
For example, an abdominal scan in a 5-year-old carries a 0.10% risk of triggering a fatal cancer, nearly 10 times the risk in adults older than 35, according to the New England Journal study.
Risks this small are not well understood, and there is a vigorous debate over what they mean.
The best data on low-level radiation exposure come from studies of about 25,000 survivors from two or three miles outside the blast zone of the Hiroshima and Nagasaki atomic bombs at the end of World War II. They received between 5 and 150 millisieverts -- equivalent of a few CT scans -- and had small but statistically significant increases in cancer and death rates, according to studies.
The National Academy of Sciences weighed in on the issue in a 2006 report, saying that there is no safe level of radiation exposure and that even small doses pose some health risks.
