Previous research had shown rising usage in patients in the government's Medicare insurance program for the elderly. The new study showed use of imaging technologies rising by about the same amount in health maintenance organizations (HMOs) in the private sector, underscoring concerns about radiation exposure.

Diagnostic imaging allows a doctor to peer inside a patient's body without using a scalpel, reducing the need for exploratory surgery. CT scans can be used to pinpoint tumors and internal injuries, and many other reasons.

Prior studies of people in Medicare had suggested that increasing use of CT scans and other technologies has been driven in part by fee-for-service payment models that reward doctors for ordering the costly tests.

The new study, published in the Journal of the American Medical Association (JAMA), suggests that even in HMOs, in which doctors are generally paid a salary or fixed rates and have little added incentive to order imaging tests, rates of imaging doubled between 1996 and 2010.

"The rate of rise is exactly the same," said Dr. Rebecca Smith-Bindman of the University of California, San Francisco, referring to increased use of imaging in studies of patients covered by Medicare and private insurance.

That may mean other factors are driving the increased use of imaging, such as a lack of uniform imaging guidelines, fear of malpractice lawsuits and the belief that imaging is a panacea, Smith-Bindman, who led the study, said in a telephone interview.

People are exposed to small amounts of radiation every day, annually adding up to about 2.4 units, known as millisieverts, a level not considered a risk to health.

The average radiation dose from a chest CT scan is 7 millisieverts, compared with 0.1 millisieverts for a typical chest X-ray, according to the American College of Radiology. But those levels can vary widely by machine, with some low-dose scanners delivering as little as 1.5 millisieverts for a chest CT, and some older machines delivering far higher doses.

Researchers say a radiation dose of 50 millisieverts starts to raise concerns about human health, and a dose of 100 millisieverts is thought to raise the risk of cancer.

In her study, Smith-Bindman's team analyzed health records of millions of patients in several Western and Midwestern U.S. states enrolled in six large HMOs from 1996 to 2010.

They found that CT imaging rates for HMO enrollees aged 65 and older rose 10.2 percent a year from 1998 to 2005, then slowed to an annual increase of 4.2 percent from 2005 to 2008.

This rate is similar to growth rates of 10.1 percent and 5.1 percent in the same two periods among Medicare enrollees, according to recent studies based on data from Medicare.

MAKE PEOPLE AWARE

Smith-Bindman said the point is not to scare people out of getting needed diagnostic tests, but to make them aware that these tests are not without risk. She has been urging U.S. lawmakers to establish national guidelines for controlling the amount of radiation a patient gets from diagnostic exams.

The Medical Imaging & Technology Alliance, which represents imaging equipment makers such as General Electric and Siemens, said the findings confirm that use of advanced imaging technologies such as CT is now actually declining, a trend seen in several recent analyses of data from Medicare and private insurance companies.

"Patients are actually receiving fewer imaging procedures, which further debunks the myth that life-saving diagnostic imaging services are growing and contributing to rising health care costs," the group said in a statement.

Smith-Bindman's team asserts that imaging rates are still too high, exposing patients to more radiation than necessary.

"A lot of people say doses are going down and manufacturers are creating new, lower-dose imaging modalities, so the problem is gone," Smith-Bindman said.

"The problem is not gone. Patients are getting high doses - higher-than-needed doses - and there are no standards for patients to know what is the right dose," she said.

Smith-Bindman cited a study last week in the journal Lancet finding among other things that exposing a child to ionizing radiation from two or three CT scans of the head can triple the risk of developing brain cancer later in life. ID:nL5E8H64O4]

Dr. George O'Connor of the Boston University School of Medicine and Dr. Hiroto Hatabu of Harvard Medical School said in an editorial in JAMA that Smith-Bindman's study does not explore the reasons for the imaging tests or whether the risk of radiation exposure is outweighed by the benefits.

It does suggest that doctors need to discuss radiation risks with their patients, taking into account the cumulative dose of radiation from prior scans, they wrote.

(Editing by Michele Gershberg and Will Dunham)

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