* Study latest example of overuse of CT scans * Half of radiation delivered was for unnecessary studies By Julie Steenhuysen CHICAGO, Nov 30 (Reuters) - More than half of patients receiving abdominal CT scans, an advanced type of X-ray, got them for tests they did not need, exposing them to excess radiation that could raise the long-term risk of cancer, U.S. researchers said on Monday. The study, presented on Monday at the Radiological Society of North America meeting in Chicago, adds to mounting evidence that Americans are exposed to an increasing amount of radiation from diagnostic imaging exams. In August, a team at Emory University in Atlanta reported in the New England Journal of Medicine that as many as 4 million Americans a year are exposed to high doses of radiation. A report in March from the National Council on Radiation Protection and Measurement found Americans were exposed to seven times more radiation from diagnostic scans than in 1980. Radiation is measured in millisieverts. The average American can expect to receive about 3 millisieverts a year from ground radon or flying in an airplane. That level is not considered a risk to health. The University of Wisconsin-Madison team led by Dr. Kristie Guite studied 978 CT scans of the abdomen and pelvis done on 500 patients that were sent to the university to be interpreted. They used American College of Radiology guidelines to determine whether they were appropriate. They found that 52.2 percent of the patients were found to have had CT scans that were unnecessary. The average excess radiation dose per patient was 11.3 millisieverts, about the equivalent of 113 chest X-rays. RAISING CANCER RISK Some patients got a lot more radiation than others. In one in five patients, the dose was 50 millisieverts, enough to raise some concerns about health problems, Guite said. Seven of the 500 got 100 millisieverts of radiation, a level known to raise the risk of cancer. "At the dose seen in our study, one in 1,000 patients could get a radiation-induced cancer," Guite told the meeting. "This could lead to up to 23,000 radiation-induced cancers per year," she said. Many of the scans they looked at involved the use of a contrast agent -- a fluid injected into a patient's veins that makes the images more clear. In that type of study, radiologists can look for different characteristics of the same organ at different times, depending on when the solution is present in the organ, and they often take multiple images of the same organ in the series. Each pass of the scanner exposes patients to radiation. Dr. Louis Hinshaw of the University of Wisconsin who worked on the study said many institutions may be doing the extra studies for good measure, or because their machines are automatically set to do them. But he said it was possible some centers were doing the extra scans because they may get paid more for them. He said patients undergoing multiple scans should ask their doctors about the risks and benefits of the exams and find out whether a smaller number of imaging studies might work just as well. Imaging equipment makers such as GE Healthcare <GE.N>, Siemens <SIEGn.DE>, Philips <PHG.AS> and Toshiba Medical Systems <6502.T> are working to develop low-dose CT scanners. (Editing by Peter Cooney) ((julie.steenhuysen@thomsonreuters.com ; +1 312 408 8131)
