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Obese Get Higher Doses of Radiation for X-Rays

Cumulative effect of that needs study, experts say,,

TUESDAY, June 30 (HealthDay News) -- People who are overweight and obese are usually given higher-than-normal doses of radiation in order to obtain usable X-ray images, even though the long-term effects are unknown, new research contends.

"You need to get a certain amount of X-rays to go through the body in order to get an informative image, and excess weight impedes that," explained the study's lead author, Jacquelyn C. Yanch, a professor at the Massachusetts Institute of Technology in Boston. "And there are very few ways around that problem, other than increasing an overweight patient's exposure to radiation to improve the image quality."

"Americans have gotten larger on average over the last half-century, and so as a result, our radiation dosages have gone up," she added. "Exposure can be sometimes 20, 30, even 40 times as much for an overweight patient as for a lean person. And in general, we're also getting more exams and more intensive exams."

"But even so, we don't actually know the impact of such high doses over time, or whether they're dangerous, because we simply haven't tracked the effect," Yanch said. "So, it's very important for us to start monitoring this exposure for each individual patient so we can get a handle on it."

X-rays account for the lion's share of people's exposure to radiation, the researchers note.

To gauge how high radiation doses must be to get effective X-rays for overweight and obese people, the researchers used computer simulations that delivered X-ray beams at various strength levels to so-called "phantom" patients representing five different fat tissue levels, from lean to obese. Different body areas, including the chest and abdominal regions, were X-rayed to see which entry points were most effective in rendering high-quality images.

The researchers then compared the dosage levels, body position options and body mass distribution numbers with the amount of radiation the computer software indicated that each imaginary patient would have had to absorb to get the desired image.

Overall, they found that directing an X-ray to the thickest part of a person's fat allowed delivery of a lower radiation dose than would be needed if a different area were targeted.

Even so, the dosage needed to get an effective X-ray via the abdominal region of a moderately overweight man was up to 10 times greater than for a lean man, the study found. And for a more severely overweight or obese person, the needed dosage was up to 40 times greater than for someone of normal weight.

The findings are reported in the July issue of Radiology.

Yanch and her team pointed out, however, that the development of adipose tissue cancer from radiation exposure has not been proven. In fact, she said, the issue is not that the jury is still out but that it has not even begun to deliberate.

"For some procedures, the standard doses are very low, and so even 20 times higher than that is likely still too low to be concerned about," she noted. "But for some procedures, the doses to a very overweight person will be quite high. Yet the scientific field does not know what this all means. We don't know if this is anything to be concerned about yet. We have models and prediction, but no real data."

"The way to find out is to begin to pay close attention to what doses are being given over time, patient by patient, and what the outcomes are," she added.

Dr. Levon Nazarian, a professor of radiology and vice chairman for education at Thomas Jefferson University Hospital in Philadelphia, agreed that the central question is how the risk for health problems goes up with repeated exposure to high-dose radiation.

"It's true that the dose from a typical X-ray is very, very small," he said. "So, yes, 40 times a very small dose is still small and probably not dangerous. However, were you to do repeated X-rays at 40 times the regular dose, eventually the additive effect could be dangerous because your body never forgets an X-ray. It's cumulative over time. So the more you do over a lifetime, the more you have a chance of causing cancer due to radiation exposure."

Dr. Raul N. Uppot, a radiologist at Massachusetts General Hospital and an assistant professor of radiology at Harvard Medical School, noted that doctors dealing with overweight patients are caught between a rock and a hard place because of the absence of definitive information about long-term side effects of high-dose radiation.

"The image quality among obese patients is not sufficient unless something is done to adjust exposure upwards to deal with the problem," he noted. "That is a bottom line. But, of course, we're always trying to balance what's safe with what's needed. So certainly I think, when you have no other option but to increase dosage exposure, you do want to come back and monitor these patients and see if their risk of cancer is any higher. Ultimately, that's the critical information needed to determine what dosages are, in fact, safe."

More information

The Radiological Society of North America has more on radiation exposure in X-rays.

SOURCES: Jacquelyn C. Yanch, Ph.D., professor, department of nuclear science and engineering and department of biological engineering, Massachusetts Institute of Technology, Cambridge, Mass.; Levon Nazarian, M.D., professor, radiology, and vice chairman, education, Thomas Jefferson University Hospital, Philadelphia; Raul N. Uppot, M.D., radiologist, Massachusetts General Hospital, and assistant professor, radiology, Harvard Medical School, Boston; July 2009, Radiology

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