DEAR MAYO CLINIC: At what level does radiation exposure from X-rays and other medical imaging increase the risk of getting cancer in the future?

ANSWER: While there are wide-ranging benefits to modern X-ray imaging technology, you are correct that there also are risks. The answer to your question isn't clear-cut, so it's important to weigh the potential risk against the known benefits of medical imaging.

Medical imaging uses radiation to generate images of the inside of your body. It can provide a quick and painless diagnosis or guide treatment, such as determining artery-opening stent placement.

The use of X-ray imaging has increased dramatically over the past 30 years, mainly due to technical advances and an increased use of CT and positron emission tomography (PET) scans. Likewise, the average amount of lifetime radiation exposure also has increased. The downside is that radiation can cause damage to, and mutation of, DNA. This might lead to the development of certain cancers.

Radiation is naturally present in the environment, coming from sources such as the sun and radon in rocks and soil. The average annual exposure in the U.S. from all sources of natural radiation is estimated to be about 3 millisieverts per person. However, you may be exposed to more or less depending on where you live. Exposure from natural radiation varies from 1 to 20 millisieverts in the U.S.

It's estimated that the average annual amount of radiation exposure -- a combination of natural and medical exposure in the U.S. -- has roughly doubled since the 1980s to about 6.2 millisieverts. This value is an average over the entire population. If you haven't had any medical exams that use radiation, your radiation exposure has not increased.

It's not known at what levels radiation begins to significantly increase cancer risk. Below 100 millisieverts, an increase in risk has not been shown to exist. However, by scaling the risk at higher doses down to lower doses, it has been estimated that 10 millisieverts of radiation exposure could increase the lifetime risk of an adult dying of cancer by about 0.05 percent. With a 21 percent overall lifetime risk of dying of cancer, 10 millisieverts of radiation exposure theoretically could raise the lifetime risk of dying of cancer to 21.05 percent. This is a small amount considering the benefits of medically needed imaging tests.

Because the potential for an increased risk of cancer due to medical exams using radiation is small, there's no reason to forgo an imaging test or X-ray-assisted procedure that could better your health or prolong your life. This is especially true for older adults, as potential radiation-caused cancer development may not occur for decades.

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Also consider that CT and positron emission tomography scanners in use today use a much lower radiation dose than was used even just a few years ago. Radiation doses are further kept to a minimum when the dose is tailored specifically to body size and diagnostic requirements.

Still, radiation experts generally err on the side of caution. They assume that there's no risk-free level of radiation exposure and recommend only medically necessary imaging tests.

To keep medical radiation exposure to a minimum, ask if the results of a scan or test are likely to change the course of your treatment. If not, it may not be necessary. If you switch health care providers, have image files transferred over to your new provider to avoid repeat X-ray exams. Finally, if the test is medically necessary, have the imaging done at a nationally accredited imaging center when possible. (Adapted from Mayo Clinic Health Letter) -- Patricia Mergo, M.D., Radiology, Mayo Clinic, Jacksonville, Fla.

(Mayo Clinic Q & A is an educational resource and doesn't replace regular medical care. E-mail a question to MayoClinicQ&A@mayo.edu. For more information, visit www.mayoclinic.org.)

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