In large doses, radiation can cause serious tissue damage and increase a person’s risk of later developing cancer. The low doses of radiation used for imaging tests might increase a person’s cancer risk slightly, but it is important to put this risk into perspective. In this section we will answer some of the more common questions people have about radiation risks linked to imaging tests.
How much does an imaging test increase a person’s radiation exposure?
Background and non-medical radiation
We are constantly exposed to radiation from a number of sources, including radioactive materials in our environment and cosmic rays from outer space. This is called background radiation.
The average person is exposed to about 3 mSv (millisieverts) of radiation from natural sources over the course of a year. (A millisievert is a measure of radiation exposure.) Much of this exposure is from radon, a natural gas with levels that vary across the country.
Because the earth’s atmosphere blocks some cosmic rays, living at a higher altitude increases a person’s exposure. For example, residents in the plateaus of New Mexico and Colorado, have an annual exposure level of about 1.5 mSv more than people living at sea level. And a 5-hour airline flight increases exposure by about 0.03 mSv.
Smoking a pack of cigarettes a day exposes the smoker to an extra 53 mSv per year.
Radiation from imaging tests
A single chest x-ray exposes the patient to about 0.1 mSv, which is about the radiation dose people are exposed to naturally over the course of 10 days. A mammogram exposes a woman to 0.4 mSv, or about the amount of exposure a person would expect to get in about 7 weeks.
Some other imaging tests have higher exposures. A lower GI series using standard x-rays exposes a person to about 8 mSv. A CT scan of the abdomen (belly) and pelvis exposes a person to about 15 mSv, this goes up to 30 mSv if the test is repeated with and without contrast. A CT colonography exposes you to about 10 mSv of radiation. Keep in mind that these are estimates, and recent studies have found that the amount of radiation you get can vary a great deal.
If you have concerns about the radiation you may get from a CT scan, check with the facility that will perform the test. (Remember that MRI and ultrasound exams do not expose a person to radiation.)
How much does the extra radiation increase a person’s cancer risk?
It is hard to know how much the radiation exposure from imaging tests increases a person’s cancer risk. Most studies on radiation and cancer risk have looked at people exposed to very high doses of radiation, such as uranium miners and atomic bomb survivors. The risk from low-level radiation exposure is not easy to calculate from these studies.
Researchers have estimated that radiation exposure from the average diagnostic x-ray may increase cancer risk very slightly (likely on the order of hundredths to thousandths of one percent). Of course, this can be affected by the type of test done, the area of the body exposed, and other factors.
We do know that children are more sensitive to radiation and should be protected from it as much as possible.
Because radiation exposure from all sources can add up over a lifetime, and radiation can, indeed, increase cancer risk, imaging tests that use radiation should only be done for a good reason. In many cases, other imaging tests such as ultrasound or MRI may be used. But if there is a reason to believe that an x-ray or CT scan is the best way to look for cancer or other diseases, the patient will most likely be helped more than the small dose of radiation can hurt.
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