Risks of Radiation

Weighing the Radiation Risks of CT, X-ray and Other Imaging

Risk of Radiation

When X-ray radiation is absorbed within our bodies, it can damage molecular structures and potentially cause harm. Very high doses of radiation cause damage to human cells, as evidenced by skin burns, loss of hair, and increased incidence of cancer. Because high doses of radiation can cause cancer, it is therefore generally assumed that low doses may also cause cancer.  However there is currently no direct scientific proof that this occurs, and some studies suggest that low radiation doses (such as those used in imaging) do not increase the risk of cancer [1, 2].

At the present time all studies of the risk of cancer from low doses of radiation have limitations. Recent articles in the scientific literature predict thousands of new cancers caused by the use of CT and other radiation-based imaging, but those predictions are derived by multiplying the large number of imaging procedures in large populations by the small assumed risk of developing cancer associated with an imaging study. If one person has a CT scan  (with a 10mSv dose) with a new cancer risk of one in 2,000, it does not mean that a second person having a CT scan will then double their risk to one in 1,000. Nor is it clear that if the same person has two CT scans, two weeks apart, that their risk will increase from one in 2,000 to one in 1,000. Among other published material on the subject, the Radiologic Society of North America and the American Association of Physicists in Medicine issued a well-respected position statement in 2011 that states:

“Risks of medical imaging at effective doses below 50 mSv for single procedures or 100 mSv for multiple procedures over short time periods are too low to be detectable and may be nonexistent. Predictions of hypothetical cancer incidence and deaths in patient populations exposed to such low doses are highly speculative and should be discouraged.”


  1. Mean Cancer Mortality Rates in Low Versus High Elevation Counties in Texas Dose Response. 2010; 8(4): 448–455.
  2. Radiation increased the longevity of British radiologists British Journal of Radiology (2002) 75, 637-639. 

Risks of Radiation vs. Benefits of Radiation

Risks of Radiation

Generally speaking, all medical procedures and tests carry both risks and benefits, and any consideration of radiation risk must be balanced against the benefits. Media reports generally focus on the risks of radiation, but there are substantial benefits to these tests that must be considered in any balanced discussion of risk versus benefit. After a radiation dose of 10 mSv (which is the kind of dose associated with many CT scans) the estimated risk of developing a fatal cancer is one in 2,000, but that risk must be considered in the context of the good that comes from receiving a dose of radiation. Undue anxiety about the cancer risks of radiation could potentially expose patients to far greater risks from delayed diagnosis or incorrect management.


Alternative Perspective Regarding Risk

Another way of thinking about risk is to focus on the likelihood that something will not happen, rather than the odds that it will happen. For example, a one in 2,000 risk of cancer means a 99.95 percent chance of not getting cancer.

Risks in Context

In addition to the benefits, the small risk from low-dose X-ray radiation should be considered in the context of other risks. The risk of death from smoking, for example, kills 440,000 Americans every year, a much greater and easily reversible proven death toll from a man-made product that produces no medical benefit.

To put the estimated risk of developing cancer from an imaging study in context, keep in mind that:

  • The annual dose of radiation we all receive from background radiation is from 3 to 5 mSv.
  • Flying from New York to Los Angeles two or three times is equivalent to receiving the dose of radiation from a chest X-ray.
  • Spending nine to 50 days in Denver, Colorado is equivalent to receiving the dose of radiation from a mammogram.

Risk Comparisons

The amount of radiation during a typical body CT scan (10 mSv) is about the same as the radiation we get every two years from background sources, and the presumed risk of getting a fatal cancer from this amount of radiation is about one in 2,000. The table and graph below put this risk in the context of other occurrences that are of similar magnitude. For example, a patient who never smoked is more than twice as likely to die from lung cancer as from a cancer caused by a typical CT scan.

Risk of Death from Common Occurrences

Relative Risk