Some studies have estimated the risk of radiation exposure from imaging tests based on the risks from similar amounts of radiation exposure in the studies of the atomic bomb survivors. Based on these studies, the US Food and Drug Administration (FDA) estimates that exposure to 10 mSv from an imaging test would be expected to increase the risk of death from cancer by about 1 chance in 2000.
It can be difficult to study cancer risks from imaging studies that use radiation. In order to see small risks (such as 1 in 2000), a study would have to look at 10s or 100s of thousands of people. Information about other exposures that could be cancer risk factors would be needed, to see if it was likely that the cancer came from the radiation exposure. Since cancers from radiation take years to develop, the study would need to follow the patients for many years.
Often, scientists use questionnaire studies to look for possible causes of cancer. These studies compare exposures among people who have a certain cancer to those who don’t. They may instead compare people who had a certain exposure (like to radiation) to those who didn’t. However, this is difficult to do for diagnostic radiation exposure since many people cannot accurately recall information about things that happened many years before (such as in childhood) and information about all the imaging tests that were done is often not available. There is also a concern that people with cancer tend to over report exposures that they worry may have caused their cancers.
Studies that have found increased risk of cancer after imaging tests that use x-rays often involve people who had multiple tests or high dose procedures, including:
Studies of women who had been imaged many times with fluoroscopy as a teenager or young woman during treatment for tuberculosis have found an increased risk of breast cancer years later.
Teenagers and young women who had many x-rays of the spine to monitor scoliosis have been found to have an increased risk of breast cancer later on.
A study compared a group of people with meningioma (a brain tumor that is most often benign) with a group without the tumors. It found that the people who had the tumors were more likely to have had a type of dental x-ray called a bite-wing, and to have had bite-wing or Panorex x-rays every year.
A study in England of exposure to radiation from CT scans found that children who received a dose of at least 30 mGy (the same as 30 mSv) to the bone marrow had 3 times the risk of leukemia compared to those who received a dose of 5 mGy or less. For brain tumors, a dose of 50 mGy or more to the brain was linked to more than 3 times the risk.
A study in Australia of exposure to radiation from CT scans in childhood and adolescence found that after an average of about 9 ½ years, those who had a CT scan had a 24% higher risk of cancer overall. The risk of cancer was higher the more CT scans the person had, and it was also higher the younger the person was at the time of the CT scan. Still, the overall risk of cancer was still low.
A study from Taiwan found that children and teens who had a CT scan of the head did not have a higher risk of brain cancer or leukemia, but were more likely to be diagnosed with a benign brain tumor.