More Americans are receiving computed tomography (CT) scans than ever before, and while this technology can save lives, some scientists are concerned that low doses of ionizing radiation could increase cancer risk.
Importantly, at an individual level, the theoretical risk of developing cancer from a CT scan is thought to be very low, if it exists at all. Patients should not hesitate to undergo these tests if they are considered medically necessary.
However, the number of CT examinations performed annually in the US has increased by more than 30 percent since 2007, and researchers suggest that unwarranted tests are exposing the population to unnecessary radiation.
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In a study published in April, a team in the US and the UK predicted that low levels of ionizing radiation from CT scans could theoretically account for 5 percent of all new cancer diagnoses in the US. CT scans conducted in 2023 could be responsible for an estimated 103,000 future cases of cancer.
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That’s based on some assumptions and historical data from high radiation events, but if right, it would put CT scans on par with other significant risk factors for cancer, like alcohol consumption, at least at a population level.
“CT is frequently lifesaving, yet its potential harms are often overlooked, and even very small cancer risks will lead to a significant number of future cancers given the tremendous volume of CT use in the United States,” writes the international team of analysts, led by epidemiologist Rebecca Smith-Bindman from the University of California, San Francisco.
As of now, these are just theoretical risks, but that doesn’t mean they aren’t worth considering. While scientists know that high doses of radiation cause cancer, conclusive evidence to link low-level radiation to cancer is lacking.
The potential association is mostly based on long-term studies of atomic bomb survivors and those exposed to nuclear power plant meltdowns. For instance, in a group of 25,000 Hiroshima survivors, who received a dose of ionizing radiation on par with three or more CT scans, there was a slight but significant increase in cancer risk across a lifetime.
Whether those results extend to CT scans remains hotly debated, and the theoretical risks must be weighed against the benefits of this technology, which can now alert patients and doctors to a whole variety of hidden diseases and injuries with very low doses of radiation (about the same amount you absorb from your environment over three years).
“Any risk from a CT scan of a sick patient is likely much less than the risk of the underlying disease,” said Cynthia McCollough, CT imaging expert and past president of the American Association of Physicists in Medicine.
In a large national trial, for instance, there was a 20 percent decrease in lung cancer deaths among smokers and ex-smokers who received low-dose CT scans compared to those who only had a chest X-ray.
The recent predictions on cancer risk are again based on historical tragedies, but compared to previous analyses, they consider more detail on the actual radiation exposure, which can depend on the type of CT device, the scanning duration, the size of the patient, and the sensitivity of their targeted body part.
The anonymous data comes from 143 hospitals and outpatient facilities across the US, catalogued in the UCSF International CT Dose Registry. Using statistics from 2016 to 2022, researchers predicted 93 million CT examinations were carried out in 2023, on roughly 62 million patients.
Based on the associated radiation risks, the team estimates that CT scans in 2023 may be tied to 103,000 future cancers.
“To empirically quantify lifetime risk would require decades-long follow-up studies of very large populations,” the authors acknowledge.
However, their results suggest that some people may be more susceptible to cancer from low-dose ionizing radiation than others. Adults receive the vast majority of CT scans, but estimated radiation-induced cancer risks were higher in children and adolescents.
Those receiving CT scans at under one year of age, for instance, seem to have a higher potential lifetime risk for thyroid cancers, and this appears more common among female patients.
Far more research is needed to confirm if low-dose ionizing radiation actually impacts a person’s cancer risk, and how.
“Estimated overall cancer risks from CT radiation doses are similarly high in Australian studies,” said medical radiation specialist Pradip Deb from RMIT University. She argues it is important to avoid unnecessary CT scans if radiation-free procedures can do the same job.
Radiographer Naomi Gibson, President of the Australian Society of Medical Imaging and Radiation Therapy, agreed.
“Although the findings highlight the need for vigilance around long-term radiation exposure, this should not discourage the use of CT imaging when clinically justified,” Gibson explained.
“In appropriately selected cases, the diagnostic and therapeutic value of CT scans significantly outweighs the potential radiation-associated risks.”
The study was published in JAMA Internal Medicine.
An earlier version of this article was published in April 2025.
