Radiation from CT scans and cancer risks

CT scans provide valuable medical information, but can also increase the risk for cancer. Mark Kostich / Adobe Stock

Computed tomography (CT) is a valuable medical imaging technique that uses x-rays and a computer to generate 3D images of a person’s insides. CT can improve health outcomes by aiding diagnosis in a variety of situations. But it also exposes patients to levels of radiation that can increase cancer risk.

Researchers have previously estimated cancer risks from CT scans in the United States for earlier time periods, when scans were less common but radiation doses may have been higher. In a new study, a team of researchers, led by Dr. Rebecca Smith-Bindman at the University of California, San Francisco (UCSF), estimated how current CT imaging might contribute to future U.S. cancer cases. To do so, they used the UCSF International CT Dose registry, funded by NIH, which has data on individual CT exams from more than 140 healthcare facilities in 20 U.S. states. This allowed them to analyze how and why CT scans were used, and how these varied by age and sex. It also allowed them to estimate how many patients underwent scans and their distribution by age and sex.

The team then used the Radiation Risk Assessment Tool (RadRAT) from NIH’s National Cancer Institute to estimate how the radiation exposure from these scans might affect lifetime cancer risk. Results appeared in JAMA Internal Medicine on April 14, 2025.

According to a national survey of hospitals and imaging facilities, 93 million CT scans were performed in 2023. Using the registry data, the researchers estimated that 96% of the people who got CTs were adults and 4% were children under 18.

Using the UCSF registry data on individual CT exams and RadRAT, the team estimated that CT use in 2023 could result in almost 103,000 future cancers. If current practices continue, CT scan use might eventually account for about 5% of all new cancer diagnoses in the U.S. per year.

Abdominal and pelvic CT scans contributed to the largest number of projected cancers—almost 40%. The most common type of projected cancer due to CT scans was lung cancer, followed by colon cancer. The projected cancer risks per CT scan were higher in children than adults, and highest in children younger than 1 year. But because the vast majority of scans were in adults, so were the vast majority of projected cancers.

These findings update the estimated projected cancers due to CT. This risk is on par with factors such as alcohol consumption and excess body weight. Yet the researchers emphasize that the benefits of CT scans often outweigh the risks.

“It’s important to note that for the individual patient, this increased risk is small, and the benefits far outweigh the risks if the scan is clinically justified,” says co-author Dr. Amy Berrington de González of the Institute for Cancer Research in London. “But when millions of CT scans are being carried out across the population, these small risks do add up.”

CT scans are a valuable diagnostic tool. But they’re sometimes used in situations where they may not be necessary. Reducing CT use in these situations could reduce cancer incidence without compromising the ability to diagnose disease. For example, ultrasound and magnetic resonance imaging, which don’t use ionizing radiation, can sometimes be used instead. Also, radiation doses for CT can be highly variable. Using the lowest dose needed for the accuracy of the scan would help reduce cancer risk.

“Reducing the number of scans and reducing doses per scan would save lives,” Smith-Bindman concludes.

—by Brian Doctrow, Ph.D.