Your next computed tomography, or CT, scan might be riskier than commonly assumed. A study out today calculates that CT scans are contributing to a substantially greater share of cancer cases than past research has suggested.
Scientists at the University of California, San Francisco, led the research, published in JAMA Internal Medicine. They estimated that out of the millions of people who received CT scans in 2023, over 100,000 would eventually develop cancer in their lifetime. The scientists argue that CT scans should be seen as a cancer risk factor on par with excessive alcohol drinking, but at least some outside experts aren’t convinced.
CT scans are commonly used to screen for and diagnose potential health problems or to help guide doctors during certain medical procedures. But they require exposing people to doses of ionizing radiation, a known carcinogen. Much of the research on how ionizing radiation increases cancer risk comes from studying the Japanese survivors of the atomic bomb attacks during World War II. But several large recent studies have found a link between the radiation exposure from CT scans and increased cancer cases among children and young adults.
The study authors wanted to conduct an updated analysis of people’s lifetime risk of cancer associated with CT scans across all ages, especially since CT scanning has become more common over the years.
Based on survey data from hospitals and imaging labs across the country, they estimated that there were 93 million CT scans performed on 61.5 million people in 2023. They then calculated that about 100,000 radiation-induced cancers would arise from these exams, based on the expected doses of radiation received and where on the body the test was performed. The risk estimates are about three to four times higher than what past studies have found, the authors say.
About two million new cancers are diagnosed in the U.S annually. So if Americans’ level of CT use and radiation dose from these tests remain constant, CT scans could eventually account for roughly 5% of all new cancer cases every year—a projected level of risk similar to obesity (7.6%) and alcohol consumption (5.4%), according to the study.
“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,” the authors wrote.
Mark Supanich, head of the division for Diagnostic Medical Physics at Rush University’s Department of Radiology and Nuclear Medicine, isn’t so sure about the team’s math, however. “I believe that the estimates of CT-related cancers predicated by this study are likely overestimated,” Supanich, who is not affiliated with the research, told Gizmodo in an email.
He argues that their numbers are based on some assumptions that might not ultimately hold up.
“A large portion of CT scans are performed on patients who are actively undergoing diagnosis or treatment of significant disease or injury,” he notes. “Including these patients in the statistical estimation of possible future cancers results in a significant overestimation of theoretical future cancer risk.”
The authors did try to account for this factor at least somewhat by removing tests performed in the last year of life from their calculations. In a secondary analysis, they also excluded tests performed up to two years before death, which did lessen their estimate of CT-related cancers. But Supanich argues that the exclusion period should be much longer (one study cited by the authors excluded CT tests performed in the last five years before death, he notes).
Supanich adds that the science is still evolving on how much low-dose radiation truly contributes to cancer risk. So until we can firmly quantify this risk, he says, it’s best to take any claims that CT scans are resulting in a huge number of cancer cases every year with “several large grains of salt.” In light of this uncertainty, he also takes umbrage with the authors’ comparison to other known cancer risk factors.
“In my opinion comparing CT scans as a risk factor for cancer comparable to those of alcohol consumption or excess body weight, as the authors do, is wholly inappropriate and carries with it a significant risk of doing more harm than good,” he says.
The authors have called for several reforms in how CT tests are conducted, which might include ordering scans less often in certain scenarios or trying to reduce the radiation dose per test. Supanich notes that there has already been some progress on these fronts. A 2019 report from the National Council on Radiation Protection and Measurements found that the average medical radiation dose per test (including CT and other imaging procedures) had declined 20% over the past decade, for instance.
But as scary as this study might look, it seems unlikely that it will be the final word on the cancer risk posed by CT scans. And Supanich says that people in general shouldn’t be too worried about getting their next CT test, given their well-known pros, which can include helping detect cancer early.
“The theoretical risks for any individual receiving a CT scan are miniscule compared to current cancer incidence rates (about 40% over an average person’s lifetime) and the benefits for CT are large,” he says.
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