By Catherine Ho

CT scans, a widely used medical imaging technology to diagnose diseases, may be more harmful than previously thought, and account for about 5% of new cancer cases annually in the U.S. population, according to new research led by UCSF scientists.

That puts CT (computed tomography) scans — which expose patients to ionizing radiation, a known carcinogen — on par with alcohol consumption and excess body weight in terms of contribution to cancer risk, according to the study, which is slated for publication Monday.

While the risk of radiation-induced cancer from one CT scan is low, people who get frequent CTs due to chronic conditions like Crohns should be aware of the benefits and risks of the procedure, said first author Dr. Rebecca Smith-Bindman, a UCSF radiologist and professor of epidemiology.

“The goal is not to scare patients, but to help them understand going forward they need to think about every time a CT is suggested,” Smith-Bindman said. “Do they need it? Do they understand why it’s done and work with their physician? Is this really a test I need now? Maybe it’s a test I can postpone.”

Based on the 93 million CTs performed on 60 million patients in 2023, an estimated 103,000 future cancers are projected to occur — three to four times more than previous studies have found. This study included more detailed data about the types of scans, and doses of radiation used in those scans, than previous analyses.

CTs are an important tool for assessing many conditions, including whether patients need emergency surgery after car accidents and falls, and to see whether some cancer treatments are working. But the technology is overused in other instances, Smith-Bindman said, such as when it’s used to diagnose a suspected pulmonary embolism, or a blood clot in the lungs, in a patient who’s at low risk for an embolism.

The use of CTs has risen about 30% since 2007, which has led to a significant number of increased projected future cancers, the researchers said.

Radiation doses for CTs vary, depending on why the scan is being done, and there are no widely used standardized doses, so patients may get different doses at different hospitals for the same type of CT. Across the 17 hospital systems analyzed in the study, about a third used an excessive dose of radiation.

This means the dose surpassed the maximum radiation threshold jointly set by the Centers for Medicare and Medicaid Services and UCSF as part of an effort to standardize doses nationally. This threshold is not a requirement for health care providers, and can be exceeded when necessary, but providers can voluntarily report the proportion of scans that exceed the threshold and receive bonuses and penalties based on how closely they adhere to the limits. It's meant to incentivize lower doses.

Decreasing CT use and establishing a standardized dose to not exceed a certain limit would both help reduce radiation-induced cancer risk, Smith-Bindman said.

It’s not clear exactly why CT use is growing so rapidly. But Smith-Bindman said it could be in part because doctors are increasingly strapped for time, and sometimes it’s easier to order a test to try to figure out the problem instead of spending more time with the patient. Another factor is that CT scans and medical imaging more broadly can be profitable for health care systems.

Additionally, many people are not aware of the potential risks of CTs, Smith-Bindman said.

“There’s very little understanding about the potential harms — they’re not known or often overlooked,” she said. “Patients sometimes ask for medical imaging for CT thinking it reflects high-quality care, not understanding there are pluses and minuses.”

The study found that for adults, the most common projected cancers were lung cancer, colon cancer, leukemia and bladder cancer. For adult women, the second most common was breast cancer.

For children, it was thyroid, lung and breast cancer.

The specific types of cancer may be linked to the body part that gets scanned — adults get a lot of head and lung CTs, and children get a lot of head CTs — and the dosage of radiation used. Lung cancer, for instance, is very susceptible to being caused by radiation, Smith-Bindman said.

The cancer risk from CTs is higher in children, especially babies, because their cells are more sensitive to radiation and because they have a longer life span within which to develop cancer, said co-author Dr. Malini Mahendra, a UCSF pediatrician.

An infant under 1 year old has a 10-fold risk of developing radiation-induced cancer compared to other age groups, the study found.

“It’s important we don’t perform unnecessary CT exams,” Mahendra said. “We hope that information in the study — quantifying the risks — allows clinicians and patients and families to have more discussions on risks and benefits associated with CT exams.”