How Much Cancer Risk Comes From Medical CT Scans?

In its strongest form, the claim is that CT scans are a hidden cause of cancer and that having one meaningfully raises your personal odds of being diagnosed. It surges after headlines about medical imaging, and it leaves people declining scans that were ordered for a reason.

A CT combines many x-ray images into cross-sections of the body, so it uses more radiation than a plain x-ray. Typical adult effective doses, per the National Cancer Institute and RadiologyInfo:

• Low-dose CT of the chest (lung cancer screening): about 1.5 mSv
• CT of the head: about 1.6 mSv
• Standard CT of the chest: about 6.1 mSv
• CT of the abdomen and pelvis: about 7.7 mSv
• Whole-body CT: 10 to 20 mSv or more

Average natural background radiation in the United States is about 3 mSv per year. So a standard chest CT is in the range of two years of background exposure, while a chest x-ray, at about 0.1 mSv, is closer to ten days.

The FDA puts the individual number this way: an exam delivering about 10 mSv "may be associated with an increase in the possibility of fatal cancer of approximately 1 chance in 2000." Set against that, the baseline chance of dying of cancer in the U.S. population is about 1 in 5 — that is, 400 chances in 2,000. The added risk is real inside the model, and small next to the risk already there.

At the population level the arithmetic looks larger. A 2025 study in JAMA Internal Medicine estimated that the roughly 93 million CT exams performed in the U.S. in 2023 could eventually lead to about 103,000 cancers — on the order of 5% of new cancer diagnoses if imaging patterns hold steady. That figure drew a lot of attention, and it is worth being precise about what it is: a projection built from dose data and risk models, not a count of cancers anyone has observed.

Here is the part usually left out. Risk at these dose levels has never been measured directly. It is extrapolated downward from populations exposed to far higher doses, using a linear no-threshold assumption — that any dose carries proportional risk and no amount is completely safe. The FDA states plainly that "there is considerable uncertainty regarding the risk estimates for low levels of radiation exposure as commonly experienced in diagnostic radiology procedures." NCI's position is that "the increase in cancer risk from one CT scan is still small" and that the benefits of a scan ordered for a genuine clinical question greatly outweigh it.

It does not mean dose is nothing. It accumulates over a lifetime, children are more sensitive than adults, and a scan that was not needed carries whatever risk it carries with no benefit to offset it. That is why the useful question is not "is CT dangerous" but "is this particular scan going to change what we do." It is entirely fair to ask that, and to ask whether ultrasound or MRI — neither of which uses ionizing radiation — could answer the same question.

What it does not mean is that declining a scan is the safe choice. When a clinician orders imaging to investigate a symptom, the risk of missing something is usually far larger, and far more immediate, than the modeled risk from the radiation.

A single medical CT carries a small, modeled, uncertain increase in lifetime cancer risk — very small compared with the cancer risk you already carry. Ask your care team what the scan is for and whether a lower-dose or radiation-free test would work. Keeping your own running list of imaging helps every clinician see the whole picture.

• FDA: What are the Radiation Risks from CT?
• NIH Research Matters: Radiation from CT scans and cancer risks (2025)
• RadiologyInfo.org: Radiation Dose from X-Ray and CT Exams
• NCI: Risk Factors — Radiation