The short answer
A CT scan uses more radiation than a plain x-ray — roughly 1.5 to 6 mSv for common chest exams, against about 3 mSv a year from natural background. The FDA estimates a 10 mSv scan may add about 1 chance in 2,000 of a fatal cancer, on top of a baseline lifetime risk of about 1 in 5. That estimate is extrapolated from higher-dose exposures using a model, not measured directly, and the FDA says the uncertainty at these doses is considerable.
Typical doses per NCI: low-dose chest CT about 1.5 mSv, standard chest CT about 6.1 mSv, whole-body CT 10 to 20 mSv or more.
Average U.S. natural background radiation is about 3 mSv per year, so a chest CT is roughly two years' worth.
The FDA estimates a 10 mSv exam may add about 1 chance in 2,000 of a fatal cancer, against a baseline U.S. risk of about 1 in 5 (400 in 2,000).
Risk at diagnostic doses has never been directly measured; it is extrapolated from higher-dose populations using a linear no-threshold model.
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The claim
In its strongest form, the claim is that CT scans are a hidden cause of cancer. Having one, it says, meaningfully raises your personal odds of being diagnosed. The claim surges after headlines about medical imaging. And it leaves people declining scans that were ordered for a reason.
What a CT scan actually delivers
A CT combines many x-ray images into cross-sections of the body. So it uses more radiation than a plain x-ray. Here are typical adult effective doses, per the National Cancer Institute:
- Low-dose CT of the chest (lung cancer screening): about 1.5 mSv
- Standard CT of the chest: about 6.1 mSv
- Whole-body CT: 10 to 20 mSv or more
Doses vary with the body part, the machine and the protocol. 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.
What the evidence shows
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 that against the baseline. The 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. It is also small next to the risk already there.
At the population level the arithmetic looks larger. A 2025 study in JAMA Internal Medicine looked at the roughly 93 million CT exams performed in the U.S. in 2023. It estimated they could eventually lead to about 103,000 cancers. If imaging patterns hold steady, that is on the order of 5% of new cancer diagnoses. The figure drew a lot of attention. It is worth being precise about what it is. It is a projection, built from dose data and risk models. It is 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. The method uses a linear no-threshold assumption: that any dose carries proportional risk, and that 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 says the benefits of a CT scan, especially one used to diagnose cancer or another serious condition in someone with signs or symptoms, "greatly outweigh any potential health risks."
What this does not mean
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. So the useful question is not "is CT dangerous". It is "is this particular scan going to change what we do". It is entirely fair to ask that. It is also fair to ask whether ultrasound or MRI could answer the same question. Neither of those uses ionizing radiation.
What it does not mean is that declining a scan is the safe choice. A clinician orders imaging to investigate a symptom. The risk of missing something is usually far larger than the modeled risk from the radiation, and far more immediate.
The bottom line
A single medical CT carries a small, modeled, uncertain increase in lifetime cancer risk. It is very small compared with the cancer risk you already carry. Ask your care team what the scan is for. Ask whether a lower-dose or radiation-free test would work. And keep your own running list of imaging: it helps every clinician see the whole picture.
Sources
Common questions
How much radiation is in a CT scan?
It depends on the body part and the protocol. NCI lists roughly 1.5 mSv for a low-dose lung screening CT, 6.1 mSv for a typical chest CT, and 10 to 20 mSv or more for a whole-body CT. Average natural background exposure in the U.S. is about 3 mSv per year.
What is my actual chance of getting cancer from one CT scan?
The FDA's estimate is that an exam delivering about 10 mSv may be associated with roughly a 1 in 2,000 increase in the chance of a fatal cancer. That sits alongside a baseline U.S. lifetime risk of dying of cancer of about 1 in 5. The added risk is small — and it is an estimate from a model, not something that has been counted in real patients at this dose.
Should I refuse a CT scan my doctor ordered?
That is a decision for you and your clinician, but the risk of missing or delaying a diagnosis is usually far larger and far more immediate than the modeled radiation risk. A more useful conversation is asking what the scan is meant to answer and whether an ultrasound or MRI, which use no ionizing radiation, could answer the same question.
Do CT scans really cause 100,000 cancers a year?
A 2025 study in JAMA Internal Medicine projected about 103,000 future cancers arising from the roughly 93 million CT exams performed in the U.S. in 2023 — potentially around 5% of new cancer diagnoses if imaging patterns continue. That is a modeled projection built from dose data and risk models. It is a reason to avoid unnecessary scans, not evidence that 100,000 cancers have been observed.
Are children at higher risk from CT radiation?
Yes. NCI says children are more sensitive to radiation than adults because their bodies are growing and their cells divide rapidly, and they have more years ahead in which a radiation-related cancer could develop. The FDA says special considerations should be applied to pediatric CT. Ask whether a child's scan is set up for a child's size.
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Written by: Cancer ExplainedSources last checked: 2026-07-30 what this meansLast updated: 2026-10-16Next planned review: 2027-01-30
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Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes, and this is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source checked — This page was written with AI assistance and then compared with the sources listed on it, looking for statements those sources do not back up. That check is not a sentence-by-sentence record and can miss errors. It is not a medical review, and it does not confirm the page is complete or right for your situation.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
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