I have zero medical knowledge (only the basic "common" ones) and I would like some opinions from you experts about CT Scan.

My girlfriend had a CT scan done some days ago, and since then she's obsessively scared about a potential cancer risk due to the X-rays she absorbed with that.

I believe (and she does it too somehow) she's over-exagerating, but she doesn't manage to get rid of that bad thought.

Could anyone with some knowledge on the subject reference the real cancer risks of having a single CT scan in life, at the age of 31, when you're in perfect health?

Note: I'm doing this just because I would like to help her easing a bit her feelings with some concrete and authoritative evidences on the "non-danger". I'm basically not looking for a real medical consultation.

  • Welcome to Health.SE :) I've tried to give some authoritative sources to confirm what you and she suspect- that the risk is small. That said, I noticed that you said that she was 'obsessively scared' about the risk, so it may be worth her talking about her fears. If there are any areas you would like more explanation on please say and I can update my answer.
    – bertieb
    Apr 10, 2017 at 15:40

3 Answers 3


CT brain at 31, what's the risk?

Short answer: Very little, compared to overall cancer risk*

Longer answer:

Your girlfriend is concerned about increased risk of brain malignancy due to having a CT at age 31. This is something that the FDA has weighed in on in a helpful summary page:

As in many aspects of medicine, there are both benefits and risks associated with the use of CT. The main risks are those associated with

  1. test results that demonstrate a benign or incidental finding, leading to unneeded, possibly invasive, follow-up tests that may present additional risks and
  2. the increased possibility of cancer induction from x-ray radiation exposure.

The probability for absorbed x-rays to induce cancer or heritable mutations leading to genetically associated diseases in offspring is thought to be very small for radiation doses of the magnitude that are associated with CT procedures.

(all emphases mine)

Hold on, the effective dose is much higher than a chest x-ray!

The article goes into further detail, and there is a comparison of radiation doses in a table. It is tempting to conclude that since the dose is much higher than a chest X-ray, the risk is also much higher. However, the article covers this:

A CT examination with an effective dose of 10 millisieverts (abbreviated mSv; 1 mSv = 1 mGy in the case of x-rays.) may be associated with an increase in the possibility of fatal cancer of approximately 1 chance in 2000.

And further:

If you combine the natural risk of a fatal cancer and the estimated risk from a 10 mSv CT scan, the total risk may increase from 400 chances in 2000 to 401 chances in 2000.

(both emphases mine again)

The absolute risk increase is relatively small.

To put it another way: if someone were to (unfortunately) develop an ultimately-fatal cancer having had a CT scan; it would be unlikely that the cancer was a result of the CT scan.

These things do matter on a larger scale, when considering populations; but your girlfriend can (hopefully) put this fear behind her.

So what do I say?

That being said, sometimes medical fears or those around adverse effects can be tricky to let go of. We are adept at spotting patterns, and if something happens after the fact, we tend to assume it happened as a result of the fact.

If she is amenable, a sensitive discussion about what happened in the lead up and around the time of the CT may let your girlfriend express some previously-unexpressed fears or emotions, and hopefully move past her worry about the risk of malignancy from the CT itself. If the vents were traumatic, or if she does feel it's 'obsessive', she may benefit from discussing it in counselling- some folks have the perception that counselling is only for the most deeply troubled or traumatic of events; but it can help in a very wide variety of circumstances.

* Astute readers will point out that there is some debate over the data used to model risk; these still tend to consider the bigger picture of population risk (eg 70 million CT scans year-1 in the US)

Further Reading:

  • 1
    Thanks a lot, much appreciated! I will read the answer in detail and let you know, meanwhile you definitely deserved my upvote!
    – Matteo NNZ
    Apr 10, 2017 at 16:47

The TL;DR answer is: yes, radiation can cause cancer, but no one knows for sure exactly what the risk is of one CT scan.

Radiation (especially in fetuses/children) increases the likelihood of cancer. Our information comes mostly from atomic bomb survivors, people exposed at Chernobyl, people treated with high doses of radiation for cancer and other conditions, and people exposed to high levels of background radiation, e.g. uranium miners.

Can one CT scan increase your chance of developing cancer? Yes, but how significant that is remains debatable. It depends on a lot of unmeasurable things: genetics, age, one's ability to repair the damage, which area of the body is being dosed, whether there are other carcinogens at work (e.g. viruses or co-carcinogens), etc.

The following are commonly found numbers. Given 10,000 people, about 2000 will die of non-radiation related cancers. If you expose those 10,000 people to 10 mSv of radiation, you can expect 5-6 additional cancer deaths in that 10,000 (or .5% increase). [2] However, if you expose someone to 10 mSv twice, with two months in between, the risk does not double.

The average dose of radiation per year from background sources is 1.5-3.5 mSv/yr.[3] [4] Because of our ability to repair damaged DNA over time, it's generally estimated that, from natural background radiation, the risk of developing cancer from background radiation is about 1% of our total cancer risk.[5]

The average CT scan is going to give you 7 mSv of exposure (that's a high estimate. The newer the machine, the lower the dose - usually.) What is your chance of getting a lethal cancer from that? The answer is: no one knows.[6]

Relative risk has be calculated by some. The following all give you a (estimated) 1 in a million chance of dying from that event[7]:

  • Smoking 1.4 cigarettes (lung cancer)
  • Eating 40 tablespoons of peanut butter
  • Spending 2 days in New York City (air pollution)
  • Driving 40 miles in a car (accident)
  • Flying 2500 miles in a jet (accident)
  • Canoeing for 6 minutes
  • Receiving 10 mrem (.1 mSv) of radiation (cancer)

So, you can estimate that (maybe) the risk of dying from a 7 mSv CT scan is about equivalent to driving 2800 miles.

The point of all this is, no one knows your exact risk of developing a lethal or relatively non-lethal cancer (yes, that does exist) from an imaging study. Most ethical radiologists will tell you there is no known safe radiation exposure, and that one should avoid unnecessary radiation exposure. While some imaging studies are unnecessary, some of them are not, and the risk of avoiding the study greatly outweighs the risk of undergoing it. What you can do to protect yourself:

  • ask if an MRI will give the doctor the same information (no radiation, but 2-3x the cost, longer wait)
  • ask the doctor if the test is absolutely necessary (a lot of tests are ordered to cover one's ass)
  • ask if there is another way to obtain the same information (e.g. colonoscopy instead of barium study)
  • get your study from the newest scanners available (radiation can be reduced up to 60% between generations of CT scanners)
  • reduce conflict of interest: any CT scan ordered in house (or by a doctor that owns a share in the imaging facility) should be questioned (but not necessarily refused.)

[1] XrayRisk.com
[2] USEPA Radiation and Health
[3] Radiation and Life
[4] Higher accumulated doses of radiation might produce a cancer which would only be observed several - up to twenty - years after the radiation exposure. This delay makes it impossible to say with any certainty which of many possible agents were the cause of a particular cancer. In western countries, about a quarter of people die from cancers, with smoking, dietary factors, genetic factors and strong sunlight being among the main causes. Radiation is a weak carcinogen, but undue exposure could certainly increase health risks.
[5] American Nuclear Society
[6] Risks of Radiation
[7] Radiation and Risk

From previously given answer on Biology.SE


The risk from a single CT exposure should realistically be estimated to be zero. The commonly used "linear no-threshold extrapolation model" to get to estimates of cancer risks due to exposure to low levels of radiation (of the order of 10 mSv or less) has no scientific basis whatsoever. E.g., observations of excess cancer cases after the Chernobyl disaster have failed to find any evidence in favor of it:

Contrary to previous claims, there was no increase in leukemia or other cancers (except thyroid cancer) in regions contaminated after the Chernobyl accident where thyroid doses ranged up to 1 Sv (123). The increase in thyroid cancer among young children is correlated with dose (124), and a threshold at 200 mSv is compatible with data (125).

However, the model is still useful to get to very rigorous safety standards for people who can be exposed to radiation. It's of course better to exaggerate the risk by a factor of a thousand than to underestimate it by even a small factor. Also, the risk is not the same for patients and medical staff, a patient may be exposed only a few times in his/her life whole the medical staff could be exposed several times a day if they would not leave the room where the patient is scanned.

The faults in the DNA that lead to cancer are overwhelmingly the result of DNA replication errors, not due to exposure to background radiation of few mSv to a few dozen mSv per year, so we don't need to worry about getting CT scans. We should worry about what we eat, whether we get enough exercise, and we should avoid stress (so, actually, we shouldn't worry too much), as these factors do impact on biological processes that are related to cancer risks. Here we need to consider that random DNA damage is not going to cause cancer, it has to be very specific damage affecting certain genes in crucial places. Also, the immune system which can then still eliminate the threat, has to fail to do so. If you stick to a healthy lifestyle, you're going to reduce most of these independent risk factors.

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