CT scans and cancer: thinking about the meaning of risk

April 3, 2010  |  General

I’ve written before about how to think about the risk of x-rays that we doctors do — here, here, and here. These posts, particularly the first one, are the most read and commented upon ones I’ve done since starting the blog over two years ago. Some recent articles in the medical literature have got me thinking about the subject again, because I order more than a few CT scans for children.

The first thing to understand is that nothing we do in medicine is without risk. All tests have risks. If the test itself is virtually risk free, there is always a risk of obtaining (and acting upon) wrong information, and sometimes that is not a trivial thing.

The next thing to understand is that ordinary x-rays, what doctors often call “plain films” cause a vanishingly small risk to the child. You should not worry about those unless your child has received hundreds of them. You can read a nice comparison of radiation exposure of the various kinds of x-rays, and what it means, here.

CT (computed tomography) scans are different, because their radiation doses are much higher than plain films. And the number of CT scans has risen dramatically: there were 62 million of them done in 2007, 4 million of which were performed on children. In comparison, there were about 2 million done on people of all ages in 1980. This thirty-fold increase has been enough to double the average radiation exposure of Americans. What do we know about the risks of that increase?

CT scans do increase the lifetime risk of cancer, especially in children. But by how much? The answer is — we don’t know for sure, although there are some studies underway to find out exactly. What we can do is calculate the radiation doses that CT scanners deliver to specific organs and combine that information with that we have from atom bomb survivors (who of course got massively greater radiation exposure) to estimate what the lifetime cancer risk is. But understand that is a sort of guesstimate. This graph, taken from this article, is a good summary of what we know.

The chart divides CT scans into the two most commonly done — head (left panel) and abdomen (right panel). It then looks at the increased risk, over a lifetime, of getting cancer that is attributable to the CT scan. For abdominal scans, that’s 0.14%, if the scan happened before the age of one. What this means is that, for all kids who get cancer at some time in their life, about one in a thousand of those cases could be attributable to a CT they had earlier in life. That’s not at all the same thing as saying the scan gives them a 0.14% chance of getting cancer — over a lifetime, all of us have a risk much, much higher than that. Rather, it says that, among the large number of us who will get cancer, as many as one in a thousand of those could be attributable to the CT scan.

How should a parent interpret all this if the doctor says their child needs a CT scan? The main thing to remember, as I’ve said in my other posts on this issue, is that several times a very, very small number is still a very, very small number. So the practical implication is that, although CT scans do increase risk, for an individual child that risk is still very small.

The practical thing for a parent to do is to ask the doctor is what the risk is of not doing the scan, of not getting the information the scan gives. If the risk of not doing the scan, which is often very large, is greater than the risk of doing the scan, which is very small, the calculation favors doing the scan.

There is no question that CT scanning has been an enormous weapon in our war against disease and injury. What we are doing now is finding out ways we can avoid doing them if possible. For example, in children the most common indication for an abdominal CT is to diagnose appendicitis, and CT does a good job at that. Some recent research has focused on determining which children need a CT for that and which don’t. The FDA also has a program to try to do what it can to reduce unnecessary scanning. Because that’s the real issue: if the increase in use of CT scanning continues at the same clip over the next couple of decades that it has over the past, then we will have a much larger problem to deal with.

If you’re interested, you can read several of the articles from the medical literature that I’ve summarized in this post — here, here, and here.


  1. I am very worried because I never realized how dangerous all these test were until now. My 11 year old daughter had a barium swallow at 2 weeks and multiple xrays. Then at 1 more xrays. At 4 more xrays not to metion dental xrays. Then at 10 she had a bad fall and had a head CT scan done. I was never informed of the risks involved. Do you think she has had to much radiation? I am not sleeping because I’m so worried I have increased her risk of cancer.

  2. Hi Sarah:

    The important thing here is to keeps things in perspective. By that I mean this: yes, CT scans and the fluoroscopy your child received with the barium swallow do increase cancer risk by a tiny, tiny amount above what her background risk would be just from living her daily life (the other x-rays don’t matter). But that additional risk really is very tiny, less than the risk of taking a drive during rush hour on an interstate highway, something we do all the time without thinking about the risk.

    The important thing to remember with any medical test is this: no medical test is completely without risk. We always need to ask ourselves: “what is the risk of NOT doing the test, of NOT getting the information the test gives.” In the case of a head injury, the risk of missing an important abnormality, such as bleeding inside the head, is typically much, much greater than the risk of the test. So we do the CT. On the other hand, we are now more aware that we need to be careful in ordering head CTs, since sometimes we get them when they’re not really needed. So that’s the question you need to ask the doctor: “what is the risk of not doing this test?”

    I know parents always worry, so telling you not to worry won’t work. But really, with that degree of radiation exposure, if she were my child I wouldn’t worry. After all, there are many other things to worry about, things that are far more likely to cause problems: for example, what about her first drive on the highway alone when she gets her license, her first trip alone to a big city, things like that.

    Thanks for stopping by, and I hope this helps put things in perspective. As a parent myself, I know it’s hard to strike a balance between vigilance and obsessive worry.

  3. My daughter was diagnosed at 15 months with nueroblastoma and since then (she’s almost 4) has had at least 10 CT scans- one of her head, one of her chest, and many to her abdomen. We are now following her by ultrasound which has been a huge relief to me.

    Should we be doing other tests to check for secondary cancers as she gets older (I read ALL is the most common type of cancer for children when it comes to radiation exposure).

    I know that these scans were unavoidable, but now that she is past the point of getting them every three months, should I be on alert for other cancers, and are there ways to be proactive with prevention?

  4. Hi Ashlee:

    I’m glad to hear that your daughter is doing well after her neuroblastoma treatments.

    Yes, second cancers are a huge concern, which is why researchers now devote a lot of effort to discovering the smallest doses of chemotherapy and radiation that can cure the first cancer. That’s a difficult tightrope to walk, of course, because the first task is to treat the initial cancer completely.

    I’m an intensive care physician, not a hematologist/oncologist. I care for many children with cancer when they’re in the PICU, but I’m not expert in what regular screening studies you should consider for your daughter; that would be a good specific question to ask your daughter’s doctor. As you already know, though, at least it would be important to minimize any further CT scans.

    Thanks for stopping by.

  5. My 1.5 year old son had a pretty hard fall on his head and we took him to the ER. The doctor recommended a full head CT scan and told us the risk of about 1 in about 11,000 or so. That seemed like a negligible number so we went back and forth but decided to do it. Because he moved they had to do it again. This has worried me sick that we may have caused him to get cancer in the future. What worries me is that no one really knows for sure what it can do to you. The numbers I have been seeing for head scans in children are generally 1 in 500 and then if he had 2 scans would that mean 1 in 250 or less? In hind sight he really didn’t need the scan so I am kicking myself every day about it. The scan could cause cataracts, thyroid cancer, learning disabilities, you name it. I don’t know if they used a lower setting for a child and who knows if more than his head was scanned. What should we look for to warn us of any problems associated with the scans. I have a radiologist practice partner friend who says not to worry that it’s negligible but that doesn’t seem to match what I’ve been seeing. Do you think that in 10-20 years there might be a cure for cancer? Do you have any numbers that show current cancer patients getting additional cancers stemming from CT scans. Thanks for the forum.

  6. Hi Manny:

    The issue really is the risk of the CT scan, which is small, but not zero, compared to the risk of NOT doing the CT scan, of missing something important. For example, a blow to the head can damage the brain. If a child’s doctor thinks that’s a real possibility, then there is a substantial risk of not doing the scan and getting that key information. When I recommend a CT scan, that’s always how I put it to parents — what would be the risk of not doing it, of not getting the information the scan gives. If that risk is significant, particularly if I am looking for serious problems, then the risk of not doing the scan is more than the risk of doing it.

    In answer to your other question, radiologists are supposed to use a lower radiation dose with children. I think a parent should ask if that is the case, because some of the older scanners don’t do that.

    The bottom line is that risk is part of life. We do what we can to reduce it, but we can never eliminate it for our children.

  7. Thanks for the response. It’s tough to look at your little child and think about those things. Hopefully as he gets older and stays healthy those thoughts will fade. I think I should try and keep him out of the sun more to try and curb the cumultive amounts if that would even help. I know there are thyroid tests that we will run on him periodically because with a full head scan I’m afraid they probably scanned it as well.

  8. I have had 2 head CT scans for my headaches (first when I was 28 and the second was 29). Thanks to God results were quite OK. Now I am 30/male. But I have seen some articles about risk coming with CT. I am over thinkink about this (as always).Acc.to radiologyinfo.com related risk is quite low but anyhow I can’t stop concerning about that. Thinking and googling make me crazy. I have just read your post and it is really tengible from my point of view. What you suggest as a professional?

  9. Hi Ralph:

    You’re correct in saying your additional cancer risk, that is, risk higher than you would have anyway, from two head CTs is very low. But successive CT scans do add a bit of risk with each one. My best advice is to do what we should do all the time and for all tests, but tend to forget: if your doctor ever suggests another scan, make sure that the risk of NOT doing the scan is significant. That is, weigh the tiny additional risk from the scan radiation against the risk of NOT getting the information the scan gives.

  10. Hi Cristopher,

    Thanks for your response.To be honest I fully trust the doctors, sure they want to do their best.As you said there is always a risk so it’s a cost/benefit issue! Hopefully unnecessary use of CT scans will be limited especially for kids. But when it’s necessary it has to be done without doubt. Once again thank you very much for your amazing effort.

    Best Wishes,


  12. It’s possible her many CTs are contributing to her illnesses, although it’s still very unlikely to be the cause because the increased cancer risk from CT scans typically happens decades later, when the child is an adult. Your daughter’s doctor should be able to tell if the scans are the cause now or not. You should ask her doctor that very question, I think.


  1. Respect diagnostic radiation, but don’t have an irrational fear of it