Thinking about risk

March 21, 2009  |  General

I’ve recently been looking over the statistics about this blog. The most popular posts are those which talk about common issues — croup and concussions, for example. But one of the most often read posts gets its popularity from people using search engines like Google to answer this question: how risky are x-rays, especially CT scans, to children? You can read my actual posts about that here and here, but what struck me most about the popularity of this topic is what it tells us regarding how we think about risk. In particular, how do we tend to think about the risk of events occurring which are very rare, but which carry grave consequences if they happen? Lawyers call these events “small probability — large loss events.” Economists have studied the subject quite a bit, too, especially as it relates to investment decisions people make.

We humans are not entirely rational when we think about risk. We tend to focus on low-probability but high seriousness events, particularly if we are thinking about them in the context of choosing to do or not do something. So, for example, if your child needs a CT scan, as a parent you many think about how the radiation in that test increases your child’s chances of getting cancer. What we don’t think about is that your child is far, far more likely to suffer harm in a car accident while you are driving to the CT scanner than he is to suffer harm from the scan. But since we drive our children around every day, we don’t think much about that risk.

According to the National Cancer Institute, a child’s overall risk of developing any form of cancer is 1-2/10,000 children, or 0.01-0.02%. Also according to the NCI, this number has changed very little, if at all, over the past 30 years. The use of diagnostic x-rays in children, especially CT scans, has increased enormously during that time, so we should be reassured by these statistics. Even so, radiologists are increasingly vigilant about how they can reduce radiation exposure when they use x-rays.

Bottom line — it is always worth asking if the risk of a test exceeds the value of the information the test will give. But for x-rays, the benefit virtually always outweighs the risk.

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9 Comments


  1. Dear Dr. Christopher,
    It happened three days ago, on 16-10-11.
    My grand daughter, 7 months old, fell down from the sofa, to the floor, which had a cushion. It was a fall head down. The child cried for some time and became normal. I felt bad because the child was under my care.
    Her father was not at home. I had to take a decision.
    It was 7 pm. I had to rule out the possibility for any complication.
    I took her to the emergency ward of the nearest public hopsital. The doctor said there was no visible complication and advised a head x ray. I asked what
    was the risk invovled. He did not give an answer and said head xray was needed.
    The Radiologist who did the xray appeared to be totally unprofessional. He was messy. He had no to handle a baby. He did not cover the areas not to be x rayed.
    He took two photos, one staight on the face, other from the side view.I do not know the ratiation level used.

    I am terribly upset. Have I caused life long problem for rhe baby.
    Can you tell be what are the possible complications and what are the precautions possible?
    Please.

  2. I wouldn’t worry about 2 simple skull x-rays. Really, there should be no life-long complications from this — the radiation received is about the equivalent of the normal background radiation of about a month of normal living at sea level or of a long airplane flight.

  3. Hi,

    I just want to say thank you for your helpful, knowledgeable posts. My 5-month-old son has a very large head (47.7cm) and my pediatrician has been vigilant about frequent measurements. He had a skull x-ray at 3.5 months to make sure there was no premature fusion. After the 47.7cm measurement yesterday my doc sent him for a CT (without contrast). He told me he was concerned about Arnold-Chiari malformation or, again, suture fusion. My husband and I were pretty sure there was nothing wrong – no symptoms, and also he is a big boy overall (21.5 lbs and 28 inches); and my husband’s noggin is so large he has to special order hats and helmets. But we went for the CT at a pediatric radiology place, got a stat read, and of course everything was fine. I then drove myself crazy last night becoming more and more upset about the unnecessary radiation we had exposed him to. My doc told me when I sort of balked at the CT yesterday that “98 out of 99 pediatricians would order a scan seeing this head circ graph,” but I feel like as a mother I should have protected my baby when I really believed nothing was wrong. Also we are supposed to be flying with the baby in a couple weeks, do you think we should cancel to prevent additional radiation exposure at this tender young age? Thank you so much for your website.
    Leslie

  4. Hi Leslie:

    As you know, these are tough questions. The big one is always weighing the risk from the radiation (which, although not zero, is still very, very small) against the risk of not having the information the scan gives. Of course I can’t speak about the details of your doctor’s decision-making because I’m not your child’s doctor. But if the doctor was concerned about the possibility of Arnold-Chiari or some other condition then I would lean toward doing the CT because the information would be important in your child’s care. So I’m not sure at all that radiation was unnecessary. It’s all a matter of relative risk. You know now that the CT was normal, which makes you wonder if you should have refused the scan, but there was a real possibility that it would not be. And the lack of any symptoms, although encouraging, doesn’t mean the scan could not be abnormal.

    I wouldn’t worry about the plane flight — the radiation exposure is tiny. There is some information regarding flying and cancer risk: airline pilots, even after 40 years of flying, have no higher risk of cancer. I realize those are adults, but you should be reassured by that research.

    I can’t tell you not to worry, because all of us worry about our children — that’s what being a parent means. But it sounds to me like you did the right thing.

  5. Thank you!

  6. Hello, and thank you for this website, it is very informative.
    I have some concerns regarding my daughter’sexposure to radiation at a very young age. Firstly, I had to undergo a brain CT when I was 32 weeks pregnant. Then, as a newborn they x-rayed her chest -2 views, and at 3 months she had several skull x-rays, I think 5 or so.
    I now live in fear that this radiation will cause her to have cancer in the future. I realize that I cannot undue what has been done, but can you please give me some insight as to how significant this exposure will be for her? Thank you.

  7. Hi Trudy:

    I wouldn’t worry about that degree of radiation exposure. The chest and skull x-rays have very small amounts of radiation. The CT had a higher dose, but you received that, not her.

  8. My son, 10 weeks old, has been having feeding issues since birth. He always cries with his feedings. The past few weeks he has only taken in half ounce to 2.5 ounce on a good feed. He is breasted and I know the ounces as I have been giving him pumped milk every so often. By breast he stays on for 5 min before he cries. The pediatrician order a milk scan and upper gi on him. The test came back normal so I fell very guilty for putting him through this radiation At 2 months old. I have been so worried about his chances of getting cancer. Should I be this extremely worried?

  9. Hi Emily:

    I wouldn’t worry about that amount of radiation at all. But your comment illustrates what I’ve been writing — always balance the risk of doing the test against the risk of not having the information the test gives. You say you feel guilty because the test came back negative. But you didn’t know before the test which way it would go, and the radiation risk (which is inconsequential) is the same either way, if it had been positive or negative. What I mean is that the way to think of it is that your doctor needed the information, and negative information is often just as helpful as a test that shows an abnormality.

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