CT scans and cancer: thinking about the meaning of risk
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.