Tooth nuking

The Herald (and media sources worldwide) is covering a research paper on brain tumours and dental x-rays.  The paper asked roughly 1500 people with meningioma, and the same number of healthy people, about their histories of dental X-rays.  The people with meningiomas were more likely than the controls to report having X-rays at least annually, and the researchers estimated a relative risk of 1.5.

Now, meningioma is pretty rare, so this increase works out to an extra lifetime risk of maybe 5 cases for each 10,000 people.  Also, if you are going to have a brain tumour, meningioma is the one to have — some are not even diagnosed, and most diagnosed ones are treated successfully.  On the other hand, brain tumours are usually something you’d like to avoid, so is the risk real?

There are at least two issues that make the relative risk of 1.5 less plausible

• Self-report of risk factors for cancer is notoriously unreliable
• Since meningiomas can be relatively minor, the time of diagnosis varies, there might be some tendency for the sort of people who have regular dental x-rays to also be the sort of people who get earlier diagnoses, which would show up as a higher rate

The Science Media Centre also has a good summary, with quotes from experts.

It’s interesting to work out whether the risk increase is in the right ballpark given general knowledge about radiation.  A 1991 paper looked at the dose from different sorts of bite-wing dental X-ray setups, and found a range from 2 microSievert to 20 microSievert.   (XKCD shows what a microSievert means).   The same paper quotes an estimated risk of 0.73 health events including cancers per Sievert of dose to a population.   We don’t know what the population size was, but we can get a rough idea from the original paper.  They found 1500 meningiomas in 5 years, so at a rate of 3 per 100,000 people per year, that means about ten million people.  Roughly a third of the controls (and so roughly a third of the population) had at least yearly X-rays, so let’s suppose we are looking at 20 x-rays exposure on average for this third of people.Multiplying all the numbers together gives about 150 extra health events at 20 microSieverts per X-ray, or about 40 at the more-typical modern value of 5 microSieverts per X-ray.  The 1.5 relative risk that the researchers found is larger than this crude extrapolation would predict, but the order of magnitude is right.

So, there may well be a small increase in risk of a rare, mostly treatable brain tumour from having yearly dental x-rays.  It’s uncertain how big the risk is, and there are reasons to expect it might be less than a 1.5-fold increase, but that increase is at least of a plausible order of magnitude.   The radiation exposure from a dental x-ray is quite a bit less than from a trans-Tasman flight, and hugely less than from a CT scan, but it’s not zero.