A small Kuwaiti study used a conditional regression to found that x-rays are an increased risk factor for thyroid cancer (n=313, odds ratio = 2.1, 95% confidence interval: 1.4, 3.1) using a case-matched methodology.

X-rays being a carcinogen isn't surprising. Every time I've gone into my dentist they've tried to image my head. I've never had a cavity, and I generally decline the imaging; their recommended cadence of every two years has seemed too aggressive to me, in the past.

I've got my first dental appointment in more than 18 months next week, and.. I'm inclined to let them image. How should one think about this sort of risk/benefit trade off? Presumably catching a cavity or other issue early is [much?] better. But that itself assumes that the dentists who'd be treating you don't do more harm than good.

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XKCD says that the dental X-ray (5 μSv) is half the average daily background radiation dose (10 μSv), and 1/8th of a cross country flight (40 μSv). To me this means that the radiation exposure is quite irrelevant in the grand scheme of things. (https://xkcd.com/radiation/)

If this were false, it would presumably be because dental X-rays are especially harmful in some way that isn't just "because of radiation".

You can drown in a river that's on average 20 centimeter deep. Understanding how radiation causes permanent damage isn't easy.

There are models that suggest it causes permanent damage if enough damage is done within one cell that the cell repair mechanisms can't easily repair the damage in a short amount of time. 

That suggests radiation for a small amount of time on a small area of the body is more problematic then average background radiation that's the same over a longer time frame and goes towards the whole body.

4Randomized, Controlled2y
A different way of saying this is: power (dE/dt) may be important.

From https://seer.cancer.gov/statfacts/html/thyro.html, new thyroid cancer cases occur at a rate of ~15 cases per 100k people per year, and the disease has a 98+% 5-year survival rate.

Compare that with whatever risk results from needing more invasive repair when your dentist can't detect the cavities as soon, and you can see if there's a net benefit. I'm not seeing any numbers on this in my 5 minutes of searching, but that doesn't mean they're not out there. But I suspect the connection between dental infections and heart disease (that any dentist will tell you all about if you ask) easily exceeds the increased risk from regular x-rays.

I think this is a pretty compelling point, but:

  • How well understood is the dental health/heart disease connection? I've heard this assertion before, including from in the classic Interventions for Longevity post, but do we know if it's causal?
  • 98% survivable sounds.. goodish, but: what sort of quality of life hit do you take?

As I mentioned above, I'm still leaning towards okaying the imaging, but also think the default policy of every two years they use may be too aggressive for me, given the absence of any cavities up till now.

To address your clarifications: Nobody seems to do proper studies on dentistry, so we don't have any gold standard evidence that I've ever seen. But, discounting institutional knowledge out of hand is foolhardy. I'd call the story the dentists tell about this "moderately strong" evidence for a causal connection, but (all together now!) more research is (obviously) needed. I know a guy who had thyroid cancer. They took the gland out and he has to take a daily pill to replicate the function, but from about two weeks after the surgery I haven't heard him complain in the years since. So, seems manageable from a quality of life angle.
2Randomized, Controlled2y
Is it? I feel like dentistry seems similar to sports science and physiotherapy in addressing super complex evolving gibs of soft and hard tissue. My general impression is that sports and physio research is marginally better than dentistry, which leaves the state of research clear enough to declare it... kind of abysmal. But also to show that a lot of practitioners swear by isn't well supported by evidence. Much of Ingraham's painscience.com [http://painscience.com] is devoted to scrutinizing physio standards of care w/r/t the state of research.. I honestly don't know how to square this with practitioners who always seem confident in their assessments and make rote noises about being evidence based, and then even have reasonable sounding noises when I ask probing questions.
Absent the preferred evidence, we have to work with what we've got. Dentists keep detailed records on their clients, including notes related to their medical situation so that they know e.g. "Alice has [condition] and should get this kind of care but not that.". Look at the state of evidence regarding flossing and gum health: I remember reading that the statement "flossing is good for your gums" is supported by exactly one study that's over 50 years old and followed a dozen people for two weeks, then had them self-report their flossing habits over that time. That's terrible, but my hygienist can accurately divine if I've been flossing or not just by looking at my gums for about 2 seconds, and I have less pain and taste less blood during cleanings if I have been. Some of dentistry certainly might have developed how it did because somebody thought it seemed like a good idea at the time and nobody's had the good sense to check the facts. But, absent studies, I don't have another way that's quite as robust of discerning that stuff from the dentistry that represents over a century of increasingly trained medical specialists noticing that their patients get better when they do X. And neither category represents a guarantee that a practice is good or bad for my health and wellbeing, nor does it identify short-term-gain-long-term-loss situations. So I have to treat everything my dentist says as at least slightly sus; but for a slightly sus claim about (e.g.) how taking care of my teeth and gums (which are connected to the blood supply) for a couple minutes every day can reduce my risk of heart problems later in life, the risk is large enough and the cost is small enough that I'm willing to brush, floss, and use that gum stimulator thinggie to get at the back surfaces where my wisdom teeth used to be every night just in case the claim is correct. "Unsupported by science" doesn't necessarily mean a practice is bad or a claim is wrong. It can also mean simply that nobody has
Or to express it another way, all the people who actually have the expertise in the topic are not interested in publishing anything but rather want to do hands-on-work. That's true. Dentists all over the world are advocating flossing, so flossing seems to be worthwhile.  When it comes to interventions such as very regular dental exams that are done much more in the US then in other countries the case is less good. Dentists have large financial incentives to overtreat their patients. Getting the x-ray increases your chances that your dentist will drill into a teeth that doesn't really need drilling. As far as dental interventions oral probiotics is one that a lot of dentists don't recommend but that actually has a few studies on it's side. 

You can reduce the risk of cavities. X-ray technology was invented in Germany, where I live, and they have had a history of regulating its use. I have my teeth checked twice a year and have never been asked to get an x-ray. This is in contrast to my experiences growing up in the American continent, where, despite all the x-rays, I developed a lot of cavities. Using an electric toothbrush, buying toothpaste with a small amount of fluoride and flossing twice a day have been effective for me in drastically reducing the risk of cavities and obviating the need for x-rays (a diet change may have also helped). That despite my teeth being so tightly packed I destroy dental floss. I think I've only had a cavity once in the last ten years and it was picked up early during a scheduled check up and dealt with promptly.

The German Bundesamt für Strahlenschutz (yep, there is one) offers this booklet to keep track of your exposures and ensure you do not exceed a threshold: https://www.bfs.de/EN/topics/ion/medicine/diagnostics/x-rays/record-card.html . Maybe it will be useful to you

I also came here to say that I've never had a dentist even suggest an x-ray in the course of a routine checkup (living in Finland). The only time I recall having had one taken was when I had an issue of grinding my teeth while sleeping, and they did some extra investigation. 

1Randomized, Controlled2y
Interesting. A quick search of dental outcomes by country turns up Evaluation of different oral care systems: Results for Germany and selected highly developed countries. An update of a former study [https://www.oatext.com/evaluation-of-different-oral-care-systems-results-for-germany-and-selected-highly-developed-countries-an-update-of-a-former-study.php]. I only have time to read the abstract right now, but it does call out the Netherlands and Finland as having somewhat worse outcomes on the measure they've created, which is: "an expanded composite indicator that measures dental health on a population basis in the form of one overall indicator – the Dental Health Index (DHI). If the DHI is combined with a Dental Care Cost Index (DCCI), an efficiency index [is created]"

I too want to say that my dentist never even suggested getting an x-ray during a routine check up.

I’ve had a dental x-ray once but it was when looking into a specific problem.

I didn’t have any cavities in years. Back when I had cavities dentist found them by looking at my teeth no x-ray needed.

[+][comment deleted]2y1
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 Case/ControlCases %Controls %OR95% CIP-value
Dental x-ray
Number of dental x-rays<0.0001*

This is the money shot, copied and pasted from the article, with the key stat in row 2.

But the tails on the data for higher exposures are insane. I think I've been imaged every two years on average--seems to be standard of care at USA dentist offices. So between ages 18 and 38, I received ~10 x-ray exposures. 

Google led me to this meta, which led me to this 2013 study, which found:

We observed a 13% increase in thyroid cancer risk for every 10 reported dental radiographs (hazard ratio = 1.13, 95% confidence interval: 1.01, 1.26), which was driven by dental x-rays first received before 1970, but there was no evidence that the relationship between dental x-rays and thyroid cancer was associated with childhood or adolescent exposures as would have been anticipated. The relationship between dental x-rays and thyroid cancer risk was surprising because we found no evidence of an association of thyroid cancer with other types of diagnostic x-rays characterized by higher radiation exposure than dental x-rays.

Per my layperson's understanding, Memon found that for each person with thyroid cancer who had had no dental x-rays, there were between 1.1 and 26.7 people with thyroid cancer who had had ten or more dental x-rays (yes, I know, low confidence interval), and Neta found that a person with ten dental x-rays had a 13% higher risk of getting thyroid cancer as someone without.

Neta cites Memon and claims various methodological advantages over Memon. Maybe they are relevant to your analysis?

I never get them -- not for two decades. I have very strong teeth and everything has been fine. But I was quie confident. If I had regular cavities I would get it done. YMMV.

Likelihood of cancer -- quite low; cost of getting it -- quite high.
Likelihood of cavities -- higher; cost of getting them -- lower.

It's hard to figure small numbers times big numbers when you don't really have either. :)

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