WDDTY July 2012 vol 23.4
Even a once-a-year bitewing dental X-ray can increase the risk of brain and thyroid cancer, research shows
For years, dentists have been reassuring patients that dental X-rays are harmless (even while diving behind protective shields themselves). The latest evidence shows that even the usual dosage—one bitewing X-ray a year—can double or even triple your chances of developing a common form of brain cancer.
Researchers discovered that meningioma, the most frequently reported brain tumour in both the US and UK, is up to three times more common in individuals who have those supposedly benign once-a-year doses of dental X-rays.
These kinds of tumours, which grow in the meninges, the three layers of tissue that enclose the brain, account for one-fifth of all brain cancers, and affect women more commonly than men. Up to seven of every 100,000 women suffer with this type of tumour compared with up to five of every 100,000 men.
Although meningiomas are slow-growing and don’t spread (metastasize), they can be deadly, causing problems if they create pressure on the brain. Even though only 2 per cent of cases are malignant, even a meningioma deemed ‘benign’ can prove lethal—and, in a third of cases, within five years of diagnosis. In the meantime, patients can suffer a litany of symptoms, including blindness and seizures. The standard medical treatment is chemotherapy or—ironically—radiotherapy.
Although medicine has carried out little research into meningiomas, ionizing radiation has emerged as one of the chief risks, along with head injury, even at low dosages (Cancer, 1993; 72: 639–48), and dental X-rays are the most common source.
In one multicentre study, researchers from Yale Univer-sity’s School of Medicine and Brigham and Women’s Hospital in Boston, MA, examined 1433 patients, aged 20 to 79, in various states in the US who’d been diagnosed with intracranial meningioma, and compared their dental records with a control group of approximately the same size.
Specifically, the researchers were examining the association between a diagnosis of meningioma and the numbers of bitewing, full-mouth and panorex dental X-rays (an X-ray displaying both the upper and lower jaws and teeth in the same film, affording the dentist a panoramic view of the patient’s teeth).
Over a full lifetime, those who’d suffered meningioma were more than twice as likely as controls to remember having had even just a single bitewing X-ray.
Nevertheless, the risk increased with frequent exposure, even at a young age. Those who’d reported receiving annual bitewing X-rays had an elevated risk (up to nearly double) of developing the brain tumour, regardless of current age. Even children under 10 had 1.4 times the risk of their contemporaries who were not exposed to these types of dental X-rays.
The most dangerous, however, was the panorex film. Those who’d had these panoramic X-rays tripled their risk overall, and the panorex was particularly risky for those whose annual panorex X-rays had started before the age of 10. Individuals in this category had nearly five times the risk of developing the potentially deadly disease (Cancer, 2012 Apr 10; doi: 10.1002/cncr.26625).
Overall, the evidence suggests that dental X-rays increase the risk of tumours by at least 50 per cent.
This latest research bolsters evidence from Brighton and Sussex Medical School in the UK showing that dental X-rays also have a devastating effect on the thyroid. In this study of more than 300 patients with thyroid cancer, the researchers discover-ed that those exposed to dental X-rays were twice as likely to develop thyroid cancer. Furthermore, the risk was dose-related: the greater the number of X-rays, the more likely the person was to have cancer (Acta Oncol, 2010; 49: 447–53).
Swedish research has also found an increased risk related to the amount of exposure. Indeed, dental assistants and personnel were 13 times more likely to have thyroid cancer, and anyone working with X-rays doubled their risk.
For patients exposed to X-rays for diagnosis, the risk was more than doubled although, again, the findings showed a clear dose-related response: those exposed to more than 10 X-rays had a three-and-a-half times greater risk (Eur J Cancer Prev, 1997; 6: 550–6).
The risk was also increased with women who’d borne children; those who’d had three or more children increased their risk by nearly five times if they’d had prior X-rays of any sort. This suggests that sex hormones are in some way adversely affected by low-dose radiation.
Although dental X-rays were suspected culprits, no firm association had been found until the recent Yale study (see above). Although a link was suspected when a University of Washington study found that a series of six or more full-mouth X-rays doubled the risk, it failed to find any link with bitewing, lateral cephalometric (the side-of-the-skull X-rays used in orthodontics) or panorex X-rays (Cancer, 2004; 100: 1026–34).
In addition, while other University of Washington research dismissed the risk of exposures in occupational settings, there was an increased risk when patients were given high doses of radiation, such as during radiotherapy for cancer, particularly to the head or neck (Neurology, 2005; 64: 350–2).
Nevertheless, there were numerous warning shots fired more than 20 years ago. For instance, a 1990 review of five epidemiological studies pointed the finger at dental X-rays as the biggest risk factor for both meningiomas and tumours of the parotid glands (the salivary glands located in front of the ears).
In fact, among the Los Angeles County residents who formed the basis of the review, some 85 per cent of the cumulative radiation in the parotid salivary region came from dental examinations (J Am Dent Assoc, 1990; 120: 151–8).
How much is too much?
Dental X-rays are measured in millisieverts (mSv), which relates to the amount of radiation absorbed. Current dental X-rays emit around 0.038 mSv of radiation for a bitewing and 0.150 mSv for a full-mouth series, which the American Dental Association (ADA) claims is the equivalent of a few days’ worth of background environmental radiation exposure, or the dose you’d receive during an airplane flight across the US.
Indeed, according to the UK’s National Radiological Protection Board, we absorb 3000 mSv from background radiation every year. However, it’s necessary to understand that, instead of absorbing that amount of radiation throughout your entire body, in a dental X-ray, it’s concentrated to one tiny area—and all of it at one go.
Nevertheless, both the Yale and the 2004 University of Washington studies pull their punches, claiming that the risk is only present with the older types of X-rays, which emitted far more ionizing radiation prior to the late 1980s than do the current models.
However, that conclusion doesn’t stack up, as much of the Yale evidence shows an elevated risk with any X-ray exposure as well as an increased risk among young people, who would have been exposed only to the newer types of X-rays.
In addition, a large percen-tage of dentists persist in using the old-style ‘D-speed’ dental film, which exposes patients to more radiation than they need to, rather than upgrading to the faster F-speed films. Further-more, a number of dentists and orthodontists are using the latest technology—cone-beam computed tomography, or CBCT—a type of scan that offers a three-dimensional view of the patient’s mouth and head. Often used to measure the mouth for braces, use of this scan in children is controversial, as children are up to 20 times more sensitive to radiation than adults.
The ADA’s guidelines now state that healthy adults should only submit to a bitewing X-ray once every two or three years, and only if something specific is suspected. Yet, given the latest findings, it may be prudent to avoid being automatically X-rayed altogether, particularly as a ‘just-in-case’ measure.
Also, if you change dentists, ask for your old X-rays to be passed on to the new dentist. And don’t worry about being considered a crank. When you do need an X-ray (and be sure that you do, such as when some-thing is suspected), always insist on a lead shield, including a thyroid shield. It may save your health, if not your life.