Wednesday April 11 2012
The link between dental X-rays and brain tumours is uncertain
Regular dental X-rays “can double or even triple the chance of developing a common type of brain tumour,” according to The Daily Telegraph.
This news is based on a large US study comparing the dental histories of people who had a brain tumour known as intracranial meningioma with a group of similar people who did not have tumours. People with brain tumours were twice as likely to report having had a specific type of dental X-ray called a “bitewing” in their lifetime, compared with people without a brain tumour. Bitewings are a common type of X-ray, where patients bite down on a small holder containing X-ray film.
What didn’t make the headlines was that the same study showed that having a series of full-mouth X-rays was not associated with any increased risk of brain tumour, which throws doubt on the proposed link between dental X-rays and brain tumour. Furthermore, participants reported their own history of X-rays rather than researchers checking their dental records. This means people with brain tumours may have been focusing on the potential causes of their cancer and therefore may have been more likely to recall dental X-rays than people without one, potentially biasing the results.
Most importantly though, the chance of developing a brain tumour is very small and, even if X-rays can double the risk, it would still be a very rare event. According to one academic quoted in the press, this doubling of risk actually translated into just a 0.07% increase in lifetime risk, once the overall rarity of brain tumours was taken into account
While this study suggests that dental X-rays may be linked to brain tumour, it falls short of proving an actual link. It is known that exposure to ionising radiation is linked to cancer (which is why X-ray use is kept to a minimum), but people should not be alarmed by today’s sensationalist headlines and should not be dissuaded from having dental X-rays when recommended by their dentist.
Where did the story come from?
The study was led by researchers from Yale University School of Medicine and was funded by grants from the US National Institutes of Health, the Brain Science Foundation and Meningioma Mommas (a not-for-profit organisation providing support for those affected by meningioma brain tumours).
The study was published in the peer-reviewed medical journal, Cancer.
It was picked up by a variety of papers and online media. Most had attention-grabbing headlines reporting that dental X-rays “raise the risk of brain tumours” while others said they could “double brain tumour risk”. Reassuringly, once past the headlines, most coverage went on to mention that the absolute risk of getting a brain tumour was tiny even after X-rays, and that the reported doubling of the risk should not be a reason to avoid necessary dental X-rays. The Sun included a reasoned quote from Dr Paul Pharoah from Cambridge University who provided a clear message for worried readers: “People who have had dental X-rays do not need to worry about the health risks of those X-rays.”
What kind of research was this?
This research was a case-control study that aimed to examine the link between dental X-rays and the risk of a brain tumour called intracranial meningioma. A case-control study compares the histories of a group of people with a particular condition (the “cases”) with a group of similar people without that condition (the “controls”). Through this process they can identify differences between the two groups and identify factors that may have caused the condition of interest. They are particularly useful for studying rare conditions such as brain tumours, which would not be detected in sufficient numbers by many study types designed to follow a population over time.
The researchers said that intracranial meningioma brain tumours are the most frequently reported primary brain tumour in the US (a primary brain tumour means that the cancer started within the brain, as opposed to secondary tumours that start in other organs and spread to the brain). The researchers also stated that ionising radiation is consistently identified as being a potential risk factor for this type of brain tumour and that dental X-rays are the most common artificial source of this radiation.
Case-control studies cannot not prove on their own that dental X-rays cause brain tumours. However, this study type is a practical method for studying rare conditions or diseases such as brain cancers.
What did the research involve?
The study enrolled 1,433 patients with intracranial meningioma diagnosed between the ages of 20 to 79. They were called the “cases”. A control group of 1,350 people without brain cancer was also assembled and selected to match the cases in age, gender and geographical location (state of residence). All participants lived in the US and were enrolled into the study between May 2006 and April 2011. People with a previous history of brain tumour were excluded from the control group.
Shortly after enrolment, both groups were contacted by telephone and interviewed by a trained interviewer. The interview included questions about the onset, frequency and type of dental care received over their lifetime. This included orthodontic work, endodontic (root canal) work, dental implants and dentures. Participants were also asked to report the number of times they had received various types of dental X-rays during four periods of life:
- aged less than 10 years old
- between 10 and 19 years old
- 20 to 49 years old
- over 50
The researchers were interested in three types of dental X-ray:
- Bitewing – a small X-ray view used to look at several upper and lower teeth simultaneously. Bitewing takes its name from the way in which the X-ray film is held in place which involves the patient biting down on a small holder filled with the X-ray film. Bitewing X-rays are often used during routine check-ups to look for tooth decay
- Full-mouth – a series of multiple X-rays is used to build a complete picture of the mouth
- Panoramic – a single X-ray that provides a broad view of the teeth, jaw and lower-skull to check dental alignment rather than find cavities
Information was also gathered on the occurrence and timing of other treatments involving radiation (such as radiotherapy for cancer) – specifically, radiation treatments applied to the face, head, neck or chest.
The researchers then compared the information on dental X-rays between the case and control group to see if there were any significant differences.
The statistical techniques used to analyse the results were appropriate. The researchers made statistical allowances for differences in a variety of factors, including age, ethnicity and educational attainment. People who had radiation of the head, neck, chest or face to treat a condition were excluded from the statistical analysis comparing differences between dental X-rays.
What were the basic results?
Among the main results from this study the researchers found that:
- Over a lifetime, cases were more than twice as likely as controls to report having had a bitewing examination (OR 2.0, 95% CI 1.4 to 2.9).
- People who reported receiving bitewing X-rays yearly or more frequently were at a significantly higher risk of having a brain tumour across all the age-groups tested, except in the over-50s. The majority of study participants (from both groups) reported having at least one bitewing X-ray in their lifetime.
- There was no significant difference between the self-reported frequency of full mouth X-rays in those with a brain tumour and those without.
- More people in the case group reported having panoramic dental X-rays at a young age, on a yearly basis or with greater frequency compared with controls. For instance, individuals in the case group (with brain tumour) were almost five times more likely to report having received panoramic X-rays before the age of 10 than people in the control group (OR 4.9 95% CI 1.8 to 13.2).
How did the researchers interpret the results?
The researchers’ cautious conclusions were that “exposure to some dental X-rays performed in the past, when radiation exposure was greater than in the current era, appears to be associated with an increased risk of intracranial meningioma [brain cancer]”.
They added, “As with all sources of artificial ionising radiation, considered use of this modifiable risk factor may be of benefit to patients.”
This large case-control study shows that people with a brain tumour report having dental X-rays (specifically bitewing and panoramic types) significantly more frequently over their lifetime than similar individuals without tumour. The differences were only significant for bitewing and panoramic type dental X-rays and not for full-mouth X-rays.
These mixed results raise the possibility that dental X-rays may be associated with brain tumours but it stops short of proving this link. The study has significant limitations which should be borne in mind when interpreting the results of the research:
- This was a case-control study that looked at people with and without a brain tumour and analysed differences in their past exposure to dental X-rays. More of the people with brain tumours recalled having dental X-rays (bitewing and panoramic) in the past and so an association was identified. However, this does not prove that dental X-rays cause brain tumours, merely that the two events may be linked. There could be many other factors at play that are contributing to this association.
- It is slightly odd that full-mouth X-rays were not shown to be linked with brain tumour in this study, which we would expect to be the case if the X-rays were indeed linked to brain tumours, particularly as they are performed using a series of X-rays. This highlights that further work is need to prove any link between dental X-rays and brain tumour. A cohort study that followed people over time to see who developed tumours and who did not would be needed to establish a causal link.
- Participants were asked to recall their own history of dental X-rays from throughout their lifetime. The accuracy of recalling this information may not be perfect and this may reduce the reliability of the results. It would have been preferable to assess the use of X-rays by examining people’s medical records.
- In particular, a specific type of bias called “recall bias” may also be at play here. There is a public perception that X-rays are linked with cancer and so people with cancer may be more likely to recall X-rays in their lifetime as it has more perceived significance to their lives than someone without cancer. This would bias the results to suggest a link between dental X-rays and cancer when there may not be one, or to show a stronger link than actually exists.
The study’s design and these limitations mean that the study cannot prove that X-rays cause brain tumours. However, even if the chance of developing brain tumours is indeed doubled by a history of regular dental X-rays (a big “if” based on this study alone) it would still be a very rare event given that the absolute risk of developing a brain tumour is very small. To put this into context, the increase in risk of meningioma over a lifetime has been estimated at 0.07% - with X-rays associated with an increase from 15 cases to 22 cases in every 10,000 people.
Therefore, people should not be alarmed by the findings of this study and should not worry about the risk of brain tumour when deciding whether to have a dental X-ray, which is a useful tool for dentists to monitor and maintain oral health.
The authors note that radiation exposure from dental X-rays in the past was stronger than is currently the case and so the results gained from X-rays in the more distant past may not be relevant to contemporary dentistry. It is known that exposure to ionising radiation is linked to cancer, which is why X-ray use is minimised, but people should not be alarmed by today’s sensationalist headlines and should not be dissuaded from having dental X-rays when recommended by their dentist.
Analysis by Bazian