Month: April 2012

Is Your Dentist Giving You A Brain Tumor?

dental x-ray
Courtesy of TheKarenD via Creative Commons.

In the United States, it is common to have dental X-rays as part of your regular checkup or when you have tooth pain. These X-rays use a small amount of ionizing radiation to take a picture of your teeth, bones and gums in order to show tooth decay, impacted teeth, bone loss, and other mouth problems.  Since ionizing radiation exposure is known to increase the risk of certain kinds of cancer, scientists have recently studied whether dental X-rays increase your risk of brain tumors.

An article was just published in the peer-reviewed medical journal, Cancer. It reported the results of a large study that examined the association between dental X-rays and the risk of the most common type of brain tumor (meningioma). The study was headed by researcher Elizabeth Claus, M.D., Ph.D. at the Yale University School of Medicine, in collaboration with the University of California at San Francisco School of Medicine, Brigham and Women’s Hospital, University of Texas M.D. Anderson Cancer Center, and Duke University of Medicine.

Recent news coverage sensationalized the results of this study, possibly alarming people and dissuading them from having dental X-rays. So here are the basics of the report. This research was a case-control study that compared the histories of 1433 people who had a confirmed meningioma brain tumor (the “cases”) with 1350 people without a brain tumor (the “controls”) who were matched to have the same age, sex and state of residence as the brain tumor cases. All participants were 20 to 79 years old, lived in the United States, and were enrolled in the study between May 2006 and April 2011. Both groups were contacted by telephone and interviewed for about an hour. This phone interview included questions about the onset, frequency and type of dental care they had received over their lifetime.

The researchers were interested in three types of dental X-rays:

  • Bitewings – a small X-ray view that shows the upper and lower back teeth simultaneously, where the patient bites down on a small holder filled with the X-ray film. Bitewings are frequently used during regular checkups to look for cavities.
  • Full-mouth – a series of about 14-21 X-ray films that are used to view the entire mouth for dental problems, usually performed during a person’s first visit to the dentist.
  • Panoramic – a single X-ray that shows a broad view of the entire mouth to provide information about the teeth, jawbones, sinuses, and other tissues of the head and neck. Panoramic X-rays are taken occasionally, often to evaluate wisdom teeth, using a machine that moves around the patient’s head.

This large case-control study showed that people with a brain tumor reported having dental X-rays significantly more frequently over their lifetime than the controls without a brain tumor. However, the differences were only significant for bitewing and panoramic type dental X-rays, and not for full-mouth X-rays which actually expose the mouth to a greater dose of radiation. This inconsistency demonstrates that further research is needed to prove any link between dental X-rays and brain tumors.

The biggest issue with this study is that participants were asked to recall their own history of dental X-rays throughout their lifetime, which makes the results less reliable. In particular, there is a fear of “recall bias” – the people with brain tumors may have been focusing on the potential causes of their cancer and therefore may have been more likely to recall dental X-rays than the control group, potentially biasing the results. Although more work, the researchers should have acquired the participants’ dental histories directly from medical records.

While this study does suggest that regular dental X-rays may be linked to an increased risk of developing a brain tumor, it does not prove an actual link. There could be other factors that contributed to this association. In order to establish a causal link, the researchers should consider performing a different kind of study that follows a group of people over time to see who develops a brain tumor.

More importantly, the recent sensationalized news headlines ignored the important fact that brain tumors are rare. Men and women in the United States have a 0.61% lifetime risk of being diagnosed with any type of primary malignant brain or central nervous system tumor, implying a 0.21% lifetime risk of developing meningioma. For instance, this is much smaller than the 12.2% lifetime risk of a woman developing breast cancer.

So this research study should not scare people away from having dental X-rays when recommended by their dentist. The American Dental Association recommends that dentists now evaluate the benefit of X-ray exposure for each patient, reducing the frequency of routine X-rays for healthy patients. In addition, dental X-rays now expose patients to less radiation than in the past.

Why Don’t We Get Cancer More Often?

See my KQED Quest blog on Dr. Mina Bissell’s pivotal breast cancer research at Lawrence Berkeley National Lab. She will be part of a free public lecture, Science at the Theater: Health Detectives. It will be held on April 23 at 7 pm at the Berkeley Repertory Theater.

science at the theater health detectives
Image courtesy of Friends of Berkeley Lab.

What A Scientist Really Looks Like

Albert Einstein photograph
Courtesy of Sebastian Niedlich via Creative Commons.

The stereotypical  image of a scientist looks something like Albert Einstein, an older white man with either wild hair or almost none. The media often reinforces this image of a “mad scientist” in a white lab coat. But in reality, scientists are just a diverse group of people that mostly look and act like everyone else.

This scientist stereotype has been informally studied by at least two major scientific laboratories, Fermilab and the Pacific Northwest National Laboratory. About 12 years ago, a group of seventh graders came for a regular field trip to Fermilab. Few young people have ever knowingly met a scientist. So Fermilab had these students draw and describe what they thought a scientist looked like, both before and after going on the tour. While at Fermilab, the students met a diverse selection of real life scientists, including speaking in small groups with a typical white male, a young female and an African American male physicist. Before their field trip, the students mostly drew the stereotypical white man wearing a lab coat. However, their drawings after the tour were much more diverse and casually dressed.

Such studies have inspired a small group of people to demonstrate what a scientist is really like. Also inspired by Science Online 2012, they recently created a website where scientists can post a photograph and short description of themselves. Their hope is to challenge the stereotypical perception of a scientist. If you are a scientist and interested in joining their efforts, you can easily submit your photograph to be uploaded on their “This Is What A Scientist Looks Like” home page. However, I hope everyone enjoys the ever-expanding collection of photographs.

Ultrafast Laser Synchronization Breakthrough

AMO experiment schematic
Set-up of a nitrogen pulse-pump experiment that uses pulse arrival time information from a cross-correlator mounted downstream from the experiment. Figure courtesy of SLAC National Accelerator Laboratory.

A journal article, just published in Applied Physics Letters, details a major breakthrough for experiments at SLAC’s Linac Coherent Light Source (LCLS).

LCLS delivers intense ultrashort x-ray pulses that can be used to study the motion of atoms as they respond to external triggers, such as an optical laser. In these “pump-probe” experiments, the optical laser “pump” pulse starts a reaction in the material, while the x-ray “probe” pulse investigates the state of the material after a defined time delay. A sequence of x-ray pulses, with different time delays between the laser and x-ray pulses, is used to “film” the reaction in the material.

LCLS ultrafast x-ray pulses basically act like high-speed flashes of a camera strobe, allowing scientists to capture images with a “shutter speed” of less than 100 femtoseconds – the time it takes light to travel the width of a human hair.

In order to be able to “film” optically-induced ultrafast processes, however, scientists need more than just ultrashort x-ray and laser pulses. They also need to synchronize the x-ray pulses to the optical laser pulses with almost femtosecond accuracy, in order to have snapshots with good time resolution (“sharp focus”). This is a major challenge, since the main laser system for the x-ray free electron laser is a kilometer away from the optical laser and experiment.

State-of-the-art synchronization is performed at LCLS by accurately measuring the arrival times of the electron bunches (and corresponding x-ray pulses) relative to the radiofrequency that drives the accelerator, since the optical laser is locked to this reference radiofrequency. The best time resolution so far achieved with this approach is 280 fs (full width at half maximum, FWHM).

Recently, scientists at the LCLS Atomic, Molecular and Optical Science Instrument (AMO) dramatically improved the time resolution for their pump-probe experiments. Their new synchronization strategy is to directly measure the relative arrival time of both the x-ray and optical laser pulses at the experiment on a shot-by-shot basis. They do this by introducing into the x-ray beam what they call a cross-correlator, which is mounted downstream of the main experiment.

AMO scientists split their laser beam, sending it to both a pump-probe experiment and the cross-correlator (with a time delay). In the cross-correlator, the laser beam is reflected off a Si3N4 thin film. The spot of the laser pulse is then imaged with a long-distance microscope on a CCD camera. X-ray pulses also hit the same surface of the Si3N4 film, quasi-instantaneously changing the surface reflectivity.

The x-ray pulse very briefly changes the surface reflectivity. By imaging and measuring the position of this reflectivity change with the reflected laser, AMO scientists can directly measure the relative arrival time of the x-ray and optical laser pulses at their experiment. The scientists then use this pulse arrival time information from the cross-correlator to correct their corresponding experimental data on a shot-by-shot basis.

The AMO team demonstrated their improved time resolution with a nitrogen pump-probe experiment. With the time information from the cross-correlator, they were able to decrease the time resolution of their nitrogen experiment down to only 50 fs (FWHM). That’s almost down to the theoretical limit, allowing scientists to investigate all sorts of new ultrafast science.