A study published this week in the journal Cancer shows that people who have had dental X-rays are more likely to develop a type of brain tumor called meningioma than those who have not.

This does not prove that dental X-rays cause tumors. But it supports previous research about the connection. Dental X-rays have also been implicated in thyroid cancer. But there's still significant doubt about the existence of any direct relationship between meningioma and dental X-rays, and dental professionals were quick to call for more research, saying the study was less than perfect.

"It’s a cautionary tale ... we do know that radiation can cause tumors, and we have to be judicious with its use," said Dr. Donald O’Rourke, an associate professor of neurosurgery at the University of Pennsylvania who was not involved in the study.

Meningiomas are the most frequently occurring tumor in the head. They are located in the meninges, the tissues covering the brain. The vast majority are "benign" - or noncancerous - but, depending on their location, could cause blindness or other serious neurological damage. Those in the skull base are more difficult to remove in their entirety. Depending on the tumor, surgery may not be required.

Dr. Elizabeth Brooks Claus, director of medical research at Yale University's School of Public Health, led the Cancer study, which focused on patients whose tumors required surgery. The patients were mostly Caucasian because of the regions from which they came; Claus' group plans a follow-up looking at more African Americans, who have a statistically increased risk for meningiomas.

The average age of the 1,433 patient participants was 57, which means their exposures to dental X-rays were likely of a higher radiation doses because of older technology, Claus said. But they ranged between 20 and 79 years old, and came from select parts of the United States. Researchers also looked at data from 1,350 people with similar characteristics who had never had a meningioma.

The meningioma patients had more than a two-fold increased likelihood of having ever experienced a dental X-ray test called a bitewing exam. Depending on the age at which the exams were done, those who'd had these exams on a yearly basis, or more often, were 1.4 to 1.9 times more likely to have had a meningioma.

Four of these X-rays is about the same amount of radiation you're exposed to in a typical day: .005 .millisieverts, according to the American College of Radiology.

Panorex exams, which involve images of all of the teeth on one film, were also linked to meningioma risks. If study participants had panorex exams when they were younger than 10 years old, their risk of meningioma went up 4.9 times. One of these around-the-head X-rays carries about twice as much radiation as four bitewing X-rays.

"My impression is that people get more dental X-rays more frequently than the American Dental Association says," Claus said.

For an adult without cavities and no increased risk for cavities, who is not new to his or her dentist, x-rays are recommended every two to three years. For a child without cavities who's not at increased risk, the interval is every one to two years, according to this chart from the Food and Drug Administration.

There's currently a low threshold for dentists to order dental X-rays, says Dr. Keith Black, director of the Maxine Dunitz Neurosurgical Institute at Cedars-Sinai Medical Center in Los Angeles, who was not involved in the study. Even if X-rays are not necessary for a procedure, dentists often request them as part an annual exam. Black hopes dentists will pay attention to this research linking the X-rays to brain tumors.

There are important uses for dental X-rays in making decisions regarding certain procedures. But if the teeth are otherwise healthy, Black recommends against the radiation.

There is a latency period - a lag time - of about 20 to 25 years with meningiomas induced by radiation, O'Rourke said. Only about 1% to 5% of meningiomas are cancerous, but in people with known increased radiation exposure, that risk can go up, he said.

But Dr. Otis Brawley, Chief Medical Officer of the American Cancer Society, which publishes the journal Cancer, points out that the study relied upon individuals' memories of how many dental X-rays they'd had, including in childhood, so there is room for error in that regard. And, again, it does not prove that X-rays directly cause tumors.

There are, however, estimates that up to 1% of all cancers in the United States are due to medical radiation, Brawley said.

In response to the study announcement, the American Dental Association also mentioned the study's reliance on individuals' memories.

"Studies have shown that the ability to recall information is often imperfect," said a written statement from the ADA. "Therefore, the results of studies that use this design can be unreliable ..."

The ADA also pointed out that the study included people who received dental x-rays decades ago from older technology that exposed them to more radiation. "The ADA encourages further research in the interest of patient safety," said the statement.

If you've already been getting annual dental X-rays, there's nothing you can do to mitigate whatever risk you already have. But Black said this research is important to keep in mind when making decisions in the future, and for children.