Frequent dental X-rays may significantly increase the risk of non-malignant brain tumours, say researchers.

Over a lifetime, having dental X-rays can double or triple the chances of developing meningioma tumours, a study has found. The tumours grow in the outer membrane covering the brain.

In one case, involving X-rays on children, a five-fold increase in risk was seen.

However, the likelihood of developing a brain tumour at all is very small. Meningiomas, which account for about one in five primary brain tumours, affect two or three in every 100,000 people in the UK each year.

The tumours are slow growing, often causing no symptoms, and usually benign.

Scientists in the US looked at the self-reported dental histories of 1,433 patients diagnosed with meningioma tumours. They were compared with a "control group" of 1,350 matched individuals who were free of the disease.

The tumour patients were consistently more likely to have been exposed to dental X-rays.

One of the most common procedures is the "bitewing" X-ray which uses an X-ray film clenched between the teeth in a tab of plastic or cardboard.

Bitewing X-rays check for decay between the teeth and can also expose bone loss caused by severe gum disease.

Over a lifetime, patients with meningioma were between 1.4 and 1.9 times more likely than controls to have undergone bitewing X-rays on a yearly or more frequent basis.

An increased risk of meningioma was also linked to "panorex", or "panoramic" X-rays which provide a broad view of the jaws, teeth and nasal area. They reveal problems such as impacted teeth, cysts, infections and bone abnormalities.

People who had panorex X-rays when they were younger than 10 years old had an almost five-fold greater chance of developing meningioma.

Having the X-rays once a year or more often was associated with a 2.7 to three times increase in risk, depending on age.

The findings appear in an early online edition of the journal Cancer, published by the American Cancer Society.

Dr Elizabeth Claus, from Yale University School of Medicine, and colleagues wrote: "Our findings suggest that dental X-rays, particularly when obtained frequently and at a young age, may be associated with an increased risk of intracranial meningioma, at least for the dosing received by our study participants."

The scientists noted that radiation doses from dental X-rays today are lower than they were in the past.

The patients taking part in the study were diagnosed at between 20 and 79 years of age. Some of them would therefore have been exposed to higher radiation doses earlier in their lives.

Ionising radiation, such as X-rays, is known to be the most important risk factor for meningioma brain tumours.

However, most research on meningioma and ionising radiation has focused on high exposure levels from atomic bombs or cancer treatments.

Previous studies had hinted at an increased risk from dental X-rays, but were limited by small numbers of patients who may have received high radiation doses, said the researchers.

British cancer expert Dr Paul Pharoah, from Cambridge University, said the study was "carefully designed and well conducted".

He added: "The authors report that dental X-rays are associated with a small relative increase in risk of disease of approximately 50% or 1.5-fold. This finding is statistically significant.

"However, as the disease is rare, the increase in absolute risk is tiny - the lifetime risk increasing from 15 in every 10,000 people to 22 in 10,000.

"People who have had dental X-rays do not need to worry about the health risks of those X-rays. Nevertheless, dental X-rays should only be used when there is a clear clinical need in order to prevent unnecessary exposure to ionising radiation."

Professor Damien Walmsley, scientific adviser to the British Dental Association, said: "X-rays are a vital tool in dentistry and reveal problems in the teeth and surrounding bone that simply cannot be observed by the naked eye, and often before patients experience any pain or discomfort. The earlier dental problems are identified, the easier they are to treat.

"Regulations governing the use of X-rays in the UK are considerably more stringent than in the US. Dentists in the UK are only permitted to take X-rays where these are absolutely necessary. This means that fewer X-rays are taken in dental practice and modern equipment has reduced exposure to radiation to extremely low levels.

"Dentists are also required to undertake regular training in the use of X-rays to ensure they are up to date with the latest techniques in deciding how, and when, these are required. This maximises safety for patients, while retaining an essential tool in the diagnosis of dental disease."

Responding to the study, a Department of Health spokesman said: "Patient safety is a priority and the UK has stringent safeguards in place covering the use of dental X-rays. All X-ray exposures must be clinically justified.

"Radiation protection is a mandatory topic for continuing professional education for dentists and must be undertaken to maintain registration with the General Dental Council."