Researchers from Brighton (England), Cambridge (England) and Kuwait have demonstrated that thyroid cancer risk increases as the number of dental x-rays taken grows. The researchers report that the incidence rates of thyroid cancer have doubled from 1.4 per 100,000 in 1975 to 2.9 per 100,000 in 2006 in the UK.

They stress that several factors are probably involved in the thyroid cancer increase, and that sensitive diagnostic techniques should not be considered to account for all of it. Further research is needed.

The research team was led by Dr Anjum Memon, senior lecturer and consultant in public health medicine at Brighton and Sussex Medical School (England), a partnership between Brighton and Sussex universities, and NHS (National Health Service) Brighton and Hove (England).

This study has been published in the medical journal Acta Oncologica.

The thyroid gland, which is located in the neck, is exposed to radiation from many dental x-rays. This gland is sensitive to ionizing radiation, especially in children. The researchers inform that dental radiography, a source of low-dose diagnostic radiation, is frequently ignored as a potential risk to the thyroid gland.

The scientists studied 313 patients in Kuwait, they all had thyroid cancer. In Kuwait dental treatment is free. Compared to other countries, such as the UK, the incidence of thyroid cancer in Kuwait is high.

They said the results of their study, although the largest case-control study on the subject, “should be treated with caution because the data were necessarily based on self-reporting by the participants. Comprehensive historical dental x-ray records were not available from the clinics.”

The researchers believe their study provides good evidence to warrant further research in settings where historical dental x-ray records are available and where radiation doses can be estimated.

Dr Memon said the findings were compatible with earlier reports of raised risk of thyroid cancer in dentists, dental assistants, and x-ray workers, suggesting that multiple low-dose exposures in adults may also be significant. He said dental x-rays have also been linked to an increased risk of brain and salivary gland tumors.

Dr Memon wrote:

The public health and clinical implications of these findings are particularly relevant in the light of increases in the incidence of thyroid cancer in many countries over the past 30 years.

It is important that our study is repeated with information from dental records including frequency of x-rays, age and dose at exposure. If the results are confirmed then the use of x-rays as a necessary part of evaluation for new patients, and routine periodic dental radiography (at 6-12 months interval), particularly for children and adolescents, will need to be reconsidered, as will a greater use of lead collar protection.

Our study highlights the concern that like chest (or other upper-body) x-rays, dental x-rays should be prescribed when the patient has a specific clinical need, and not as part of routine check-up or when registering with a dentist.

(conclusion) The notion that low-dose radiation exposure through dental radiography is absolutely safe needs to be investigated further, as although the individual risk, particularly with modern equipment is likely to be very low, the proportion of the population exposed is high.

“Dental x-rays and the risk of thyroid cancer: A case-control study”
Anjum Memon , Sara Godward , Dillwyn Williams , Iqbal Siddique & Khalid Al-Saleh
Acta Oncologica May 2010, Vol. 49, No. 4, Pages 447-453

Written by Christian Nordqvist