Multiple exposures to dental X-rays may be associated with an increased risk of developing thyroid cancer, according to a collaborative study by scientists from Brighton, Cambridge and Kuwait.

In the study, the risk of thyroid cancer increased with increasing numbers of dental X-rays taken.

In Great Britain about 1,900 new cases of thyroid cancer are diagnosed each year and the incidence rates more than doubled, from 1.4 to 2.9 per 100,000 people between 1975 and 2006. Increasing use of sensitive diagnostic techniques is not considered to account for all of this increase and the researchers believe that other causes need investigation.

The thyroid gland, situated in the neck, is known to be highly sensitive to ionising radiation, particularly in children, and dental radiography, a source of low-dose diagnostic radiation, is often overlooked as a potential hazard to the gland.

Dr Anjum Memon of Brighton and Sussex Medical School led the research team, which studied 313 thyroid cancer patients in Kuwait, where the incidence of thyroid cancer is relatively high compared to Britain. The team said that the results of the study provide good evidence to warrant more research in settings where historical dental X-ray records are available and where doses of radiation can be estimated.

The results of the Kuwait study should be treated with caution as the data were necessarily based on self-reporting by the participants.

'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,' said Dr Memon.

The study was published in Acta Oncologica (2010; 49: 447–453).