Dental x-rays and the risk of thyroid cancer and meningioma: a systematic review and meta-analysis of current epidemiological evidence

Background: Exposure to moderate-to-high doses of ionising radiation is the only established environmental risk factor for thyroid cancer and brain and CNS tumours. Considering the high life-time prevalence and frequency of exposure to dental x-rays, the most common source of diagnostic radiation exposure in the general population, even a small associated increase in cancer risk would be of considerable public health importance. With the objective to inform clinical practice and guidelines, we synthesised the current epidemiological evidence on the association between dental x-rays and the risk of thyroid cancer, meningioma and other cancers of the head and neck region. Methods: The Medline, Embase and Web of Science databases were searched to identify eligible studies. Summary odds ratio/relative risk estimates and confidence intervals were extracted, and pooled risk ratios (RR) for each cancer were calculated using random effects meta-analysis. Results: The literature search identified 5537 publications; of these, 26 studies including 10,868 cancer patients were included in the synthesis. The random effects meta-analyses, based on seven studies of thyroid cancer (six case-control, one cohort) and eight studies of meningioma (all case-control), showed that multiple (or repeated) exposures to dental x-rays were significantly associated with an increased risk of thyroid cancer (pooled RR = 1.87, 95% CI 1.11–3.15) and meningioma (pooled RR = 1.53, 95% CI 1.26–1.85). There was no association with glioma, and there were too few studies of other cancers of the head and neck region to conduct a meaningful meta-analysis. Conclusions: Based on a meta-analysis of retrospective case-control studies, these findings provide some support to the hypothesis that multiple (or repeated) exposures to dental x-rays may be associated with an increased risk of thyroid cancer and meningioma. These studies did not include individual organ doses and ages at exposure, and are subject to recall bias and other limitations. Furthermore, the thyroid exposure has decreased dramatically over time from the use of thyroid shields and improved technology/equipment. Prospective studies, based on dental x-rays records and patient follow-up, are needed to test the hypothesis further and clarify the possible cancer risk associated with dental radiography, as although the risk at the individual level, particularly with improved technology/equipment, is likely to be very low, the proportion of the population exposed is high. Considering that about one-third of the general population in developed countries is routinely exposed to one or more dental x-rays per year, these findings manifest the need to reduce diagnostic radiation exposure as much as possible.