Radiotherapy for childhood cancer and subsequent thyroid cancer risk: a systematic review.

Most of the pooled analyses and reviews reported an association between radiotherapy for childhood cancer and an increased thyroid cancer risk. Up to now this article presents the first systematic literature review on this association combined with a critical assessment of the methodological quality of the included articles. PubMed and Web of Science databases were searched for relevant articles until May 2016. We included peer-reviewed cohort and case-control studies that investigated an association between radiotherapy for childhood cancer and the occurrence of subsequent thyroid cancer. A systematic overview is presented for the included studies. We identified 17 retrospective cohort studies, and four nested case-control studies, representing 100,818 subjects. The age range at first cancer diagnosis was 0-25.2 years. Considerable variability was found regarding study sizes, study design, treatment strategies, dose information, and follow-up periods. 20 of the 21 identified studies showed increased thyroid cancer risks associated with childhood radiation exposure. The large majority showed an increased relative risk or odds ratio confirming the association between radiotherapy and thyroid cancer although the variation in results was large. Additionally to a pooled analysis that has been published recently, we systematically included 17 further studies, which allowed us to cover information from countries that were not covered by large-scale childhood cancer survivor studies. The methodological limitations of existing studies and inconsistencies in findings across studies yielded a large study heterogeneity, which made a detailed comparison of study results difficult. There is a need to strengthen standardisation for reporting.