Exposure to persistent organic pollutants and thyroid cancer risk: a study protocol of systematic review and meta-analysis

Introduction The thyroid cancer incidence has been increasing all over the world. However, the aetiology of thyroid cancer remains unclear. A growing body of evidence suggested exposure to persistent organic pollutants (POPs) may play a role in the initiation of thyroid cancer, but the results are generally inconsistent across studies. This review aims to synthesise the evidence for the health effects of POPs on the risk of thyroid cancer. Methods and analysis This protocol was reported in accordance to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses Protocols (PRISMA) statements. A comprehensive search, including electronic database search (eg, PubMed, Embase, ProQuest and CNKI), website search and manual search, will be performed to identify all eligible studies. The Population, Exposure, Comparator and Outcome framework was used to clarify the inclusion and exclusion criteria. The Newcastle-Ottawa Scale will be used to assess the quality of included studies. Maximally adjusted effect estimates from individual studies will be summarised with random-effect models in a conservative manner. I-2 statistics and Q-tests will be used to test the heterogeneity across studies. We will perform extensive sensitivity analyses, such as confounding risk ratio (confounding), E-value, fixed-effect models, excluding the most relatively weighted study, including only the high-quality studies and many predesigned subgroup analyses, etc. The findings will be reported in accordance to the PRISMA guidelines. Ethics and dissemination Ethical approval is not required in this systematic review of published literatures. The results will be published in a peer-reviewed journal and presented at relevant conferences. PROSPERO registration number CRD42020181343.