Challenges in investigating risk factors for thyroid cancer

Abstract

Thyroid cancer has become the fifth most commonly diagnosed cancer in adult women worldwide, and the second most common in women older than 50 years. 1 Bray F Ferlay J Soerjomataram I Siegel RL Torre LA Jemal A Global cancer statistics 2018: GLOBOCAN estimates of incidence and mortality worldwide for 36 cancers in 185 countries. CA Cancer J Clin. 2018; 68: 394-424 Crossref PubMed Scopus (41701) Google Scholar The rapid increase in incidence compared with mortality trends—which generally have remained stable at low levels (around two orders of magnitude lower than incidence) or have even declined 2 Li M Brito JP Vaccarella S Long-term declines of thyroid cancer mortality: an international age–period–cohort analysis. Thyroid. 2020; 30: 838-846 Crossref PubMed Scopus (23) Google Scholar —strongly suggests that the thyroid cancer epidemic has been largely driven by overdiagnosis (the diagnosis of cancers that would not have caused symptoms in a person's lifetime). 3 Vaccarella S Franceschi S Bray F Wild CP Plummer M Dal Maso L Worldwide thyroid-cancer epidemic? The increasing impact of overdiagnosis. NEJM. 2016; 375: 614-617 Crossref PubMed Scopus (543) Google Scholar The increasingly intense scrutiny of the thyroid gland has led to the discovery of a large reservoir of subclinical thyroid cancers. 4 Furuya-Kanamori L Bell KJL Clark J Glasziou P Doi SAR Prevalence of differentiated thyroid cancer in autopsy studies over six decades: a meta-analysis. J Clin Oncol. 2016; 34: 3672-3679 Crossref PubMed Scopus (106) Google Scholar Overdiagnosis might account for as much as 70–90% of all thyroid cancer diagnosed in adult women in some countries including South Korea, the USA, and Italy, and around 40–60% in the Nordic countries—in which there has been a smaller increase in thyroid cancer incidence compared with other countries. 3 Vaccarella S Franceschi S Bray F Wild CP Plummer M Dal Maso L Worldwide thyroid-cancer epidemic? The increasing impact of overdiagnosis. NEJM. 2016; 375: 614-617 Crossref PubMed Scopus (543) Google Scholar , 5 Li M Dal Maso L Vaccarella S Global trends in thyroid cancer incidence and the impact of overdiagnosis. Lancet Diabetes Endocrinol. 2020; 8: 468-470 Summary Full Text Full Text PDF PubMed Scopus (64) Google Scholar The peak in thyroid cancer incidence is found in age groups that have the highest surveillance pressure, with young adult women mostly affected because of their better access to health care for reproductive and gynaecological reasons and consequently increased chance of having their thyroid inspected. Overdiagnosis might also account for a substantial part of the increase in thyroid cancer diagnoses in adolescents, particularly girls, worldwide. 6 Vaccarella S Lortet-Tieulent J Colombet Davies L et al. Thyroid cancer in children and adolescents: global patterns and trends of incidence and mortality. Lancet Diabetes Endocrinol. 2020; (in press). PubMed Google Scholar Maternal health, in-utero, and perinatal exposures and risk of thyroid cancer in offspring: a Nordic population-based nested case-control studyIn-utero exposures, particularly those related to maternal thyroid disorders, might have a long-term influence on thyroid cancer risk in offspring. Full-Text PDF