Epidemiology of thyroid cancer in Tuscany (Central Italy) 2013–2017: not just overdiagnosis

Background: Thyroid cancer (TC) incidence, particularly the papillary histotype (PTC), has been increasing worldwide, and some of the highest incidence rates (IRs) have been reported in Italy, although with intra-country heterogeneity. The increasing incidence of TC is predominantly due to overdiagnosis, but environmental and lifestyle causative factors cannot be excluded. The aim of this study was to report TC incidence and mortality rates in the Tuscany region (Central Italy) and to review the histological records of TC patients born and living in the region, according to the province of residence, particularly in municipalities where heavy metal pollution has been well documented. Methods: The Tuscany Cancer Registry (ISPRO), certified by the Italian Association of Cancer Registries (AIRTUM), provided us with the number of cases and European age-standardized IRs of TC patients for all 10 Tuscany provinces from 1 January 2013 through 31 December 2017. In addition, we collected the TC histological records and diagnosed them in the same period. Results: A total of 4459 cases, 3209 (72%) women and 1250 (28%) men, were diagnosed with TC and reported by ISPRO. In women, the age-standardized IRs ranged from 22.3 to 45.4 cases per 100,000 inhabitants; in men, this ranged from 8.4 to 16.5 cases per 100,000 inhabitants. The histological records of TC patients living in the Tuscany provinces of the coastal areas (Livorno, Grosseto, Pisa, Massa-Carrara, and Lucca) appeared phenotypically more aggressive (higher extrathyroidal invasion, higher rate of lymph node metastases, higher rate of tumor bilaterality and multicentricity, lower rate of microcarcinomas) compared to those of the other provinces. When evaluating the relative risk (RR) of diagnosing TC in relation to environmental sources of heavy metal pollution exposure, we found that exposed patients had an RR of 1.16 (95% CI: 1.04–1.29), significantly greater compared to non-exposed patients. Conclusions: The overdiagnosis phenomenon is the main explanation for the increased incidence of TC in Tuscany. However, in some geographical areas of the region, the presence of environmental pollution, especially that characterized by the release of heavy metals, might influence TC incidence rates and the phenotype.