Rising incidence of follicular thyroid carcinoma and oncocytic carcinoma of the thyroid in Australia

The incidence of follicular thyroid carcinoma (FTC) and oncocytic carcinoma of the thyroid has seldom been studied. However, thyroid cancer incidence has experienced significant increases globally over recent decades. This study aims to investigate the incidence of FTC and oncocytic carcinoma of the thyroid in Australia with particular attention to the impact of changes in the World Health Organization (WHO) endocrine tumour classification. Using incidence data from the Australian Institute of Health and Welfare cancer registry (spanning 1982-2019), this descriptive epidemiological study employed joinpoint regression analysis to assess temporal trends in FTC and oncocytic carcinoma of the thyroid. Results were then compared with WHO endocrine tumour classification changes over the same period to identify potential impact(s). FTC and oncocytic carcinoma of the thyroid accounted for 9.4 and 3.2% respectively, of all thyroid carcinomas. Oncocytic carcinoma of the thyroid incidence steadily increased across the study period. Subtype analysis of FTC showed the incidence of widely invasive FTC initially increased significantly before declining and was found to affect older adults primarily. On the other hand, minimally invasive FTC incidence rose sharply for both sexes. Encapsulated angioinvasive FTC, a recently classified subtype, was found predominantly in younger age groups, particularly the 30-34 age bracket. Changes in the incidence of FTC and oncocytic thyroid carcinoma are noted to be influenced by classification updates. To conclude, there is a steady rise in Australian FTC and oncocytic carcinoma of the thyroid incidence, influenced to some degree by WHO classification changes.