Temporal trends in thyroid cancer incidence worldwide and in BRICS countries: A Bayesian age-period-cohort analysis and 20-year projections based on GBD 2021 data

The incidence of thyroid cancer has increased globally over recent decades, while mortality has remained stable or declined. However, integrated assessments of temporal trends, age-period-cohort (APC) effects, and future projections, particularly in BRICS countries, are limited. This study evaluated long-term incidence trends of thyroid cancer worldwide and across BRICS nations from 1990 to 2021 and projected future patterns. Data were derived from the Global Burden of Disease 2021 study. Age-standardized incidence rates and estimated annual percentage changes were calculated to assess temporal trends. Countries were grouped using hierarchical cluster analysis. Conventional APC analysis and Bayesian APC models were applied to identify dominant temporal effects and project incidence from 2022 to 2041 by sex and age group. Globally, the age-standardized incidence rate of thyroid cancer increased from 2.06 to 2.91 per 100,000 population between 1990 and 2021 (estimated annual percentage change = 1.25%). Considerable heterogeneity was observed among BRICS countries. China and India experienced the fastest increases, with particularly steep rises among Chinese males. The Russian Federation showed persistently high but relatively stable incidence, while Brazil and South Africa exhibited moderate upward trends. APC analyses indicated that period effects predominated globally and in Brazil and South Africa, cohort effects were dominant in China and India, and age effects were more pronounced in the Russian Federation. Bayesian APC projections suggested a continued global increase in thyroid cancer incidence, especially among middle-aged and older women, with sustained female predominance. Thyroid cancer incidence is projected to rise further worldwide, with heterogeneous APC-driven patterns across BRICS countries, underscoring the need for targeted surveillance and prevention strategies.