Risk of Second Primary Cancer Among Patients with Thyroid Carcinoma in Finland: A Nationwide Population-Based Study

Purpose: Thyroid cancer (TC) survivors face an increased risk of second primary cancers (SPCs). While previous research on the risk of SPCs among TC patients has been conducted in other regions, comprehensive data from Northern Europe remain limited. Our register-based cohort study aimed to evaluate the risk of SPCs among Finnish TC survivors. Patients and Methods: We obtained data on all patients diagnosed with TC in Finland in 1953–2022 from the Finnish Cancer Registry (FCR). Standardized incidence ratios (SIRs) of SPCs were calculated relative to the risk of cancer in the general population. Results: We identified 14,520 patients with TC (3,212 males, 22%; 11,308 females, 78%), with a median follow-up of 12.1 years. A metachronous SPC was diagnosed in 16.4% (n=527) of males and 15.5% (n=1,744) of females. The overall SPC risk was elevated for both males (SIR 1.23, 95% CI 1.12–1.34; EAR 2.39/1,000 PYs) and females (1.24, 1.19–1.30; EAR 1.92/1,000 PYs). Female patients with papillary TC (PTC) displayed increased SIRs for breast (SIR 1.26, 95% CI:1.15–1.37), urinary organ (1.50, 1.19–1.86), brain (1.85, 1.48–2.29), and hematolymphoid cancers (1.31, 1.10–1.54). Males with PTC showed an increased risk of urinary organ (SIR 1.39, 1.01–1.85), brain (2.73, 1.69–4.17), and hematolymphoid cancers (1.52, 1.12–2.01). The elevated SPC risk of the breast and brain persisted even after 20 years of follow-up. An increased incidence of SPCs of the urinary organs, brain, and hematolymphoid tissues was observed in females diagnosed with follicular TC (FTC) or medullary TC (MTC) in 1953–1985. Conclusion: Our results indicate an excess risk of cancers of the breast, urinary organs, brain and hematolymphoid tissues for Finnish patients with a history of PTC. For FTC and MTC risk of cancers of the urinary organs, brain, and hematolymphoid tissues is specific for female patients diagnosed in 1953–1985.