[Risk of differentiated thyroid cancer as the first and second primary tumor]

Certain second primary malignancies have been reported to be more common after well differentiated thyroid cancer than it would be expected in the general population. Conversely, second primary thyroid cancers are more frequent following certain malignancies. In this review, the incidence of second primary malignancies after differentiated thyroid cancers and the risk of second primary thyroid cancer following the most common malignancies have been analyzed based on recent literature. The reported incidence of second primary malignancies might be influenced by study design, genetic predisposition, lifestyle, environmental factors, oncogenic treatment of the first malignancy and detection bias. Nevertheless, after thyroid cancer, a higher incidence of breast, prostate, colorectal, renal cancer, and certain hematological malignancies can be observed compared to the general population. Thyroid cancer occurs more frequently after breast, prostate, colorectal, lung, renal, and bladder cancer, certain hematological malignancies, and neuroblastoma. Long-term clinical follow-up is advised after thyroid cancer to ensure early detection of a possible second primary malignancy. Increased adherence to cancer screening programs is also highly recommended for survivors of the first primary thyroid cancer. Orv Hetil. 2025; 166(7): 253–262.