Objective: Most studies on second primary malignancies (SPMs) after primary thyroid cancer were conducted in USA or Europe. The discrepancy between SPMs in these studies could be attributed to geographical and ethnic heterogeneity. Thus, there is a clear need for another large-scale epidemiological study, particularly in Asian countries, to define the incidence and risk of SPMs in thyroid cancer survivors. Design: A population-based study was conducted using the nation-wide database from Taiwan Cancer Registry between 1979 and 2006. Methods: We quantified standardized incidence ratios (SIRs) and cumulative incidence of SPMs among 19068 individuals (4205 males and 14 863 females) with primary thyroid cancer. Results: A total of 644 cases (3.38%) developed at least a SPM during 134 678 person-years of follow-up. The risk for subsequent SPMs was significantly greater than that of the general population (SIR=1.33, 95% CI 1.23-1.44). There was a greater risk of developing major salivary glands, nasopharyngeal, lung, thymus, breast (females), bladder, and brain cancers, and leukemia and lymphoma. We observed that the risk was highest within the first 5 years of diagnosis of thyroid cancer (SIR = 5.29, 1.68, and 0.68 for ≦5, 5-10, and > 10 respectively) and in the younger patients (SIR = 1.81 vs 1.61 for <50 and ≧50 respectively). The median overall survival for primary thyroid cancer patients was 23.28 years, but it was only 4.73 years for those who developed SPMs. Conclusion: Thyroid cancer is associated with a 33% risk increment of SPMs, which had a negative impact on survival. There are sites of SPMs in the Asian population that are distinctive from those in the Western population, suggesting that other genetic predisposition or environmental factors may play a role.
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