Incidence rate and factors associated with the development of secondary cancers after radioiodine therapy in differentiated thyroid cancer: a multicenter retrospective study.

Purpose: The objective of this study was to estimate the incidence of secondary cancers and the factors associated with their development among patients who underwent radioiodine therapy (RIT) with differentiated thyroid cancer. Methods: We retrospectively collected medical records for patients who underwent first RIT between January 1, 2000, and December 31, 2005, from seven tertiary hospitals in South Korea after total thyroidectomy for differentiated thyroid cancer. Cancer incidence and calculated standardized rate ratio were compared with Korean cancer incidence data. The association between the development of secondary cancers and various parameters was analyzed by Cox-proportional hazard regression. Results: A total of 3106 patients were included in this study. Mean age at the time of diagnosis of thyroid cancer was 45.7 ± 13.3 years old, and 2669 (85.9%) patients were female. The follow-up period was 11.9 ± 4.6 (range, 1.2–19.6) years. A total of 183 secondary cancers, which included 162 solid and 21 hematologic cancers, occurred in 173 patients (5.6%). There was no significant difference between solid cancer incidence in our study population who underwent RIT and the overall Korean population, but the incidence of hematologic cancers and total cancer in our study was significantly higher compared with that of the Korean population. A multivariate analysis identified independent prognostic factors for the development of secondary cancer including age at 1st RIT, male, and total cumulative dose over 200 mCi. Conclusion: We need to assess the risk benefit for patients who receive over 200 mCi of a total cumulative dose. [ABSTRACT FROM AUTHOR]