Abstract
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Objectives Radioactive iodine therapy (RIT) is one of the important therapeutic strategy for thyroid cancer, there still remains, however, a concern of secondary primary malignancy (SPM) risk. The purpose of the present study is to assess the risk of SPM in patients who have ever treated with radioactive iodine.
Methods The risk of SPM after total thyoidectomy was retrospectively investigated in 4964 patients (male:female = 3851:1113, mean age = 48±12 years) diagnosed between 2003 and 2013 from Seoul National University Bundang Hospital by using electronic medical record and clinical data warehouse. The mean period of follow-up was 44 months (range, 2-121 months). Of all, 2685 patients were treated with 131I (mean cumulative dose = 4.0 GBq, range, 1.11-54.76 GBq). Standardized incidence ratios (SIRs) were calculated by dividing the number of observed cancers (O) by expected cancers. The expected numbers (E) of SPM were calculated by multiplying age and calendar-year-specific person-years at risk with the corresponding incidence rates in Korea.
Results In total, 51 patients developed a SPM, which was significantly more than that expected in the Korean general population [SIR (O/E) = 1.29; 95% confidence interval (CI) 1.08-1.52; absolute excess risk (AER) = 16.06]. 26 of the 2685 patients who had ever been treated with 131I had a SPM, following 2 months after 131I therapy and 25 of the 2279 patients who had never received RIT had a SPM, following 6 months post the day of operation for thyroid cancer. However, there was no significant risk of SPM incidence over that expected in the Korean general population for 131I therapy cohort (SIR = 1.17; 95% CI 0.91-1.50; AER = 9.53). Also, no significant association was found between exposure to 131I and risk of SPM (RR = 0.88, 95% CI 0.51-1.52).
Conclusions The overall risk of SPM was not significantly increased in thyroid cancer patients by 131I therapy. Rather, the increased risk of SPM in thyroid cancer survivors may be related to disease itself or genetic predisposition.