Objectives: The association between second primary malignancy (SPM) and radioactive iodine (RAI) is controversial. We examined the association between RAI and SPM after treatment of differentiated thyroid carcinoma (DTC) using a large cohort from a national cancer database.
Methods: From the Surveillance, Epidemiology and End Results (SEER) database, all index cases of DTC (papillary or follicular) were extracted for the years 1988 to 2001. Two cohorts were constructed: 1) patients with DTC who were not treated with RAI, and 2) patients with DTC who were treated with RAI. For each cohort, we tabulated all subsequent malignancies for each patient, identifying patients in each group with 1 or more SPMs.
Results: According to inclusion criteria, 18,882 cases of DTC treated without RAI (mean follow-up, 55.5 months) and 10,349 cases treated with RAI (mean follow-up, 61.8 months) were identified. The most common SPM sites were breast or prostate followed by colon or lung for both groups. On univariate analysis, SPMs developed in 6.7% of patients without RAI versus 4.8% of those with RAI (p < .001, univariate chi-squared). However, on multivariate analysis, only age and male gender had statistically significant hazard ratios (1.052 and 1.438, respectively; p < .001); follicular carcinoma histology and use of RAI did not influence occurrence of SPM after DTC (p = .180 and p = .789, respectively).
Conclusions: Use of RAI does not elevate the risk of SPM. Concern about SPM induction should not adversely affect the decision to administer RAI for DTC.