Background: The occurrence of nonthyroidal primary malignancy (NTPM) and the potential association of with radioiodine (RAI) treatment are important concerns in patients with differentiated thyroid cancer (DTC), but incidence data are conflicting. The aims of the present study were to investigate the incidence of NTPM and its association with RAI treatment in a cohort of DTC patients treated at a single tertiary medical center.
Methods: The data of 1943 patients with DTC recorded in the Rabin Medical Center Thyroid Cancer Registry were cross-matched with data from the Israeli National Cancer Registry to identify those diagnosed with an NTPM. Patient medical files were reviewed. Second primary malignancy (SPM) was defined as new malignancy diagnosed at least two years after DTC diagnosis.
Results: For 1434 of the 1943 patients (73.8%), the American Joint Committee on Cancer TNM stage was 1-2. The mean follow-up was 9.3 years. Of the 1943 patients, 1574 (81%) were treated with RAI, and 1467 were followed for at least 2 years, and of these, 1145 patients (78%) received a cumulative dose of ≤200 mCi. A total of 409 NTPMs were diagnosed in 368/1943 patients with DTC (18.9%; 265 female, mean age 53.9 ± 15 years), including 173 SPMs (42.3%) in 166/368 patients. The most common NTPM and SPM was breast cancer followed by hematologic malignancies. In patients followed for ≥2 years, SPMs were diagnosed in 9% of RAI-treated patients and 10.5% of non-RAI-treated patients. SPM rates were 10.2% and 7.8% for a cumulative RAI dose of ≤100 mCi and >100 mCi respectively. Hazard ratios for SPM in patients that received/did not receive RAI treatment was 1.27 (95% confidence interval 0.88-1.82; p = 0.1). There was no correlation between first or cumulative RAI dose and diagnosis of SPM.
Conclusions: NTPMs are not uncommon in patients with DTC and usually antecede the DTC. In a population of mostly low-risk DTC patients, in whom limited activities of RAI are usually administered, this treatment is apparently not associated with an overall increased risk of SPMs compared with subjects not receiving RAI treatment.