Risk and Outcome of Secondary Primary Malignancies After Radioactive Iodine Treatment in Differentiated Thyroid Cancer Patients

Background: To investigate the relative risks(RRs) of secondary primary malignancies (SPMs) by different sites associated with radioactive iodine (RAI) treatment in differentiated thyroid cancer (DTC) survivors, and the attributable risk proportion of RAI treatment for developing different SPMs. We identified DTC survivors from SEER registries and performed Poisson regression to calculate the RRs of SPMs by different sites associated with RAI treatment, as well as the attributable risk of RAI for different SPMs. Results: The RRs of developing SPM associated with RAI varied from 0.86 (0.53-1.40) for SPMs of nervous system to 1.25 (1.05-1.50) for hematologic SPMs. The RRs of developing all cancer combined SPMs generally increased with age at DTC diagnosis and decreased with the latency time. We estimated that the attributable risk proportion of RAI treatment is only 0.9% for all cancer combined SPMs and 20% for hematologic SPMs, which is the highest among all SPMs. The tumor features and mortality of patients treated with and without RAI are generally comparable. Conclusion: The RR of developing SPMs associated with RAI treatment in DTC survivors increase only in hematologic systems, with the highest attributable risk proportion of RAI treatment among all cancer combined SPMs. However, in consideration of its low incidence in all DTC survivors, the absolute number of hematologic SPMs would be low. For all cancer combined SPMs in DTC survivors, a very low proportion is associated with RAI treatment, indicating patients and physicians should not be panicked with the RAI treatment in DTC patients.