The risk of second primary malignancy is increased in differentiated thyroid cancer patients with a cumulative 131I dose over 37 GBq

BackgroundThe aim of this study was to investigate the risk factors for second primary malignancy (SPM) diagnosed after differentiated thyroid cancer (DTC). MethodsA total of 2468 DTC patients who underwent thyroidectomy were reviewed. SPM was defined as a non-thyroidal malignancy, diagnosed at least 1year after the diagnosis of thyroid cancer. Patients were divided into five groups according to cumulative I-131 dose: very high-activity (370 GBq), high-activity (223-369 GBq), intermediate-activity (556-222 GBq), low-activity (11-555 GBq) and no RAI. ResultsAmong the 2468 patients, 61 (25%) had SPMs during 70 (10-330)years of median follow-up. Age above 40years, male sex and very high-activity RAI were independent risk factors for the development of SPM. SPM-related mortality was highest in the very high-activity group, while DTC-related mortality was highest in the high-activity group. The overall mortality both from SPM and DTC was highest in the high-activity group. ConclusionA cumulative I-131 dose <370 GBq did not increase the risk of SPM. A cumulative I-131 dose 370 GBq increased the risk of SPM and SPM-related mortality and decreased the DTC-specific mortality, resulting in a similar all-cause mortality compared with the low-activity RAI group. Using repeated high-dose RAI for treating RAI-responsive but persistent DTC patients needs careful consideration of the individual benefits from RAI vs the risk of developing SPM.