The number of 131I therapy courses needed to achieve complete remission is an indicator of prognosis in patients with differentiated thyroid carcinoma.

To assess the risk of differentiated thyroid cancer (DTC) recurrence, DTC-related mortality and life expectancy in relation to the number of courses of (131)I therapy (RIT) and cumulative (131)I activities required to achieve complete remission (CR). The study was a database review of 1,229 patients with DTC, 333 without and 896 with CR (negative TSH-stimulated thyroglobulin and negative (131)I diagnostic whole-body scintigraphy) after one or more courses of RIT. The median follow-up was 9.0 years (range 0.1 - 31.8 years) after CR. Recurrence rates at 5 years, 10 years and the end of follow-up were 1.0 ± 0.3%, 4.0 ± 0.7 % and 6.2 ± 1.1 %, and DTC-related mortality was 0.1 ± 0.1%, 0.5 ± 0.3% and 3.4 ± 1.1%, respectively. Recurrence rates also increased with an increasing number of RIT courses required (p = 0.001). DTC-related mortality increased from four RIT courses. In patients with CR after one RIT course, there were no differences in recurrence or DTC-related mortality rates between low-risk and high-risk patients. In patients requiring two RIT courses these rates remain elevated in high-risk patients. Recurrence and DTC-related mortality rates were only significantly elevated in those requiring a cumulative activity over 22.2 GBq (600 mCi) from multiple RIT courses for CR. Regardless of the number of RIT courses or activity needed, life expectancy was not significantly lowered. If more than one RIT course is needed to achieve CR, higher recurrence and DTC-related mortality rates are observed, especially in high-risk patients. Patients requiring >22.2 GBq (131)I for CR should be followed in the same way as patients in whom CR is never reached as long-term mortality rates are similar.