Long-term relative survival with and without radioiodine in patients with low-risk thyroid cancer: a SEER based analysis of histologic subtypes and risk factors

The ESTIMABL2-trial demonstrated non-inferiority of avoiding radioiodine therapy in low-risk differentiated thyroid cancer (DTC, i.e. papillary- and follicular thyroid cancer (PTC, FTC)). Yet, follow-up duration was limited and FTC were underrepresented. We therefore assessed long-term relative survival by comparing observed and expected cancer-free survival. This robust measure of net survival was used to evaluate the long-term effects of radioiodine therapy, including in previously underrepresented histological subgroups. Using the SEER-database, we identified 18,645 patients with DTC according to the criteria in the ESTIMABL2-trial (low risk PTC and FTC with pT1am-pT1b, N0-NX). Long-term relative survival (> 10 years) was analysed retrospectively, subdivided based on histology (PTC and FTC) and TNM-status, and additionally in 5,171 patients with lymph node involvement (N1). Relative survival was compared at 3 -, 5 -, and 10-years follow-up with and without radioiodine. Radioiodine therapy was associated with higher long-term relative survival in specific subgroups: Among FTC patients, relative survival was higher with radioiodine after 5- and, 10-years by 0.3% (p = 0.029), and 2.3% (p = 0.055), not after three years (0.2%, p = 0.150). For PTC without lymph node involvement, no survival difference was detected. With lymph node involvement, relative survival was higher in the radioiodine-group after 10 years (pT1b group with 2.6% difference, p = 0.023). Depending on histology, subgroups of patients with low-risk DTC selected from the SEER-database according to ESTIMABL2-criteria, have a higher survival after radioiodine therapy, notably in FTC patients after more than 5 years. Furthermore, PTC patients with lymph node involvement revealed higher relative survival after 10 years.