Estimabl2: Is There a Need for Radioiodine Ablation in Low Risk Differentiated Thyroid Cancer (DTC) Patients?: Results From the French Randomized Phase III Prospective Trial on 776 Patients (NCT 01837745)
Authors: Leboulleux Sophie , Bournaud Claire , Chougnet Cecile N , Zerdoud Slimane , Catargi Bogdan Nicolescu , Cao Christine Do , Kelly Antony , Barge Marie-Luce , Dygay Inna , Vera Pierre , Rusu Daniela , Schneegans Olivier , Benisvy Danielle , Klein Marc , Roux Julie , Eberle Marie-Claude , Bastie Delphine , Nascimento Camila , Giraudet Anne-Laure , Moullec Nathalie Le , Bardet Stephane , Drui Delphine , Roudaut Nathalie , Godbert Yann , Morel Olivier , Anne Drutel , Schwartz Claire , Velayoudoum Fritzline , Schlumberger Martin Jean , Leenhardt Laurence , Borget Isabelle 
Published: May 2021

Background: The benefits of post-operative radioactive iodine (RAI) administration have not been demonstrated in patients with low risk differentiated thyroid cancer (DTC). The objective of this randomized phase III trial is to assess in low risk DTC patients the non-inferiority of a follow-up strategy as compared to a systematic adjuvant post-operative RAI administration.

Methods: ESTIMABL2 is a French multicentric randomized phase III trial in patients with low-risk DTC treated with total thyroidectomy with or without prophylactic neck lymph node dissection (pT1am N0 or Nx with a sum of the diameters of tumor lesions ≥ 10mm, pT1b N0 or Nx). Two to five months after surgery, in the absence of suspicious lateral neck lymph node on ultrasonography (US), patients were randomized either to the follow-up group (FU, no RAI administration) or to the ablation group and received post-operative RAI (1.1 GBq following rhTSH stimulation). Yearly controls under levothyroxine treatment consisted in thyroglobulin (Tg) and Tg antibodies (TgAb) determinations and neck-US. The primary objective was to assess at 3 years after randomization the non-inferiority of the proportion of patients without tumor-related event in the FU group as compared to the ablation group. Non-inferiority is demonstrated if the rate of patients without event at 3 years does not differ by more than ΔL=-5%. A tumor-related event was defined by the occurrence of subsequent treatment (RAI administration or surgery) for abnormal RAI uptake on the post-therapeutic WBS or by elevated Tg or TgAb levels and/or abnormal neck US during controls. Tg levels on levothyroxine treatment were considered elevated if &gt; 2ng/mL in the FU group and &gt; 1ng/mL in the ablation group. TgAb were considered elevated if &gt; the upper limit range with an increase above 50% on 2 consecutive determinations performed 6 months apart.

Results: 776 low-risk DTC patients were included between 2013 and 2017 in 35 French centers within the TUTHYREF network; 83% females, mean age: 52 years, papillary TC: 96%, pT1bNx: 43.6%, pT1bN0: 37.5%, pT1amNx: 12.6%, pT1amN0: 6.3%. Among the 729 patients evaluable at 3 years after randomization, tumor-related events occurred in 18/367 patients (4.9% IC95%=[2.9; 7.6]) in the FU group and in 15/362 patients (4.1% IC95%=[2.3; 6.7]) in the ablation group. Thus, 95.1% of patients in the FU group had no event at 3 years and this percentage is not inferior from the 95.9% of patients observed in the ablation group (difference = -0.8% [95% CI:-3.3%; 1.8%]. The number of subsequent surgery and/or RAI administration was 6 (1.6% IC95%=[0.6; 3.5]) in the FU group and 9 (2.5% IC95%=[1.1; 4.7]) in the ablation group.

Conclusion: this phase III trial demonstrates the non-inferiority of a follow-up strategy compared to a systematic adjuvant post-operative administration of RAI (1.1GBq following rhTSH) in low risk DTC patients (PHRC 2012; NCT01837745).