Systematic Radioiodine Administration Versus Decision of Radioiodine Treatment Guided by a Post-operative Work-up

Inclusion Criteria:

* Subgroup of patients with differentiated thyroid cancer and intermediate-risk defined as follows according to TNM 2017:

  * Papillary thyroid cancer (PTC) without aggressive subtype, follicular thyroid cancer (FTC) (with \< 4 foci of vascular invasion) or Hürthle cell carcinoma (HCC)
  * T1b or T2 with minimal extra-thyroid extension into the perithyroidal soft tissues and/or pN1 with largest nodal dimension between 2 and 10 mm, without extra-capsular invasion and with a number of metastatic nodes ≤ 10
  * T1aN1 with largest nodal dimension between 2 and 10 mm, without extra-capsular invasion and with a number of metastatic nodes ≤ 10
* Patient treated by total thyroidectomy with macroscopically complete tumor resection (R0 or R1) ± neck dissection
* Total thyroidectomy performed within 6 to 14 10 weeks before randomization
* Patient with or without anti-thyroglobulin antibodies (TgAb)
* No known distant metastases
* Normal post-operative neck ultrasound (US) or if doubtful US, negative cytology and normal Tg value (\<10 ng/ml) in FNA washout fluid
* Post-operative LT4 treatment initiated at least 6 weeks before randomization
* Performance Status 0 or 1
* Patients aged 18 years or older
* Signed informed consent form
* Patient who agrees to be followed annually during 5 years
* Patient affiliated to the French social security system

Exclusion Criteria:

* • Patients with:

  * medullary or anaplastic thyroid cancer
  * or poorly differentiated carcinoma
  * or well differentiated FTC with at least more than 4 foci of vascular invasion
  * or PTC with aggressive variants (tall cell or columnar cell carcinoma, diffuse sclerosing papillary, hobnail variant)
  * NIFTP (Noninvasive follicular thyroid neoplasm with papillary-like nuclear features)

    • Low-risk or high-risk DTC patients according to ATA 2015, and intermediate-risk patients with extra-thyroid extension into the perithyroidal muscles (pT3b according to pTNM 2017), and/or pN1 with nodal largest dimension \>10 mm or with extra-capsular invasion or more than 10 metastatic nodes. This excludes the following patients:
  * All pT1a, pT3 or pT4
  * pT1aN0/x with or without minimal extra-thyroid extension
  * pT1bN0/x, pT2N0/Nx without minimal extra-thyroid extension
  * pT1aN1 or pT1bN1 or pT2N1 without extra-thyroid extension and with nodal largest dimension \<2mm
  * pT1aN1 or pT1bN1 or pT2N1 without extra-thyroid extension and with nodal largest dimension \>10mm
  * pT2N0/Nx without extra-thyroid extension
  * pT2N1 without extra-thyroid extension and with nodal largest dimension \<2mm
  * pT2N1 without extra-thyroid extension and with nodal largest dimension \>10mm
  * Surgery considered as macroscopically incomplete (R2)

    * Patients who have undergone lobectomy only
    * Post-operative neck US with metastatic lymph-nodes confirmed by cytology or by increased Tg (\>10 ng/ml) in FNA washout fluid
    * Drugs affecting thyroid function including iodinated contrast agents in the 6 weeks prior to randomization. Amiodarone should have been stopped at least 1 year before randomization.
    * Previous RAI treatment for thyroid cancer
    * Pregnant or lactating women
    * Any associated geographical, social or psychopathological condition that could compromise the patient's ability to participate in the study
    * Patient deprived of liberty or placed under the authority of a tutor
    * History of malignancy in the past 3 years, except skin cancer excluding melanoma, carcinoma in situ of the cervix. Any other solid tumor or lymphoma (without bone marrow involvement) must have been treated and not have shown signs of recurrence for at least 3 years