Reinducing Radioiodine-sensitivity in Radioiodine-refractory DTC Using Lenvatinib (RESET)

Inclusion Criteria: * Age ≥ 18 years at the time of informed consent * Histologically or cytologically confirmed DTC (including papillary, follicular or Hürthle Cell carcinoma) * Progressive (biochemical or anatomic) disease for which lenvatinib is started as standard treatment at the discretion of the treating physician * Measurable disease at baseline imaging (F-18 FDG PET) according to the definition of the Positron Emission Tomography Response Criteria in Solid Tumors (PERCIST) 1.0 with at least one lesion ≥1.0 cm in the longest diameter for a non-lymph node or ≥1.5 cm in the short axis for a lymph node. * RAI-R disease on structural imaging, defined as any one of the following: * Metastatic lesions that are not RAI-avid on a diagnostic or intra-therapeutic RAI scanperformed prior to enrolment in the current study * RAI-avid metastatic lesions which remained stable in size or progressed according to RECIST 1.1 criteria despite RAI treatment. Absence of response is observed during 6-9 months after high dose I-131 therapy. * No recent treatment for thyroid cancer: * No prior I-131 therapy is allowed \<6 months prior to initiation of therapy on this protocol (a diagnostic study using \<400 MBq of I-131 is not considered 131I therapy) * No external beam radiation therapy is allowed \<4 weeks prior to initiation of therapy on this protocol. (Previous treatment with radiation for any indication is allowed if the investigator judges that the previous radiation does no…