**5. Conclusions**

In conclusion, our study suggests that in patients with advanced radioiodine refractory DTC undergoing Lenvatinib treatment, tumor response evaluation by 18F-FDG-PET/CT outperforms morphological response evaluation using CT and furthermore appears to be stronger correlated with outcome analysis. Monitoring tumor response with 18F-FDG-PET/CT in these patients has the potential to improve treatment individualization and avoid ineffective therapies by selecting patients who will most likely benefit from Lenvatinib treatment. Therefore, tumor response assessed by 18F-FDG-PET/CT is a highly promising modality in order to increase diagnostic accuracy in these patients and should be further investigated.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/2072-669 4/13/2/317/s1, Table S1: Patient Characteristics, Table S2: Single-Patient Course of Disease.

**Author Contributions:** Conceptualization, F.A., L.B. and A.T.; methodology, F.A., L.B., C.B. and A.T.; software, F.A. and C.B.; validation, F.A., L.B. and A.T.; formal analysis, T.G.; investigation, F.A. and V.F.K.; data curation, F.A., C.B. and V.F.K.; writing—original draft preparation, F.A., L.B. and A.T.; writing—review and editing, F.A., L.B., V.W., H.I., M.P.F., C.S. and A.T.; supervision, P.B., C.S. and A.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** Vera Wenter was financially supported by the Kuhbier-Langewiesche foundation and the Bavarian Equal Opportunities Sponsorship (BGF). The foundation had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. No other potential conflicts of interest relevant to this article exist.

**Institutional Review Board Statement:** The study was conducted according to the guidelines of the Declaration of Helsinki, and approved by the Ethics Committee of the Medical Faculty, University Hospital, LMU Munich, Munich, Germany (IRB #20-736, 21.09.2020).

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Data Availability Statement:** The data presented in this study are available upon reasonable request from the corresponding author.

**Acknowledgments:** This study is part of the doctoral thesis of Freba Ahmaddy. Vera Wenter was financially supported by the Bavarian Equal Opportunities Sponsorship (BGF) to promote equal opportunities for women in research and teaching. The sponsorship did not influence the study design, data collection and analysis, decision to publish, or preparation of the manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.
