*4.1. Study Population*

For this retrospective study, we selected patients with advanced, radioiodine-refractory PTC, FTC and PDTC undergoing Lenvatinib treatment at the department of endocrinology (University Hospital, LMU Munich, Munich, Germany) between May 2015 and August 2019. Patients were eligible for inclusion in this study when the time span between baseline 18F-FDG-PET/CT and initiation of treatment was less than 4 months and follow-up imaging was performed after 3 ± 3 months and/or 6 ± 3 months at the Department of Nuclear Medicine (University Hospital, LMU Munich, Munich, Germany).

All patients had extended disease, were radioiodine-refractory and had at least one FDG-positive lesion, except for one patient at baseline PET scan. Lenvatinib treatment was initiated based on a multidisciplinary tumor board decision.
