Redifferentiation of a RET-Fusion, Radioiodine-Refractory, Differentiated Thyroid Cancer with Selpercatinib: Flip-Flop Between [¹⁸F]FDG PET/CT and ¹³¹I Posttreatment Scanning

During first radioiodine treatment, [¹⁸F]FDG PET/CT scan showed hypermetabolism in left cervical region due to tumor residue (green arrows) (A), and postradioiodine scan revealed right laterocervical (blue arrows) fixations with no uptake at tumor site (B). During second radioiodine treatment with selpercatinib, [¹⁸F]FDG PET/CT scan (C) and postradioiodine (D) scan showed [¹⁸F]FDG-negative, radioiodine-positive neck residue and metastatic miliary lung disease (green arrows).