Fig. 2

a-c Patient with ¹⁸F FDG positive sarcoidosis and TC: Patient #10 presented with hilar lymphadenopathy in the ¹⁸F FDG PET/CT (SUVmax 8.3; a) and elevated Tg of 9.0 ng/ml one year after RIT. Histopathologically and clinically acute sarcoidosis was confirmed. The elevated Tg remained morphologically and functionally unclear. After 4 months follow-up PET/CT showed successful treatment of sarcoidosis with reduced 1¹⁸F FDG uptake (SUVmax 4.2) and decreased size of bi-hilar lymph nodes (not shown). During follow-up, Tg steadily increased and an additional ¹⁸F FDG PET/CT during follow-up revealed positive pulmonary lesions (SUVmax 3.4; b) which were iodine-refractory. The patient showed progressive disease during follow-up with local recurrence (SUVmax 19.9), progressive pulmonary metastases (SUVmax 14.5) and hilar lymph node metastases (SUVmax 55.3; c) in the ¹⁸F FDG PET/CT. Tg correspondently increased to 2434 ng/ml. After resection of central lymph node metastases, external radiation therapy of mediastinal lymph nodes and systemic therapy with Lenvatinib (Lenvima® 14 mg/d, Eisai GmbH, 60,528 Frankfurt, Germany) the patient showed partial remission in the PET/CT scan (not shown) and a significant decrease of Tg (12.3 ng/ml)