18F-DOPA-PET/CT role in medullary thyroid cancer patients with biochemical incomplete response

Objective Several patients with medullary thyroid carcinoma (MTC) are not cured after surgery. Disease persistence/recurrence is monitored by measuring calcitonin (CTN) and carcinoembryonic antigen (CEA) and by imaging. The role of 18F-fluoro-dihydroxyphenylalanine positron emission tomography/computed tomography (18F-DOPA-PET/CT) in identifying structural disease in patients with biochemical incomplete response (BiR) remains debated. This study aimed to evaluate the capability of 18F-DOPA-PET/CT to identify structural disease in MTC patients with BiR. Methods We performed 18F-DOPA-PET/CT in 91 MTC patients with BiR without evidence of structural disease or indeterminate lesions on neck ultrasound (US) and total-body computed tomography (CT). Results 18F-DOPA-PET/CT showed pathological uptake in 16/37 (43.2%) CT-negative cases and in 43/54 (79.6%) CT-indeterminate cases. Of these, 23/43 (53.5%) cases showed concordance between 18F-DOPA-PET/CT uptake and CT-scan indeterminate lesions, 4/43 (9.3%) cases showed discordance, and 16/43 (37.2%) cases showed partial concordance. Moreover, 18F-DOPA-PET/CT showed pathological uptake in cervical lymph nodes in 33/91 (36.3%) cases, of whom 9 (27.3%) had no lesions at neck US, and 24 (72.7%) had indeterminate findings. Patients with positive 18F-DOPA-PET/CT showed higher median CTN and CEA values. Management of patients with positive ¹⁸F-DOPA PET/CT uptake did not change, except for those with bone metastases. Conclusions 18F-DOPA-PET/CT demonstrates good diagnostic accuracy for identifying structural disease in MTC patients with BiR, particularly when indeterminate lesions are present on neck US and CT. However, in most BiR patients, the results of 18F-DOPA-PET/CT did not change their management.