Rationale: In patients with medullary thyroid cancer (MTC), molecular imaging is used to assess the extent of disease in the primary diagnostic process and follow-up period to determine possible therapeutic options. The currently most used tracer in clinical practice, F-18 labelled fluorodeoxyglucose (18F-FDG), does not accurately detect MTC tumors with an indolent growth rate. A new, complimentary tracer is warranted to detect different subtypes.

Objective: The primary objective is to assess the feasibility of using the F-18 labelled prostate specific membrane antigen (18F-PSMA) PET/CT for (re)staging patients with medullary thyroid cancer. The secondary objective is to compare the ability to detect MTC with the 18F-PSMA PET/CT to that of the 18F-FDG PET/CT.

Study design: Prospective, single-centre, feasibility study.

Study population: Patients (18 years of age or older) with biochemically and cytological/histological confirmed MTC, for whom the indication of an 18F-FDG PET/CT for tumor staging has already been determined on clinical grounds.

Main study parameters/endpoints: The primary outcome of this study is the performance (lesion-based//patient-based sensitivity) of the 18F-PSMA PET to detect MTC lesions in patients with cytologically/histologically confirmed disease. Secondarily, the performance of the 18F-PSMA PET will be compared to the 18F-FDG PET/CT.

Inclusion Criteria:

• 18 years of age or older
• Histological or cytological proven MTC
• Biochemical evidence of disease activity (elevated/increasing calcitonin and/or CEA)
• Clinical indication for an 18F-FDG PET/CT
• Able to follow instructions to participate in the study
• Able to give informed consent

Exclusion Criteria:

• Patients with prostate cancer or renal cell carcinoma
• Pregnant patients
• Recent neck surgery (<3 months ago)