Histopathology of a surgically removed adrenal mass by an expert surgeon is considered to be the gold standard for the differential diagnosis of an adrenal neoplasia. Thus we will only recruit patients scheduled for surgery for an adrenal mass of uncertain origin. Patients will be eligible for the trial when an adrenal mass has been discovered and standard imaging (CT and/or MRI) has not led to a clear characterization of the malignant potential of the lesion (Hounsfield units in unenhanced CT ≥10). In addition, patients with adrenal neoplasia will be only recruited when a hormonally active lesion has been excluded and surgery is planned because of a perceived risk of malignancy. Patients will be evaluated by both [123I]Iodometomidate SPECT/CT and [18F]fluorodeoxyglucose PET/CT. Computed tomography will be performed as low dose CT of the adrenal region. Imaging can be started with either [123I]Iodometomidate SPECT/CT or [18F]fluoro- deoxyglucose PET/CT and can be performed at two consecutive days. The time interval between [123I]Iodometomidate SPECT/CT and [18F]fluorodeoxyglucose PET/CT should not exceed 6 weeks. Patients will have a follow up visit 2-4 weeks after [123I]Iodometomidate SPECT/CT and [18F]fluorodeoxyglucose PET/CT (or immediately before surgery, if earlier than 2 weeks post imaging). Thirty days after surgery patients will be contacted by phone for assessment of adverse events related to surgery.