Adrenal masses are highly prevalent and detected with high frequency by conventional imaging. Conventional imaging often fails to rule out a malignant lesion. Accordingly, most hormonally inactive adrenal masses removed by surgery are benign adenomas for which surgical removal is unnecessary and poses an avoidable risk to the patients. We hypothesize that the combination of FDG-PET and 123I-Iodometomidate imaging has the potential to noninvasively identify benign adrenocortical adenomas with high accuracy, thereby avoiding unnecessary surgery. Uptake of 123I-Iodometomidate by the adrenal mass demonstrates the presence of CYP 11B enzymes which specifically bind metomidate with high avidity establishing the adrenocortical origin of the lesion, while low uptake of FDG-PET in an adrenocortical lesion establishes its benign nature and excludes the presence of adrenocortical cancer (ACC). The proposed trial will assess the sensitivity, specificity, positive and negative predictive value of the combined imaging for the diagnosis of adrenocortical adenoma. A secondary focus is on the performance of the combined test for differentiating ACC from non-ACC lesions. We expect that the results of our trial will help to greatly reduce the need for surgery in hormonally inactive adrenal masses.