Imaging of adrenal incidentalomas with PET using (11)C-metomidate and (18)F-FDG

Our aim was to evaluate the use of PET with (11)C-metomidate and (18)F-FDG for the diagnosis of adrenal incidentalomas.

Twenty-one patients underwent hormonal screening before dynamic imaging of the upper abdomen with (11)C-metomidate, and for 19 of these 21 patients, static (18)F-FDG imaging followed. Uptake of (11)C-metomidate and (18)F-FDG in incidentalomas was quantified and correlated with the hormonal work-up and the mass size on CT (median, 2.5 cm; range, 2-10 cm).

The final diagnoses were hormonally active adenoma (n = 7), nonsecretory adenoma (n = 5), adrenocortical carcinoma (n = 1), pheochromocytoma (n = 2), benign noncortical tumor (n = 2), normal adrenal (n = 1), and malignant noncortical tumor (n = 3). Diagnosis was established at surgery (n = 9), percutaneous biopsy (n = 4), or follow-up (n = 8). The highest uptake of (11)C-metomidate, expressed as standardized uptake value (SUV), was found in adrenocortical carcinoma (SUV = 28.0), followed by active adenomas (median SUV = 12.7), nonsecretory adenomas (median SUV = 12.2), and noncortical tumors (median SUV = 5.7). Patients with adenomas had significantly higher tumor-to-normal-adrenal (11)C-metomidate SUV ratios than did patients with noncortical tumors. (18)F-FDG detected 2 of 3 noncortical malignancies but failed to detect adrenal metastases from renal cell carcinoma. All inactive and most active adenomas were difficult to detect with (18)F-FDG against background activity, whereas both pheochromocytomas and adrenocortical carcinoma showed slightly increased uptake of (18)F-FDG. There was no correlation between uptake of (11)C-metomidate or (18)F-FDG and mass size.

(11)C-Metomidate is a promising PET tracer to identify incidentalomas of adrenocortical origin. (18)F-FDG should be reserved for patients with a moderate to high likelihood of neoplastic disease.