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Imaging of Adrenal Incidentalomas with PET Using ¹¹C-Metomidate and ¹⁸F-FDG

¹ Turku PET Centre, University of Turku, Turku, Finland
² Department of Oncology and Radiotherapy, Turku University Central Hospital, Turku, Finland
³ Department of Internal Medicine, Tampere University Hospital, Tampere, Finland
⁴ Department of Internal Medicine, Helsinki University Hospital, Helsinki, Finland
⁵ Department of Internal Medicine, Turku University Central Hospital, Turku, Finland

View larger version (96K): [in a new window]   FIGURE 1. 11C-Metomidate (left column) and 18F-FDG (right column) PET images of patients with adrenal incidentalomas: a left-sided hormonally inactive adenoma (patient 11) (A and B), a right-sided cortisol-secreting adenoma (patient 2) (C and D), a right-sided adrenocortical carcinoma (patient 13) (E and F), and a right-sided hepatocellular carcinoma described as "pseudoadrenal mass" (patient 20) (G and H). The images are coronal sections through the midabdomen obtained between 25 and 45 min (11C-metomidate) or 45 and 65 min (18F-FDG) after injection. Tumor is indicated in each case by a white arrow, and normal adrenal, when visible in the same section, by a black arrow. All adrenocortical tumors (first 3 rows) have much higher uptake of 11C-metomidate than of 18F-FDG, whereas the opposite is true for the noncortical neoplasia shown in the lowest row. In this case (G), uptake of 11C-metomidate is not confined to tumor but occurs in a rim of adrenal tissue (black arrow) displaced by the mass. Initially, the high 18F-FDG uptake of the adenoma in 1 d was misjudged as metastasis, with an average SUV of 3.9 (maximum, 6). Liver has invariably high uptake of 11C-metomidate, and tracer excreted in the urinary tract presents the highest radioactivity in 18F-FDG images.

View larger version (13K): [in a new window]   FIGURE 2. Representative HPLC chromatogram of a venous plasma sample at 6 min after tracer injection showing 3 peaks designated as metabolites 1 and 2 and native 11C-metomidate (MTO). The retention times for peaks 1, 2, and 3 are 2.3, 3.2, and 6.6 min, respectively.

View larger version (17K): [in a new window]   FIGURE 3. Time–activity curve for venous plasma and selected tissues (A) and Patlak graphical analysis for adenoma, normal adrenal, and liver (B) in a patient undergoing dynamic 11C-metomidate PET. The high contribution of blood metabolites of 11C-metomidate in the uncorrected plasma curve make it inappropriate for graphical analysis.

View larger version (17K): [in a new window]   FIGURE 4. Average 11C-metomidate SUV in incidentaloma. Dotted lines show variability within 12 normal adrenals, excluding a single exceptionally low value of 4.6. The SUV in lipoma was relatively high, most likely because of the contribution of normally functioning adrenal tissue.

View larger version (12K): [in a new window]   FIGURE 5. Box plots of SUV ratios for patients with active and inactive adenomas and noncortical tumors: tumor to normal adrenal for 11C-metomidate (MTO) (A) and tumor to liver for 18F-FDG (B). The error bars and lines within boxes represent the highest and lowest quartiles and the medians, respectively, of the data distribution. The 11C-metomidate SUV ratio well distinguishes adenomas from other tumors, whereas 18F-FDG shows clearly higher values for noncortical tumors. Because noncortical tumors have a very mixed character, they do not take up significantly more 18F-FDG than do adenomas.