Rationale and Objectives
We sought to assess the ability of 18 F-fluoro-2-deoxy- d -glucose (FDG) PET/CT to distinguish adrenal adenomas from nonadenomas in patients with suspected malignancy.
Methods
Fifty-nine adrenal masses were evaluated with coregistered 18 F-FDG PET (PET/CT). Two reviewers independently graded uptake of 18 F-FDG by visual inspection of the adrenal mass in comparison to liver and background. CT attenuation value of the adrenal mass and the standardized uptake value (SUV) of the mass, liver, background, and primary neoplasm (when visible) were measured. Mean SUVs of the adrenal mass, liver, and background and ratios of the SUVs were calculated.
Results
Of 47 adenomas, 43 measured <10 HU on unenhanced CT. Of 12, 12 nonadenomas measured >10 HU on unenhanced CT. Using qualitative assessment of FDG activity in the adrenal mass compared with liver, adenomas were less than, equal to, or more active than the liver in 51%, 38%, and 10%, respectively. Nonadenomas were less than, equal to, or more active than liver in 0%, 25%, and 75%, respectively. The mean SUV of adenomas (4.2) was significantly lower ( P = .002) than that of their primary malignancies (9.2) but not that of liver (4.3). The mean SUV of adenomas was not significantly different than that of nonadenomas (5.2), but the mean adrenal/liver ratio (1.0) for the adenomas was significantly lower ( P = .006) than that of the nonadenomas (2.1).
Conclusion
Adrenal adenomas were better differentiated from nonadenomas using unenhanced CT measurements in combination with ratios of the SUVs. Adrenal mass activity, which was visibly less than liver, was more specific for adenoma, whereas adrenal mass activity visibly greater than liver was more specific for malignancy.
Increasing experience with noninvasive imaging examinations such as CT and MR has led to the successful ability to characterize adrenal masses. Dedicated imaging protocols have been developed in order to distinguish adrenal adenomas from nonadenomas, particularly adrenal metastases. Prior investigations have shown that 18 F-fluoro-2-deoxy- d -glucose (FDG) PET is highly sensitive in detecting adrenal metastases revealing 19–34% additional metastatic foci compared with traditional imaging methods ( ). Recent literature has also suggested that 18 F-FDG PET may be effective in identifying benign adrenal lesions ( ); however, several investigators reported that benign adrenal adenomas demonstrated variable activity, which led to falsely positive diagnoses for malignancy ( ). In addition, different diagnostic criteria were used in these prior studies; activity in adrenal masses was compared with either background activity or liver activity.
Our objective was to determine the ability of 18 F-FDG PET/CT to differentiate adenomas from nonadenomas by comparing 18 F-FDG uptake in adrenal masses with both background and liver activity using subjective qualitative assessment of the adrenal mass, semiquantitative analysis with standardized uptake values (SUVs), and normalized or relative SUVs (RUVs). Our goal was not to compare CT and 18 F-FDG PET characterization of an adrenal mass as CT characterization of adrenal masses has been previously investigated ( ). We used previously published and widely accepted CT criteria to distinguish benign from malignant adrenal masses in patients in whom no histology was obtained. Furthermore, we sought to determine the incremental value of 18 F-FDG PET combined with the ability of unenhanced CT in the evaluation of an adrenal mass.
Materials and methods
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Results
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Qualitative Assessment
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Quantitative Assessment
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Table 1
Comparison of the mean attenuation values for adenoma and nonadenoma
Adenoma (n = 47) Nonadenoma (n = 12)P value Adrenal mass 1.3 ± 9.0 33.4 ± 6.6 <.0001 Liver 54.1 ± 13.5 57.2 ± 15.5 .5 Background −103.7 ± 17.6 −98.6 ± 24.3 .5
Table 2
Comparison of mean SUVs for adenoma and nonadenomas
Adenoma (n = 47) Nonadenoma (n = 12)P value Adrenal mass 4.0 ± 2.9 4.8 ± 3.1 .4 Background 2.3 ± 1.6 1.7 ± 1.1 .1 Liver 4.4 ± 2.4 3.0 ± 1.7 .1 Primary (adenoma, n = 34; nonadenoma, n = 9) 9.2 ± 8.7 5.7 ± 3.5 .1
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Discussion
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Conclusion
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