Objective: There is limited evidence on the diagnostic accuracy of imaging tests in patients at high risk for adrenal malignancy. Our objective was to determine the performance of computed tomography (CT) and 18FDG-PET/CT imaging in diagnosing a malignant adrenal mass in a high risk population of patients referred for CT-guided adrenal biopsy.
Methods: We retrospectively reviewed the medical records of 378 patients who had percutaneous adrenal core biopsies performed at Mayo Clinic Rochester between 1994 and 2014. Reference standard was based on histology in all patients. For FDG-PET, the standardized uptake value (SUV) was measured in the adrenal mass (SUV max) and liver (SUV liver). The SUV max to SUV liver ratios (ALR) were calculated.
Results: The median age of our cohort was 68 years (range 1891) and 237/379 (62.5%) were men. Patients were referred for adrenal biopsy mainly due to suspected or confirmed extra-adrenal malignancy (303/378, 80%). Malignant adrenal lesions were found in 237 patients (62.5%). Benign adrenal cortical adenomas were diagnosed in 136 patients (36%). Unenhanced CT was performed in 352 patients (225 malignant, 127 benign lesions). All malignant adrenal lesions demonstrated a radiodensity of >10 Hounsfield units (HU) (sensitivity 100%, specificity 33%, positive predictive value (PPV) 73%, and negative predictive value (NPV) 100%). 18FDG-PET/CT was performed in 91 patients. SUV max was higher in malignant lesions when compared to benign lesions (median 10.1 (range 1.929.4) vs 3.7 (range, 1.424.5), P<.001). Similarly, ALR in malignant lesions was higher than in benign lesions (median 3 (range, 0.513.4) vs 1.15 (range, 0.66.6), P<0.001). An ALR cutoff of 1.8 performed best in diagnosing adrenal malignancy (sensitivity 83%, specificity 84%, PPV 85%, and NPV 82%).
Conclusion: Noncontract CT HU of 10 or below is a good initial imaging approach as it excludes a malignant lesion. For lesions >10HU, 18FDG PET/CT could be considered in a population at high risk for adrenal malignancy. However, both sensitivity and specificity of 18FDG PET/CT are not perfect; therefore clinical judgment is warranted.