"Adrenal masses that are not FDG avid are likely to be benign with a high negative predictive value. Especially in patients undergoing therapy, however, there is a small but statistically significant false negative rate. A considerable proportion of benign nodules have increased FDG activity."

Utility of PET/CT in Differentiating Benign from Malignant Adrenal Nodules in Patients with Cancer
Vikram R et al
AJR 2008;191:1545-1551