FDG-Uptake in the Adrenal Glands

The adrenal glands are a common challenge for the PET/CT radiologist because:

• They are a very common site of metastatic disease; and 
• Benign adrenal lesions are common (≈ 5% incidence in general population) and can be hypermetabolic.

The normal adrenal glands typically demonstrate little or no FDG uptake.

It is not uncommon for one adrenal to be mildly FDG-avid, while the other is non-avid.

Assessment of adrenal lesions is as much art as science. While the PET/CT radiologist may not always be able to provide a definitive diagnosis, we are expected to strongly suggest a diagnosis in the majority of cases.

Based on our experience in assessing thousands of adrenal lesions, we utilize the following diagnostic algorithm (which must always be applied in the context of the patient’s primary malignancy and specific presentation – “oncologic plausibility”) for the following presentations:

• Lipid-Rich or Fat-Containing Nodules/Nodularity (Unilateral or Bilateral)
• FDG Uptake in NORMAL Appearing Adrenal Gland – Unilateral or Bilateral
• Unilateral or Bilateral Adrenal Nodule (or “Nodularity”, “Fullness”, “Enlargement”), with INDETERMINATE CT Appearance

Lipid-Rich or Fat-Containing Nodules/Nodularity (Unilateral or Bilateral):

• “Lipid-Rich” (H.U. <10):  If an adrenal nodule demonstrates a Hounsfield measurement of less than 10 units, we report it as a benign adenoma, irrespective of its metabolic activity.  
  • Caveat: If only a portion of an FDG-avid adrenal nodule demonstrates a Hounsfield measurement of less than 10 units, we occasionally recommend MRI characterization (as this could represent a necrotic metastasis). 
• “Fat-Containing” Nodule: If an adrenal nodule contains any measurable focus of fat-equivalent density, we report it as a benign myelolipoma, irrespective of its metabolic activity.
• While most adrenal adenomas and myelolipomas demonstrate little or no FDG-uptake, they can occasionally be hypermetabolic. Even when intensely-avid, however, they are considered benign.

FDG Uptake in NORMAL CT Appearing Adrenal Glands (Unilateral or Bilateral):

• If Adrenal Uptake ≤ Liver Uptake:
  • In these cases, we simply report that such metabolic activity “typically reflects normal physiologic uptake.”

• If Adrenal Uptake > Liver Uptake:
  • While such intense uptake is often physiologic, we are more cautious in our assessment: “While this often represents normal physiologic uptake, follow-up may be warranted to exclude an early adrenal metastasis.”

FDG Uptake in Adrenal Nodule (or “Nodularity” or “Fullness”), with INDETERMINATE CT Appearance (Unilateral or Bilateral):

• If Adrenal Uptake ≤ Liver Uptake:  “Its lack of significant metabolic activity suggests this represents a benign finding, such as…. ”
  • (If appears as a nodule on CT: “…such as a benign adenoma.”)
  • (If appears as fullness on CT: “such as adenomatous change or benign hyperplasia”.)
• If Adrenal Uptake > Liver Uptake:
  • If Clinical Suspicion of Metastatic Disease: “The appearance is highly suspicious for metastatic disease.  MRI characterization may be of diagnostic value, if clinically warranted.”
  • If Low Clinical Suspicion of Metastatic Disease (Incidental Finding): “While this can reflect hyperplasia or adenomatous change, malignancy must be excluded in this case. MRI characterization may be of diagnostic value, if clinically warranted.”