Adrenal Mass Characterization

Fig 1: Precontrast axial CT image shows a 3-cm left adrenal mass with an attenuation of 1.3 HU, indicating a lipid-rich adenoma.
Fig. 2: Precontrast axial CT image of a different patient shows a right adrenal mass with an attenuation of 20.8 HU, which is indeterminate. Note that the region of interest (ROI) measurement should cover at least half the size of the mass.

Facts:

• Lipid-rich adenomas contain intracellular fat (microscopic level) that can be shown on CT as low attenuation
• Adenomas wash out contrast more rapidly than malignant masses
• Extension into IVC suggests malignancy, particularly adrenocortical carcinoma

Adrenal Mass: Adenoma Vs. Others

1. Size: can not be definitively used to distinguish benign from malignant adrenal masses. In general, patients without known malignancy and a mass greater than 5 cm, surgical resection is advised
2. Precontrast attenuation: less than 10 HU (sensitivity 70%, specificity 98%)
3. Postcontrast (venous phase) attenuation: at 60-70 second delay after contrast injection, absolute enhancement of a mass of > 110-120 HU likely pheochromocytoma
4. Delayed postcontrast attenuation: used to calculate absolute and relative percentage washout. Absolute Percentage Washout (APW) >60% or Relative Percentage Washout (RPW) >40% suggests an adenoma.

Our cases: Both cases are adrenal adenomas, the first is a lipid-rich adenoma and the second is a lipid-poor adenoma (confirmed with dynamic scans, not shown).

Reference:

Johnson PT, Horton KM, Fishman EK. Adrenal imaging with multidetector CT: evidence-based protocol optimization and interpretative practice. RadioGraphics 2009;29:1319-1331.