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.
- 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
- 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
- Precontrast attenuation: less than 10 HU (sensitivity 70%, specificity 98%)
- Postcontrast (venous phase) attenuation: at 60-70 second delay after contrast injection, absolute enhancement of a mass of > 110-120 HU likely pheochromocytoma
- 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.
Johnson PT, Horton KM, Fishman EK. Adrenal imaging with multidetector CT: evidence-based protocol optimization and interpretative practice. RadioGraphics 2009;29:1319-1331.