Adrenal Adenoma
Submitted by Shuchi Rodgers, MD

• Incidence in the population is 2-8%
• Diagnosis is often made as an incidental finding on CT examination.
• In patient with no known primary, an adrenal mass is almost always a benign adenoma
• In a patient with a known neoplasm, especially lung cancer, an adrenal mass is problematic and diagnosing a metastasis versus an adenoma is critical for prognosis

 Imaging findings

• CT
  • Size greater than 4 cm tend to be metastases or adrenal carcinoma
    • Heterogeneous appearance and irregular shape are malignant characteristics 
  • Homogeneous and smooth are benign characteristics.
    • Intracellular lipid in adenoma results in low attenuation on CT
  • Little intracytoplasmic fat in metastases results in high attenuation on non-enhanced CT
  • Non-enhanced CT (NECT)
    • Threshold 10 HU
    • Sensitivity 79%, specificity 96%
  • Contrast-enhanced CT (CECT)
    • Because majority of CT examinations in oncology use IV contrast, the % washout is useful after 10 minutes. 
    • Adenomas have greater than 50% washout after 10 minutes
    • Washout can also be used on adrenal masses that measure > 10 HU on NECT
    • Alternative is to do MR or PET
• MR
  • Chemical Shift
    • Most sensitive method for differentiating adenomas from metastases
    • Sensitivity 81-100%.  Specificity 94-100%. 
    • The difference in resonance rate of protons in fat and water is exploited in chemical shift. 
      • Intracellular lipid and water in same voxel result in summation of signal on "in-phase" and canceling out of signal on "out of phase"
  • Spleen or muscle is used as an internal standard to visually quantify signal drop-off
    • Liver is not a reliable standard because of steatosis

CT shows left adrenal mass. In-phase T1 shows adrenal mass is hyperintense relative to the spleen.
T1 out-of-phase shows adrenal mass is hypointense to the spleen and compared to the
in-phase, there is a drop in signal intensity