Adrenal lesions show characteristic density patterns on unenhanced scans:

• Adrenal adenomas are typically small, homogeneous, and well-defined with HU values below 10, owing to their high intracellular lipid content.

• Adrenocortical carcinomas often present as large (>5 cm), irregular, heterogeneous masses with HU values above 20, and frequently show central necrosis or hemorrhage due to rapid growth and limited vascular support.

• Pheochromocytomas generally have HU >10 and appear heterogeneous, reflecting dense cellularity and low lipid content, which often places them in the indeterminate range on non-contrast imaging.

Further lesion characterization is achieved using multiphase CT, including non-contrast, portal venous (≈ 60–70 s post-contrast), and delayed (≈ 10–15 min post-contrast) phases. From these, washout is calculated using HU values across time points:

In clinical practice, washout CT serves as a second-line test when non-contrast findings are inconclusive. A relative washout >40% or absolute washout >60% strongly suggests a benign lipid-rich adenoma. In contrast, adrenocortical carcinomas and pheochromocytomas typically show slower or variable washout (<40% relative, <60% absolute), although up to 30–50% of pheochromocytomas can demonstrate rapid washout similar to adenomas.

The physiologic basis of these imaging differences lies in tumor composition and vascular architecture.

• Adenomas contain abundant intracellular lipid and possess a simple, well-organized capillary network, leading to rapid contrast uptake and clearance (high washout).

• Pheochromocytomas consist of densely cellular, highly vascular neuroendocrine tissue. Although they enhance strongly, their complex microvasculature and extracellular matrix slow contrast clearance.

• Adrenocortical carcinomas, being heterogeneous and necrotic with disorganized vasculature, exhibit delayed and incomplete washout.

Take Home Messages

• CT imaging features of adrenal tumors reflect their underlying biology; tissue composition and vascular structure determine attenuation and contrast behavior.

• Lipid-rich adenomas appear homogeneous with low attenuation (<10 HU) due to abundant intracellular fat.

• Adrenocortical carcinomas are large, heterogeneous, and irregular, showing high HU values, necrosis, and slow contrast washout because of poor vascular organization.

• Pheochromocytomas are highly vascular and cellular, leading to high attenuation and variable washout, often mimicking malignant lesions.

• Washout analysis helps differentiate benign from malignant lesions; rapid washout suggests adenoma, while slow or variable washout indicates other tumor types.

• In summary: The CT appearance of an adrenal mass is not arbitrary, it mirrors its pathophysiologic makeup, offering key diagnostic clues.

References

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