Adrenal Incidentaloma

Principles of CT Diagnostics

Computed tomography is the primary method for evaluating incidentally found adrenal masses.

The main criterion of benignity on unenhanced (non-contrast) CT is low tissue density, which is due to a high lipid content inside the cells (typical for adenomas).

• If the density of the mass is < 10 HU, this indicates a lipid-rich adenoma with high specificity (98%).
• If the density is > 10 HU, the mass is considered indeterminate (lipid-poor). This group includes up to 30% of benign adenomas, as well as malignant tumors and pheochromocytomas.

For indeterminate masses, contrast-enhanced CT with evaluation of contrast washout is required.

• Adenomas (both lipid-rich and lipid-poor) rapidly accumulate and rapidly wash out the contrast agent.
• Malignant tumors and pheochromocytomas rapidly accumulate but slowly wash out the contrast.

To evaluate washout, scanning is performed 60 seconds (venous phase) and 15 minutes (delayed phase) after contrast administration, and then the washout percentage is calculated.

Calculation of Contrast Washout

There are two main indicators used to confirm the diagnosis of an adenoma based on density dynamics:

1. Absolute Contrast Enhancement Washout (ACEW). Used if unenhanced CT data is available. A value of $\ge$ 60% is considered characteristic of adenomas.
2. Relative Contrast Enhancement Washout (RCEW). Used when the unenhanced phase was not performed. A value of $\ge$ 40% (at 15 minutes) strongly indicates an adenoma.

Important: measurements are taken using a region of interest (ROI) covering at least 2/3 of the volume of the homogeneous mass.

MRI Capabilities

Magnetic resonance imaging is the method of choice for children, young adults (under 40), and pregnant women.

The key MRI technique for differential diagnosis is chemical shift imaging (CSI). It is based on the difference in resonance frequencies of water and fat protons in a magnetic field. Scanning is performed in-phase (in-phase) and out-of-phase (out-of-phase).

• Adenomas containing many intracellular lipids noticeably lose signal intensity on out-of-phase images.
• Malignant masses and pheochromocytomas that do not contain lipids do not change their signal intensity.

To accurately assess signal loss, special indices are calculated, such as the signal intensity index (SII), or the signal from the adrenal gland is compared with that from the spleen.

Management tactics

The general approach to managing a patient with an incidentaloma is based on its size and imaging characteristics:

• Less than 4 cm with typical benign features (density < 10 HU on unenhanced CT): further imaging is not required if hormonal activity is excluded.
• Greater than 4 cm or indeterminate imaging features: additional evaluation (contrast-enhanced CT, MRI, PET) or surgical removal is required.
• Small masses without typical features: dynamic CT follow-up at 6-12 months is recommended to assess growth rate.

Pheochromocytoma can mimic both an adenoma (having low unenhanced density or a rapid washout profile) and cancer. At the slightest doubt, it must be ruled out using laboratory methods.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Formulas for calculating absolute (ACEW) and relative (RCEW) contrast washout.
• Histogram analysis method of unenhanced CT in indeterminate masses.
• Formulas and threshold values for MRI indices (SII, ASR).
• Features of FDG PET/CT application in incidentalomas.
• Pathognomonic CT signs of myelolipomas and adrenal cysts.
• The role of dual-energy CT (DECT) in differential diagnosis.

Other topics in the section "Diagnostic Radiology of the Abdominal Organs"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.