Residency · Residency · Diagnostic Radiology

Adrenal Mass Characterization

Overview

Clinical Significance

Adrenal incidentalomas are found in approximately 4-5% of abdominal CT scans. The two key questions for any adrenal mass are whether it is malignant and whether it is hormonally active. CT washout protocol and chemical shift MRI are the primary imaging tools for characterization, and most incidentalomas prove to be benign, non-functioning adenomas.

Epidemiology

Adenoma is the most common adrenal lesion, accounting for approximately 80% of incidentalomas. Metastasis is the most common malignant adrenal mass, with lung, breast, melanoma, renal, and colon being the most frequent primary tumors. Pheochromocytoma accounts for approximately 5% of incidentalomas and must be excluded before biopsy. Adrenocortical carcinoma (ACC) is rare but important, typically presenting as a large mass exceeding 4 cm.

CT Washout Protocol

Unenhanced CT Attenuation

A lipid-rich adenoma demonstrates unenhanced attenuation of 10 HU or less, with a sensitivity of approximately 71% and specificity of approximately 98%. A lipid-poor adenoma has unenhanced attenuation exceeding 10 HU and requires washout analysis for characterization. A myelolipoma contains macroscopic fat measuring less than -30 HU and is often heterogeneous, with its appearance being pathognomonic.

Contrast-Enhanced Washout

The washout protocol is performed with dedicated 60-second (portal venous) and 15-minute delayed images. The absolute percentage washout (APW) is calculated as [(enhanced - delayed) / (enhanced - unenhanced)] x 100, with an APW of 60% or greater indicating adenoma. The relative percentage washout (RPW) is used when unenhanced images are unavailable and is calculated as [(enhanced - delayed) / enhanced] x 100, with an RPW of 40% or greater indicating adenoma. Metastases and pheochromocytomas typically show slow washout, with APW less than 60% and RPW less than 40%.

Size Criteria

Lesions smaller than 1 cm are typically ignored or followed only if there is clinical concern. Lesions measuring 1-4 cm should be characterized with CT washout or MRI. Lesions exceeding 4 cm raise increased suspicion for ACC or pheochromocytoma, and surgical resection should be considered. Lesions exceeding 6 cm have a high likelihood of malignancy, and surgery is generally recommended.

Chemical Shift MRI

Principle

Chemical shift MRI exploits the difference in resonant frequency between water and fat protons using in-phase and opposed-phase gradient echo imaging. Intracellular lipid within adenomas causes signal dropout on opposed-phase images.

Interpretation

Signal loss on opposed-phase images compared to in-phase images indicates intracellular lipid and is diagnostic of adenoma. Quantitatively, the signal intensity index (SII) is calculated as [(in-phase SI - opposed-phase SI) / in-phase SI] x 100, with an SII greater than 16.5% suggesting adenoma. The spleen should be used as an internal reference because its signal remains unchanged between sequences.

Limitations

Clear cell renal cell carcinoma metastases may contain intracellular lipid and show signal dropout, producing a false positive for adenoma. Lipid-poor adenomas may not show signal dropout. Hemorrhagic lesions can be confounding.

Specific Adrenal Lesions

Pheochromocytoma

Pheochromocytoma arises from adrenal medulla chromaffin cells. Classic teaching describes it as "light bulb bright" on T2-weighted MRI, though the signal intensity is variable. It shows avid enhancement with slow washout on CT. The traditional 10% rule (now outdated) held that approximately 10% are bilateral, 10% are extra-adrenal (paraganglioma), and 10% are malignant. Pheochromocytoma must be excluded biochemically before biopsy to avoid the risk of hypertensive crisis. Associated syndromes include MEN 2A/2B, VHL, SDH mutations, and NF1.

Adrenocortical Carcinoma

ACC presents as a large heterogeneous mass, often exceeding 6 cm at diagnosis. Internal necrosis, hemorrhage, and calcification are common. Invasion of the renal vein or IVC may occur. It does not show significant washout on CT. ACC may be hormonally active, producing Cushing syndrome or virilization.

Metastasis

Adrenal metastases are bilateral in approximately 50% of cases. They show heterogeneous enhancement and may demonstrate central necrosis. They do not wash out (APW less than 60%, RPW less than 40%). History of a known primary malignancy is critical context. On PET/CT, metastases are FDG-avid, which helps distinguish them from adenomas.

Myelolipoma

Myelolipoma contains macroscopic fat with attenuation less than -30 HU on CT, producing a pathognomonic appearance that requires no further workup. On MRI it shows high signal on T1-weighted images with signal loss on fat-saturated sequences. It is usually asymptomatic, though large lesions may hemorrhage.

Adrenal Hemorrhage

Acute adrenal hemorrhage is hyperdense on CT at 50-70 HU. In the subacute phase it becomes T1 hyperintense on MRI due to methemoglobin. Causes include trauma, anticoagulation, stress (Waterhouse-Friderichsen syndrome in meningococcemia), and post-surgical changes. Follow-up imaging should show decreasing size; if the lesion does not resolve, an underlying lesion should be considered.

Adrenal Lesion Characterization Summary

Adrenal Cyst

Adrenal cysts are thin-walled, demonstrate homogeneous fluid attenuation, and show no enhancement. Types include endothelial (lymphangiomatous), epithelial, parasitic, and pseudocyst (post-hemorrhage).

<image>A CT washout protocol illustration showing three axial CT images of the same adrenal mass at different time points: (1) Unenhanced CT showing a left adrenal nodule measuring 15 HU. (2) 60-second post-contrast image showing enhancement to 100 HU. (3) 15-minute delayed image showing washout to 40 HU. A calculation box demonstrates APW = (100-40)/(100-15) x 100 = 70.6%, confirming adenoma. Below, a comparison case shows a metastasis with unenhanced 30 HU, enhanced 90 HU, delayed 80 HU, with APW = 16.7%.</image>

<image>A side-by-side MRI comparison showing chemical shift imaging of an adrenal adenoma. The left panel shows the in-phase gradient echo image with a right adrenal nodule of intermediate signal intensity. The right panel shows the opposed-phase image where the same nodule demonstrates marked signal dropout (dark), confirming intracellular lipid content consistent with adenoma. The spleen is circled as a reference showing no signal change between sequences. Arrows point to the adrenal nodule on both images.</image>

<image>A composite image panel showing four different adrenal lesions on CT: (1) A small homogeneous left adrenal adenoma measuring 5 HU on unenhanced CT. (2) A right adrenal myelolipoma with macroscopic fat measuring -45 HU. (3) A large heterogeneous right adrenal mass (8 cm) with areas of necrosis representing adrenocortical carcinoma with IVC invasion. (4) Bilateral heterogeneous adrenal masses in a patient with known lung cancer representing metastases. Each panel is labeled with the diagnosis and key measurement.</image>

Clinical Pearls

An unenhanced CT attenuation of 10 HU or less is diagnostic of a lipid-rich adenoma, and no further workup is needed. Pheochromocytoma must always be excluded biochemically (with plasma metanephrines) before percutaneous biopsy of an adrenal mass. The CT washout protocol requires dedicated timing with unenhanced, 60-second enhanced, and 15-minute delayed images; "borrowing" phases from a routine CT is insufficient. Size exceeding 4 cm and interval growth are the strongest predictors of malignancy in an adrenal incidentaloma. Chemical shift MRI is most useful for lipid-poor adenomas that are indeterminate on unenhanced CT (greater than 10 HU). Bilateral adrenal masses in a patient with known malignancy are metastases until proven otherwise, but bilateral adenomas are also common.

References

• Mayo-Smith WW, et al. "Management of Incidental Adrenal Masses: A White Paper of the ACR Incidental Findings Committee." Journal of the American College of Radiology, 2017
• Blake MA, et al. "Adrenal Imaging." Radiologic Clinics of North America, 2015
• Song JH, et al. "The Incidental Adrenal Mass on CT: Prevalence of Adrenal Disease in 1,049 Consecutive Adrenal Masses." AJR, 2008
• ACR Appropriateness Criteria: Adrenal Mass Evaluation, 2021

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