Adrenal Adenoma versus Metastasis

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Adrenal Adenoma versus Metastasis

Adrenal adenomas are the most common adrenal lesions. As an incidental finding, a small homogeneous adrenal lesion is highly likely to be an adenoma. In patients with cancer, it is often important to determine whether an adrenal lesion is an adenoma or metastasis. The methods below should be only applied to relatively homogeneous adrenal lesions. Adrenal adenomas generally cannot be diagnosed by imaging in masses with substantial areas of necrosis or hemorrhage.¹

Diagnosis of Lipid-Rich Adrenal Adenomas

Adrenal adenomas have intracytoplasmic lipid that is not present in metastases. Detection of lipid content differentiates adenomas from metastases. This can be evaluated by noncontrast computed tomography (CT) or chemical shift magnetic resonance imaging (MRI).

Noncontrast Computed Tomography

Adenomas have lower attenuation values than metastases. Using a threshold attenuation value of 10 Hounsfield units (HU), the sensitivity is 71% and the specificity 98%.²

Chemical Shift Magnetic Resonance Imaging

Adenomas demonstrate signal loss on opposed phased chemical shift imaging. Both visual and quantitative analysis can be used to evaluate whether there is signal loss. Two quantitative parameters that have been used to diagnose lipid content are

Lesion signal loss >20% on opposed phase imaging
Adrenal-to-spleen ratio ≤0.70:³ The adrenal-to-spleen ratio is the adrenal mass-to-spleen signal intensity ratio on opposed phase images divided by the adrenal mass-to-spleen signal intensity ratio in phase images.

Diagnosis of Lipid-Poor Adrenal Adenomas

Ten to 40% of adrenal adenomas will be lipid poor⁴ and cannot be diagnosed as adenomas on noncontrast CT. Although adenomas can have noncontrast CT attenuation values >10, a non-calcified nonhemorrhagic adrenal lesion measuring >43 HU is suspicious for malignancy.⁵ In lesions with an attenuation value >10 but <43 on noncontrast CT, chemical shift MRI and contrast-enhanced CT may be helpful for diagnosis.