If the CT or MRI features of an incidentally detected adrenal gland nodule are nondiagnostic, what are the next steps to determine its etiology?
If specific diagnostic features of a lipid-rich adrenal adenoma or other benign cause of an adrenal nodule (e.g., myelolipoma, hematoma, cyst) are not present, and no prior cross-sectional imaging studies are available to demonstrate long-term stability in size over time, then dedicated thin-section adrenal gland CT imaging with acquisition of unenhanced, venous phase contrast-enhanced, and 15-minute delayed phase contrast-enhanced images is performed. This is because adrenal adenomas are vascular lesions that enhance and wash out contrast on delayed phase images more than do malignant lesions. The washout of contrast from an adrenal nodule is calculated in one of two ways:
1. The absolute percentage washout (APW) = [venous phase attenuation (in HU) − 15-minute delayed phase attenuation (in HU)] / [venous phase attenuation (in HU) − unenhanced attenuation (in HU)] × 100%. An APW >60% is diagnostic of an adrenal adenoma
- 2. The relative percentage washout (RPW) = [venous phase attenuation (in HU) − 15-minute delayed phase attenuation (in HU)] / [venous phase attenuation (in HU)] × 100%, which is used when unenhanced CT images were not obtained. An RPW >40% is also diagnostic of an adrenal adenoma.
The below table provides a summary of CT and MR imaging features that are useful to distinguish benign from malignant adrenal gland nodules. When these imaging features are not sufficient to determine whether or not an adrenal gland nodule is benign or malignant in nature, short-term follow-up CT or MR imaging may be performed to document size stability over time, which would favor a benign etiology. Fluorodeoxyglucose (FDG) positron emission tomography (PET) may also be performed, as benign adrenal nodules tend to have FDG uptake less than or equal to that of the liver. Otherwise, percutaneous biopsy or surgical removal is performed to obtain a definitive diagnosis.
CT and MR Imaging Features to Distinguish Benign from Malignant Adrenal Gland Nodules
| SUGGESTIVE BENIGN FEATURES | SUGGESTIVE MALIGNANT FEATURES |
|---|---|
| Small size ( ≤ 3-4 cm) | Large size (>4-5 cm) |
| Stability in size (over >6-12 months) | Growth in size (over >6-12 months) or new |
| Homogeneous attenuation or signal intensity | Heterogeneous attenuation or signal intensity |
| Smooth margins and no invasion of adjacent structures | Irregular margins or invasion of adjacent structures |
| Presence of macroscopic fat (myelolipoma) | |
| Presence of microscopic lipid (lipid-rich adenoma) | |
| APW >60%/RPW >40% (adenoma) | APW ≤ 60%/RPW ≤ 40% |
| Lack of enhancement (cyst, hematoma) |
APW = absolute percentage washout; RPW = relative percentage washout
