The question of whether a lipid-poor adenoma represents cancer is a common concern arising from modern medical imaging. Most masses discovered in the adrenal glands are benign and are known as adrenal incidentalomas because they are found accidentally during scans for unrelated issues. The primary challenge for physicians is distinguishing the common, harmless adrenal adenoma from the rare, but aggressive, adrenocortical carcinoma. The specific imaging characteristic of being “lipid-poor” introduces diagnostic uncertainty that requires focused investigation.
Understanding Adrenal Adenomas
The adrenal glands are small, cap-like organs situated immediately above the kidneys, playing a significant role in the endocrine system. They consist of the outer cortex and the inner medulla, which produce hormones like cortisol, aldosterone, and adrenaline. An adenoma is a benign tumor that originates from the glandular tissue of the adrenal cortex.
These growths are highly prevalent, increasing in frequency with age. Up to 7% of people may have an adrenal mass by age 70, with the adenoma being the most frequent type of lesion found. The term “adrenal incidentaloma” describes a finding on a cross-sectional imaging study, such as a computed tomography (CT) scan, performed for another medical reason. The majority of these adenomas are non-functional, meaning they do not produce excess hormones or cause symptoms.
The Significance of “Lipid-Poor”
The key to characterizing an adrenal mass lies in its fat or lipid content, which is assessed using a non-contrast CT scan. Benign adrenal adenomas are characterized by an abundance of intracellular lipids, or fat droplets, within their cells. This high fat content makes the mass appear relatively dark on a CT image.
Tissue density is quantified using Hounsfield Units (HU). A benign, lipid-rich adenoma typically exhibits a low attenuation value, usually \(\le\)10 HU, which strongly suggests a benign finding. The term “lipid-poor” is applied when the mass measures higher than this threshold, often between 10 and 30 HU. This higher density is concerning because malignant tumors, such as adrenocortical carcinomas, also lack significant intracellular fat, causing them to appear dense and lipid-poor on the initial scan.
Diagnostic Tools to Determine Malignancy
When an adrenal mass is deemed lipid-poor, further diagnostic steps are necessary to resolve the ambiguity. Size is a primary consideration, as larger lesions inherently carry a higher risk of malignancy. Masses exceeding 4 centimeters are generally considered highly suspicious and are often recommended for surgical removal.
Contrast-Enhanced CT and Washout
For masses smaller than 4 centimeters with indeterminate HU values, a contrast-enhanced CT scan is typically performed to measure “washout.” This test involves injecting an iodine contrast dye and taking sequential images to measure how quickly the dye leaves the mass. Benign adenomas have a distinctive, rapid washout due to their unique vascular structure.
Rapid washout is confirmed if the mass shows an Absolute Washout Percentage (AWP) of \(\ge\)60% or a Relative Washout Percentage (RWP) of \(\ge\)40%. These rapid washout characteristics are shared by both lipid-rich and lipid-poor adenomas, allowing for confident differentiation from malignant tumors, which tend to retain the contrast dye for a longer duration. If the mass fails to meet both the low HU and rapid washout criteria, it remains indeterminate, and other imaging modalities like chemical-shift Magnetic Resonance Imaging (MRI) may be used for further evaluation.
Management and Follow-Up
Once the risk of malignancy has been assessed using imaging, a comprehensive hormonal workup is performed to check for functionality. Even a benign adenoma may produce an excess of hormones, which can have significant health effects, such as causing Cushing’s syndrome (excess cortisol) or Conn’s syndrome (excess aldosterone). This evaluation typically involves blood and urine tests, such as a low-dose dexamethasone suppression test for cortisol.
If the mass is confirmed to be non-functional and benign based on imaging criteria, surveillance is the most common course of action. This involves repeating the imaging study, typically a non-contrast CT, in 6 to 12 months to monitor for growth. A lesion that remains stable or grows very little (less than 5 millimeters in a year) is generally considered safe and requires less frequent follow-up.
Surgical removal, or adrenalectomy, is usually reserved for lesions that exhibit signs of hyperfunction, show significant growth during surveillance, or are large and suspicious for malignancy, such as those greater than 4 to 6 centimeters.