A 68-year-old man undergoes CT abdomen for evaluation of chronic back pain. An incidental adrenal mass is discovered. Non-contrast CT shows the lesion marked **A** with an attenuation of 8 Hounsfield units. Which of the following statements best characterizes the imaging finding at **A**?
A. The lesion requires MRI chemical-shift imaging to exclude pheochromocytoma before any further intervention
B. The lesion is highly specific for a lipid-rich adenoma and does not require adrenal washout protocol imaging
C. The lesion is suspicious for malignancy and warrants immediate adrenalectomy regardless of size
D. The lesion requires adrenal washout protocol with absolute washout >60% to confirm benignity
Explanation
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An attenuation of <10 Hounsfield units on unenhanced CT is 98% specific for a lipid-rich adenoma, because benign adenomas contain abundant intracytoplasmic lipid that attenuates X-rays minimally. This imaging characteristic alone is sufficient to diagnose a lipid-rich adenoma and does not require the adrenal washout protocol (which is reserved for lesions ≥10 HU). Per Harrison's and Endocrine Society guidelines, the presence of <10 HU on unenhanced CT reliably excludes malignancy and lipid-poor lesions. However, biochemical testing (1-mg overnight dexamethasone suppression test, plasma metanephrines) remains mandatory to assess for hormone secretion.
Why each distractor is wrong
Option 2: The adrenal washout protocol (absolute washout >60% or relative washout >40%) is used for lesions ≥10 HU to confirm a lipid-poor adenoma. A lesion already <10 HU does not require this protocol because the low attenuation itself confirms lipid richness.
Option 3: Lesions suspicious for malignancy show features such as size >4 cm, irregular margins, heterogeneity, necrosis, calcification, or HU >20 on unenhanced CT. An 8 HU lesion is benign in character and does not warrant immediate adrenalectomy unless it is >4 cm or functionally active.
Option 4: MRI chemical-shift imaging showing signal drop on out-of-phase sequences confirms intracellular lipid and is an alternative confirmatory method, but it is not mandatory when unenhanced CT already shows <10 HU. Pheochromocytoma is excluded biochemically (plasma metanephrines), not by imaging alone.
High-YieldNEET PG
Unenhanced CT attenuation <10 HU = lipid-rich adenoma (98% specific); no washout protocol needed. Always do biochemical screening regardless.
Harrison's Principles of Internal Medicine, 21st ed; Endocrine Society Guidelines 2023
