Lipid-rich adrenal adenoma is the most common benign neoplasm of the adrenal gland, accounting for approximately 80% of all adrenal incidentalomas. The tumor cells contain abundant intracellular lipid (cholesterol, cholesterol esters, and neutral fats). This lipid content causes low attenuation on CT (<10 HU) and marked signal loss on MRI chemical-shift sequences. It typically presents as a well-circumscribed, homogeneous mass measuring 2-4 cm in diameter. It may be functional (secreting cortisol or aldosterone) or non-functional. The most common clinical scenario is incidental detection, and a measurement of <10 HU on non-contrast CT is pathognomonic for diagnosis.
Age Range
30-80
Peak Age
55
Gender
Female predominant
Prevalence
Very Common
Lipid-rich adrenal adenoma originates from adrenal cortex zona fasciculata or zona glomerulosa cells. These cells store and process cholesterol as part of steroid hormone synthesis. During adenomatous transformation, cells accumulate excessive intracellular lipid while maintaining their normal steroidogenic capacity. This lipid accumulation consists predominantly of cholesterol esters and neutral fats, forming numerous lipid droplets (vacuoles) in the cellular cytoplasm. The low attenuation on non-contrast CT (<10 HU) results from this lipid content attenuating X-ray photons less than surrounding soft tissue — fat molecules have lower electron density, reducing Compton scattering and photoelectric absorption. On MRI chemical-shift imaging, dramatic signal loss occurs on opposed-phase sequences; the physical basis is the phase cancellation phenomenon caused by the difference in magnetic resonance frequencies of water and fat protons within the same voxel. On in-phase images, water and fat signals are additive, while on opposed-phase images they cancel each other, producing a net signal drop. This mechanism reliably distinguishes intracellular lipid-containing adenomas from myelolipomas containing macroscopic fat and from lipid-poor metastases.
Non-contrast CT attenuation <10 HU of an adrenal lesion is a pathognomonic finding for lipid-rich adrenal adenoma. This criterion diagnoses adenoma with 96-100% sensitivity and 98% specificity and requires no additional imaging or washout analysis. Measurement should be performed with an ROI placed centrally covering at least 2/3 of the lesion. On MRI chemical-shift sequences, a thin dark line around the lesion (India Ink artifact) may be visible, confirming the presence of a water-fat interface as an additional clue. This single criterion is the cornerstone of all adrenal incidentaloma algorithms and has been endorsed by the ACR Incidental Findings Committee.
On non-contrast CT, the adrenal lesion demonstrates attenuation <10 HU. This is a direct result of the intracellular lipid content attenuating X-ray photons less than surrounding soft tissue. Measurement should be performed with an ROI (region of interest) covering at least 2/3 of the lesion area, placed centrally. Edge artifacts and surrounding tissue should be avoided. This is a standalone diagnostic criterion and requires no additional imaging.
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A well-circumscribed, homogeneous adrenal lesion measuring approximately ___ cm in the left/right adrenal gland demonstrates ___ HU attenuation on non-contrast CT, consistent with a lipid-rich adrenal adenoma.
In the arterial phase, lipid-rich adenoma shows homogeneous, moderate enhancement. Enhancement is typically a 30-40 HU increase. The lesion remains well-circumscribed and does not demonstrate peripheral nodular enhancement or heterogeneous enhancement pattern. This homogeneous enhancement reflects the orderly vascular architecture of the adenoma.
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The lesion demonstrates homogeneous moderate enhancement in the arterial phase (___ HU), maintaining its well-defined margins.
On delayed phase (10-15 minutes later), lipid-rich adenoma demonstrates significant washout. Absolute washout value is >60% and relative washout value is >40%. Washout calculation: Absolute washout = (arterial HU - delayed HU) / (arterial HU - non-contrast HU) × 100. Relative washout = (arterial HU - delayed HU) / arterial HU × 100. This rapid washout reflects the rich venous drainage of adenoma and rapid elimination of contrast. In lipid-rich adenoma, non-contrast CT is usually sufficient, so washout analysis is more diagnostically important in lipid-poor adenoma.
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On delayed phase (15 minutes), the lesion has decreased to ___ HU, with calculated absolute washout of ___% (>60%), consistent with adrenal adenoma.
On MRI chemical-shift imaging, marked signal loss is seen on opposed-phase sequences compared to in-phase images. Signal drop is typically >20% and in most lipid-rich adenomas exceeds 30-40%. Signal loss can be quantitatively measured using the adrenal signal intensity index (ASI): ASI = (SIin - SIopp) / SIin × 100. ASI >16.5% has high sensitivity and specificity for adenoma diagnosis. This finding is the most reliable adenoma-specific diagnostic criterion after non-contrast CT.
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On MRI chemical-shift sequences, the adrenal lesion demonstrates marked signal drop on opposed-phase images compared to in-phase images (ASI: ___%); this finding is consistent with intracellular lipid content and supports a lipid-rich adrenal adenoma.
On T1-weighted images, lipid-rich adenoma shows isointense or slightly hyperintense signal compared to the liver. This slight hyperintensity results from the T1-shortening effect of intracellular lipid content. The lesion appears homogeneous without significant internal heterogeneity. T1 hyperintensity may be more pronounced in the presence of hemorrhage, but this is a rare complication.
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The adrenal lesion shows isointense/slightly hyperintense, homogeneous signal on T1-weighted images compared to the liver.
On T2-weighted images, lipid-rich adenoma shows isointense or slightly hyperintense signal compared to the liver. Unlike the markedly hyperintense T2 signal of pheochromocytoma ('light bulb brightness'), adenoma shows only mild increase. This feature reflects the solid architecture and relatively low water content of adenoma. Markedly hyperintense T2 signal should raise alternative diagnoses such as pheochromocytoma or cystic lesion.
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On T2-weighted images, the lesion shows isointense/slightly hyperintense signal compared to the liver, without the marked hyperintensity expected in pheochromocytoma.
On diffusion-weighted imaging, lipid-rich adenoma typically does not show significant diffusion restriction. ADC (apparent diffusion coefficient) values may be normal or mildly reduced, but the marked restriction seen in malignant lesions is absent. Typical ADC values range from 1.2-1.8 × 10⁻³ mm²/s. This finding reflects the low cellularity and normal interstitial architecture of adenoma. It should be noted that lipid content can affect DWI signal and create artifacts.
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On diffusion-weighted imaging, the lesion does not show significant diffusion restriction, with ADC value measured at ___ × 10⁻³ mm²/s.
On FDG PET-CT, lipid-rich adenoma typically does not show significant FDG uptake. SUVmax is usually below or equal to the liver SUV. This low metabolic activity reflects the benign nature and low cellular proliferation of adenoma. However, functional adenomas (especially cortisol-producing) may show mildly increased FDG uptake. Significant FDG uptake (SUVmax > liver SUV) should raise suspicion for metastasis or adrenocortical carcinoma.
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On FDG PET-CT, no significant FDG uptake is observed in the adrenal lesion (SUVmax: ___), consistent with benign adenoma.
Criteria
No hormone secretion; incidentally detected. Normal hormonal profile (cortisol, aldosterone, catecholamines normal). CT <10 HU, size typically <3 cm. Most common adenoma type (85%).
Distinct Features
No clinical symptoms, no follow-up needed (if <4 cm and <10 HU). Annual follow-up CT recommended only for size >4 cm or atypical appearance. Functional assessment (1 mg dexamethasone suppression test) recommended in some guidelines for all incidentalomas.
Criteria
Autonomous cortisol secretion; cortisol >1.8 μg/dL on 1 mg dexamethasone suppression test. Clinical Cushing syndrome features may be mild or absent. CT <10 HU, variable size. Prevalence 5-20% of incidentalomas.
Distinct Features
Metabolic complications: diabetes, hypertension, osteoporosis, obesity. Contralateral adrenal atrophy may be seen (due to ACTH suppression). Adrenalectomy should be considered especially in young patients with metabolic comorbidities. Requires postoperative steroid replacement (hypothalamic-pituitary axis suppression).
Criteria
Primary hyperaldosteronism; elevated aldosterone-to-renin ratio (ARR >30). Usually small (<2 cm), unilateral. CT <10 HU. Presents with hypertension and hypokalemia. Prevalence 5-10% of hypertensive patients.
Distinct Features
May be missed on CT due to small size — thin-section CT or MRI may be needed. Adrenal venous sampling is the gold standard for lateralization (to exclude bilateral disease). Laparoscopic adrenalectomy is curative. Medical treatment: spironolactone or eplerenone (for non-surgical candidates).
Criteria
Oncocytic cell morphology (abundant eosinophilic granular cytoplasm, numerous mitochondria). Variable lipid content — some are lipid-rich, others may be lipid-poor. CT may show >10 HU (lipid-poor oncocytic adenoma). Size typically 3-8 cm. Rare (<1%). Classified as benign, borderline, or malignant by Lin-Weiss-Bisceglia criteria.
Distinct Features
May be confused with adrenocortical carcinoma due to larger size and potentially higher CT attenuation. Surgical indication usually exists (size + histological uncertainty). Immunohistochemistry: inhibin positive, Melan-A positive, cytokeratin positive. Risk of malignant transformation is low but possible.
Distinguishing Feature
Lipid-poor adenoma shows 10-30 HU attenuation (>10 HU); chemical-shift signal drop may be less pronounced or absent. Differentiation is made by washout analysis (absolute >60% / relative >40% = adenoma).
Distinguishing Feature
Metastasis shows >10 HU attenuation, no chemical-shift signal drop (no intracellular lipid), slow washout (absolute <60%), usually with known primary malignancy and may be bilateral.
Distinguishing Feature
Myelolipoma contains macroscopic fat (-30 to -100 HU), different from adenoma's intracellular fat (<10 HU but rarely below -30 HU). On MRI, shows signal loss on frequency-selective fat suppression; chemical-shift signal drop may not be prominent (macroscopic fat rather than voxel-level water-fat mixture).
Distinguishing Feature
Pheochromocytoma shows markedly hyperintense T2 signal ('light bulb brightness'), >20 HU attenuation, no chemical-shift signal drop. Elevated catecholamines are diagnostic. Biopsy is contraindicated (catecholamine crisis risk).
Distinguishing Feature
Adrenocortical carcinoma is typically >4 cm, heterogeneous, irregular margins, may contain necrosis and calcification. >20 HU attenuation, slow washout, local invasion signs (renal vein, IVC). Weight loss and virilization may accompany.
Urgency
routineManagement
conservativeBiopsy
Not NeededFollow-up
no-follow-upLipid-rich adrenal adenoma is a benign lesion and the combination of <4 cm size and <10 HU attenuation confirms the diagnosis — no additional imaging or biopsy is needed. Functional evaluation (1 mg dexamethasone suppression test, plasma metanephrines, aldosterone/renin ratio) is recommended in some guidelines for all incidentalomas. No follow-up is needed for non-functional, <4 cm, <10 HU adenoma. Annual CT follow-up for 1-2 years is recommended for 4-6 cm lesions. Surgery should be considered for lesions >6 cm or showing rapid growth. Adrenalectomy is curative for functional adenomas (Conn syndrome, subclinical Cushing).
Lipid-rich adenoma can be confidently diagnosed on unenhanced CT (≤10 HU). It constitutes the majority of incidental adrenal masses and requires no follow-up if hormonally inactive.