Lipid-poor adrenal adenoma accounts for approximately 30% of all adrenal adenomas. Due to insufficient intracellular lipid content, it shows >10 HU attenuation on non-contrast CT and may not demonstrate signal loss on chemical-shift MRI. Diagnosis is established by contrast-enhanced CT washout analysis: absolute washout >60% or relative washout >40% has high sensitivity and specificity for adenoma. This adenoma subtype may show imaging overlap with metastasis and pheochromocytoma, making accurate washout analysis critical. It may be functional or non-functional; incidental detection is the most common presentation.
Age Range
30-80
Peak Age
55
Gender
Female predominant
Prevalence
Common
Lipid-poor adrenal adenoma originates from the same adrenal cortex cells as lipid-rich adenoma but has significantly less intracellular lipid accumulation. Cells contain fewer cholesterol ester and neutral fat droplets, instead having more compact cytoplasm. The reason for this decreased lipid content is not fully understood, but may be related to differences in lipid metabolism, lipid synthesis-storage balance disruption, or different cell differentiation. The >10 HU attenuation on CT is because the reduced intracellular lipid allows the soft tissue component to dominate, causing greater X-ray photon attenuation. The absent or minimal signal loss on MRI chemical-shift sequences results from insufficient water-fat proton mixture at the voxel level — with fewer fat protons, the phase cancellation effect is minimal. However, washout kinetics are the same as lipid-rich adenoma because the vascular architecture and venous drainage are similarly orderly. Therefore the absolute washout >60% criterion also applies to lipid-poor adenoma and plays a critical role in differentiation from metastasis.
The most critical finding for lipid-poor adenoma diagnosis is contrast-enhanced CT washout analysis. In adrenal lesions not meeting the <10 HU threshold on non-contrast CT (10-30 HU), absolute washout is calculated by measuring arterial and 15-minute delayed phase attenuation. >60% absolute washout or >40% relative washout diagnoses adenoma with 98% sensitivity and 92% specificity. This analysis is the most reliable method distinguishing lipid-poor adenoma from metastasis and pheochromocytoma and is a cornerstone of the ACR incidentaloma guideline.
On non-contrast CT, lipid-poor adenoma shows attenuation of 10-30 HU. This is above the <10 HU threshold for lipid-rich adenoma and is not diagnostic by itself. Malignant lesions can also fall in this attenuation range (metastasis: 20-45 HU, pheochromocytoma: 20-40 HU). ROI measurement must be careful, covering 2/3 of the lesion. Lesions in this attenuation range require further evaluation with washout analysis or MRI.
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A well-circumscribed adrenal lesion measuring approximately ___ cm in the left/right adrenal gland demonstrates ___ HU attenuation on non-contrast CT, above the lipid-rich adenoma threshold; washout analysis or MRI characterization is recommended.
In the arterial phase, lipid-poor adenoma shows homogeneous enhancement. Enhancement amount may be higher than lipid-rich adenoma (can reach 40-80 HU). It does not show heterogeneity or peripheral enhancement pattern. This homogeneous enhancement reflects the orderly vascular architecture of adenoma and provides the arterial phase attenuation needed for washout calculation.
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The lesion demonstrates homogeneous enhancement in the arterial phase, with attenuation increasing to ___ HU.
On delayed phase (10-15 minutes), lipid-poor adenoma demonstrates significant washout — this is THE MOST CRITICAL finding for lipid-poor adenoma diagnosis. Absolute washout >60% and relative washout >40% thresholds are diagnostic. Absolute washout = (arterial HU - delayed HU) / (arterial HU - non-contrast HU) × 100. Relative washout = (arterial HU - delayed HU) / arterial HU × 100. If non-contrast phase is unavailable, relative washout is used. This washout analysis is the most reliable method distinguishing lipid-poor adenoma from metastasis and pheochromocytoma. Washout analysis diagnoses adenoma with 98% sensitivity and 92% specificity.
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On delayed phase (15 minutes), the lesion has decreased to ___ HU, with calculated absolute washout of ___% (>60%) / relative washout of ___% (>40%); this finding supports lipid-poor adrenal adenoma with high probability.
On MRI chemical-shift sequences, lipid-poor adenoma may show minimal or no signal drop on opposed-phase images compared to in-phase. ASI (adrenal signal intensity index) may be <16.5%. This is the key distinguishing feature from lipid-rich adenoma. However, some lipid-poor adenomas may show mild signal loss (<20%) — therefore limited signal loss does not completely exclude adenoma. If chemical-shift is negative, washout analysis is mandatory.
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On MRI chemical-shift sequences, the adrenal lesion does not show significant signal drop on opposed-phase compared to in-phase images (ASI: ___%); this finding is consistent with lipid-poor adenoma, and further evaluation with washout analysis is recommended.
On T1-weighted images, lipid-poor adenoma shows isointense signal relative to liver and spleen. The mild T1 hyperintensity seen in lipid-rich adenoma is usually absent because the lipid-related T1-shortening effect is insufficient. The lesion is homogeneous, and significant heterogeneity should suggest malignancy.
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On T1-weighted images, the adrenal lesion shows isointense, homogeneous signal compared to surrounding parenchymal organs.
On T2-weighted images, lipid-poor adenoma shows isointense or mildly hyperintense signal compared to liver. Must be distinguished from the marked T2 hyperintensity of pheochromocytoma ('light bulb brightness'). T2 signal may overlap with metastasis; however, homogeneity and well-defined margins support adenoma.
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On T2-weighted images, the lesion shows isointense/mildly hyperintense, homogeneous signal, without the marked hyperintensity expected in pheochromocytoma.
On diffusion-weighted imaging, lipid-poor adenoma typically does not show significant diffusion restriction. ADC values range from 1.0-1.8 × 10⁻³ mm²/s. Metastasis and adrenocortical carcinoma may show lower ADC values (<1.0 × 10⁻³ mm²/s). DWI can be used as a complementary tool to washout analysis but is not diagnostic alone.
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On diffusion-weighted imaging, the lesion does not show significant diffusion restriction, with ADC value of ___ × 10⁻³ mm²/s, consistent with benign adenoma.
On FDG PET-CT, lipid-poor adenoma typically shows FDG uptake equal to or below liver SUV. SUVmax is usually <3.1. However, some lipid-poor adenomas may show mildly increased FDG uptake, so low-to-moderate FDG uptake does not exclude adenoma. If SUVmax > liver SUV, metastasis or adrenocortical carcinoma should be considered. PET-CT is helpful in differentiating benign from malignant adrenal lesions in the presence of known malignancy.
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On FDG PET-CT, the adrenal lesion shows FDG uptake at or below liver SUV level (SUVmax: ___, Adrenal/Liver ratio: ___), consistent with benign adenoma.
Criteria
No hormone secretion, incidentally detected. CT 10-30 HU, chemical-shift signal loss minimal/absent. Diagnosis by washout >60%. Normal hormonal values.
Distinct Features
Unlike lipid-rich adenoma, non-contrast CT alone is not diagnostic — washout analysis is mandatory. Follow-up and management are the same as lipid-rich adenoma: no follow-up needed if <4 cm and washout positive.
Criteria
Hormone production present (cortisol, aldosterone). CT >10 HU, washout >60%. Abnormal hormonal tests (dexamethasone suppression test, ARR).
Distinct Features
Functional evaluation is recommended for all incidentalomas; it is particularly important in lipid-poor adenoma because imaging findings are less specific. If functional, adrenalectomy should be considered.
Criteria
CT >10 HU, chemical-shift negative, washout <60%. Rare (2-5% of adenomas). Diagnosis is challenging — MRI DWI, PET-CT, or biopsy may be needed. Some authors recommend repeat CT or MRI follow-up.
Distinct Features
Differentiation from metastasis and pheochromocytoma is most difficult in this subgroup. Low FDG uptake on PET-CT, no restriction on DWI, and size stability support a benign lesion. Clinical decision should be multifactorial.
Distinguishing Feature
Lipid-rich adenoma shows <10 HU attenuation and pronounced chemical-shift signal loss (>20%). Lipid-poor adenoma is >10 HU and chemical-shift may be negative. Both show washout >60%.
Distinguishing Feature
Metastasis shows slow washout (absolute <60%, relative <40%), no chemical-shift signal drop, usually with known primary malignancy, and may have irregular margins. Bilateral involvement is common. FDG uptake is usually high.
Distinguishing Feature
Pheochromocytoma shows marked T2 hyperintensity ('light bulb brightness'), slow washout (<60%), chemical-shift negative. Plasma metanephrines elevated. Biopsy contraindicated. In lipid-poor adenoma, T2 is isointense/mildly hyperintense and washout >60%.
Distinguishing Feature
Adrenocortical carcinoma is typically >4-6 cm, heterogeneous (necrosis, hemorrhage, calcification), irregular margins, slow washout (<60%), local invasion signs. Lipid-poor adenoma is typically <4 cm, homogeneous and well-circumscribed.
Distinguishing Feature
Lymphoma typically presents as bilateral, large, homogeneous hypodense masses. Washout is slow (<60%). Accompanied by systemic lymphoma findings (LAP, splenomegaly, B symptoms). Shows marked restriction on DWI.
Urgency
routineManagement
conservativeBiopsy
Not NeededFollow-up
6-monthLipid-poor adrenal adenoma is a benign lesion when diagnosed by washout analysis, and the same management algorithm as lipid-rich adenoma applies. If washout >60%, diagnosis is confirmed and no additional follow-up is needed for <4 cm lesions. If washout is indeterminate (40-60% range), size check at 6-12 months or further evaluation with MRI/PET-CT is recommended. Functional evaluation should be performed in all cases — 1 mg dexamethasone suppression test, plasma metanephrines, and aldosterone/renin ratio are standard screening tests. Biopsy or surgery should be considered for lesions with negative washout and chemical-shift, >4 cm, or showing rapid growth.
Lipid-poor adenoma is more challenging to diagnose than lipid-rich adenoma. Washout CT protocol (15-minute delayed phase) is the gold standard. PET-CT or biopsy may be needed in indeterminate cases.