Adrenal metastasis is the most common malignant tumor of the adrenal glands and is far more frequent than primary adrenal malignancies. It most commonly originates from lung (35%), breast (15%), melanoma (10%), kidney (10%), and colon (5%) carcinomas. In autopsy series, adrenal metastasis is found in 27% of advanced cancer patients. Bilateral involvement occurs in 10-15% and is one of the most important diagnoses along with lymphoma in the context of bilateral adrenal masses. Clinically usually asymptomatic, detected incidentally on staging CT or PET-CT. Bilateral massive metastasis rarely can cause adrenal insufficiency. Differentiation from adenoma on imaging is critical — absence of chemical-shift signal loss, slow washout (<60%), and known primary malignancy suggest metastasis.
Age Range
40-85
Peak Age
65
Gender
Equal
Prevalence
Common
Adrenal metastasis occurs when primary tumor cells reach the adrenal glands via hematogenous (most common) or lymphatic routes. The adrenal glands' rich arterial blood flow (one of the highest blood flow organs relative to body weight) and wide sinusoidal vascular architecture provide a favorable environment for metastatic cell lodging and proliferation. Tumor cells enter the circulation (intravasation), lodge in the capillary bed (arrest), exit the vasculature (extravasation), and begin growing in the adrenal parenchyma. The pathophysiological basis of imaging findings: high attenuation on CT (>20 HU) results from tumor cells lacking intracellular lipid — the cholesterol ester storage seen in adenomas is absent in metastatic cells. The absence of chemical-shift signal loss is explained by the same reason. Slow washout results from tumor neovascularization being irregular and permeable — contrast pools in the interstitium longer. High FDG PET-CT uptake reflects the increased glycolysis rate of metastatic cells (Warburg effect).
In a patient with known malignancy history, an adrenal mass showing >10 HU on non-contrast CT, no chemical-shift signal loss, and slow washout (<60%) on washout analysis is highly likely metastasis. This triple combination (clinical + MRI + CT kinetic) provides the strongest evidence for metastasis diagnosis. FDG PET-CT completes confirmation and staging. Biopsy should only be considered in uncertain cases that will change treatment decisions and after biochemical exclusion of pheochromocytoma.
On non-contrast CT, adrenal metastasis typically shows attenuation >20 HU. Does not meet the lipid-rich adenoma threshold (<10 HU). The lesion may be homogeneous or heterogeneous — in large lesions, necrosis, hemorrhage, and calcification areas contribute to attenuation heterogeneity. Bilateral masses increase suspicion for metastasis. May be well-circumscribed or irregular — irregular margins favor malignancy.
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An adrenal mass measuring approximately ___ cm in the left/right adrenal gland demonstrates ___ HU attenuation on non-contrast CT; in the context of known ___ carcinoma, metastasis should be considered.
In the arterial phase, adrenal metastasis shows variable enhancement — dependent on the primary tumor's vascularity. Hypervascular metastases (RCC, melanoma, thyroid, neuroendocrine) show avid enhancement, hypovascular metastases (lung, colon) show less enhancement. In large lesions, heterogeneous enhancement, rim enhancement (peripheral enhancement + central necrosis), or irregular enhancement pattern may be seen.
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In the arterial phase, the lesion demonstrates heterogeneous enhancement, with central hypodense areas suggesting necrosis.
On delayed phase, adrenal metastasis shows slow washout — absolute washout <60% and relative washout <40%. This kinetic profile is distinctly different from adenoma's rapid washout (>60%) and is the most reliable CT criterion for adenoma-metastasis differentiation. Slow washout results from the irregular structure of tumor neovascularization and increased vascular permeability.
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On delayed phase, the lesion shows slow washout (absolute washout: __%, <60%), this kinetic profile argues against adenoma and is consistent with metastasis.
On MRI chemical-shift sequences, adrenal metastasis does not show signal drop on opposed-phase images. This reflects the absence of intracellular lipid in metastatic cells and is the key differentiating finding from lipid-rich adenoma. Rare exceptions: clear cell RCC metastasis and HCC metastasis may contain intracellular lipid and show chemical-shift signal loss — these situations create diagnostic difficulty.
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On MRI chemical-shift sequences, no opposed-phase signal loss is seen in the adrenal lesion, indicating absence of intracellular lipid; consistent with metastasis in the context of known malignancy.
On T2-weighted images, adrenal metastasis shows variable signal — usually mildly to moderately hyperintense. Lower intensity than the marked T2 hyperintensity of pheochromocytoma ('light bulb sign'). Necrotic/cystic areas may be markedly hyperintense on T2. Melanoma metastasis may be T1 hyperintense and T2 hypointense due to paramagnetic effect of melanin (but amelanotic melanoma does not show this feature).
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On T2-weighted images, the lesion shows mildly to moderately hyperintense signal, without the marked hyperintensity expected in pheochromocytoma.
On DWI, adrenal metastasis typically shows significant diffusion restriction. ADC values are low (typically 0.6-1.2 × 10⁻³ mm²/s) — significantly lower than adenoma (1.2-1.8 × 10⁻³ mm²/s). This finding complements washout analysis and contributes to differentiation in indeterminate lesions. ADC may be increased in necrotic areas.
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On DWI, the lesion shows significant diffusion restriction (ADC: ___ × 10⁻³ mm²/s), consistent with a malignant lesion.
On FDG PET-CT, adrenal metastasis typically shows high FDG uptake — SUVmax is above liver SUV. This reflects the increased glycolysis rate of metastatic cells. PET-CT has 95-100% sensitivity and 80-95% specificity for differentiating benign (adenoma) from malignant (metastasis) adrenal lesions in the presence of known malignancy. Adrenal-to-liver SUV ratio >1.5-2.0 is a strong finding favoring metastasis. PET-CT also reveals other metastatic foci.
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On FDG PET-CT, increased FDG uptake is observed in the left/right adrenal lesion (SUVmax: ___, Adrenal/Liver ratio: ___), consistent with metastasis in the context of known ___ carcinoma.
Criteria
Most common source of adrenal metastasis (35%). Small cell and non-small cell lung cancer. Usually hypovascular, at soft tissue density.
Distinct Features
Detected on staging imaging. May be bilateral. Adrenal metastasis indicates stage IV disease and changes treatment strategy.
Criteria
T1 hyperintense (melanin paramagnetic effect), T2 hypointense. Hypervascular, avid enhancement. High FDG uptake.
Distinct Features
T1/T2 signal not typical in amelanotic melanoma. Melanoma metastasis can appear even very late (years later). PET-CT critical for staging and follow-up.
Criteria
Hypervascular, avid arterial enhancement. Clear cell type may contain intracellular lipid — chemical-shift signal loss in 2-5% of cases. May even show <10 HU on CT.
Distinct Features
Metastasis type most commonly confused with adenoma (may contain lipid). History of ipsilateral nephrectomy or contralateral renal mass is critical clue. Washout analysis may be misleading.
Distinguishing Feature
Lipid-rich adenoma shows <10 HU attenuation and chemical-shift signal loss; metastasis >10 HU and chemical-shift negative. Washout: adenoma >60%, metastasis <60%.
Distinguishing Feature
Lipid-poor adenoma is >10 HU but shows rapid washout (>60%), low FDG uptake. Metastasis shows slow washout (<60%) and high FDG uptake.
Distinguishing Feature
Pheochromocytoma shows marked T2 hyperintensity ('light bulb'), MIBG positive, catecholamines elevated. Metastasis mild-moderate T2 hyperintensity, MIBG negative.
Distinguishing Feature
Adrenocortical carcinoma is primary adrenal malignancy, usually >6 cm, may be functional (virilization, Cushing), local invasion. Metastasis usually smaller, known primary malignancy.
Distinguishing Feature
Lymphoma usually bilateral, large, homogeneous, low enhancement. B symptoms may be present. Metastasis usually heterogeneous (necrosis), unilateral or bilateral.
Urgency
urgentManagement
medicalBiopsy
NeededFollow-up
specialist-referralDetection of adrenal metastasis means the cancer is stage IV and significantly changes treatment strategy. In solitary adrenal metastasis, surgical resection (adrenalectomy) in some tumor types (especially isolated NSCLC metastasis) may provide long-term survival benefit — 5-year survival 25-35%. Differentiating adenoma from metastasis in adrenal masses with known malignancy is critical because 50-75% of incidentalomas may be adenoma. PET-CT is the most reliable imaging method for this differentiation. Biopsy should be considered in uncertain cases that will change treatment decisions, after biochemical exclusion of pheochromocytoma. In bilateral massive metastasis, adrenal insufficiency risk exists (1-5%) — cortisol levels should be checked.
When an adrenal mass is found in patients with known malignancy, metastasis should be the primary consideration. PET-CT is helpful in metastasis-adenoma differentiation. Biopsy may be needed in indeterminate cases.