Adrenal myelolipoma is a rare, benign adrenal tumor composed of mature adipose tissue and hematopoietic bone marrow elements (erythroid, myeloid, and megakaryocyte series). It constitutes 6-16% of all adrenal incidentalomas and is the second most common benign adrenal tumor. The macroscopic fat content produces distinctly negative HU values on CT (-30 to -100 HU), which is pathognomonic — this finding is not seen in any other adrenal mass (adenoma contains intracellular fat, not macroscopic). Size can range from a few mm to 30 cm. Large lesions carry hemorrhage risk. Most are asymptomatic and incidentally detected.
Age Range
40-80
Peak Age
60
Gender
Equal
Prevalence
Common
Adrenal myelolipoma arises from proliferation of metaplastic bone marrow elements and mature adipocytes within the adrenal gland. Although the pathogenesis is not fully understood, several theories have been proposed: (1) metaplasia of adrenal cortex cells into mesenchymal stem cells, (2) persistence of embryonal bone marrow remnants, (3) transformation of adrenal reticuloendothelial cells in response to chronic stress or ACTH stimulation. The macroscopic fat component causes distinctly negative attenuation on CT (-30 to -100 HU) because fat tissue has very low electron density and minimally attenuates X-ray photons. T1 hyperintensity on MRI is related to the short T1 relaxation time of fat protons; signal loss on frequency-selective fat suppression confirms the presence of this macroscopic fat. Important distinction: Fat in myelolipoma is macroscopic (organized as adipocytes), while fat in adenoma is intracellular (cytoplasmic lipid droplets) — therefore chemical-shift signal loss is not expected in myelolipoma because there is pure fat tissue rather than water-fat mixture at the voxel level.
The presence of macroscopic fat in an adrenal mass (-30 to -100 HU) is pathognomonic for myelolipoma. No other adrenal lesion contains macroscopic fat — adenoma contains intracellular fat (>-30 HU), liposarcoma is extremely rare in the adrenal gland. This finding alone is diagnostic and requires no additional imaging or biopsy. If MRI confirmation is needed, a T1 hyperintense component showing signal loss on frequency-selective fat suppression is sought.
On non-contrast CT, myelolipoma shows distinctly negative attenuation (-30 to -100 HU). This is the same attenuation as surrounding retroperitoneal or subcutaneous fat. The lesion usually appears heterogeneous — fat components show very low attenuation while bone marrow components may be at soft tissue density (20-40 HU). Areas of high attenuation (50-80 HU) may be seen if hemorrhage is present. Diagnostic ROI measurement should be placed on the fat-containing area. Detection of macroscopic fat in any adrenal mass establishes the diagnosis of myelolipoma.
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A heterogeneous mass measuring approximately ___ cm in the left/right adrenal gland containing macroscopic fat (ROI: ___ HU) is seen, pathognomonically consistent with adrenal myelolipoma.
In the arterial phase, the bone marrow components of myelolipoma show moderate enhancement, while fat components do not enhance. This accentuates the heterogeneous appearance. Enhancing areas correspond to hematopoietic bone marrow regions. Septa and peripheral capsular structures may also enhance. The heterogeneous enhancement pattern may be more pronounced in large lesions with internal hemorrhage or necrosis.
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In the arterial phase, the soft tissue components of the lesion show moderate enhancement, while fat components remain unenhanced.
On T1-weighted images, myelolipoma shows markedly hyperintense signal in fat components — same signal intensity as subcutaneous fat. Bone marrow components may be at intermediate signal intensity. Hemorrhage areas show stage-dependent signal (acute: isointense-hypointense, subacute: hyperintense methemoglobin). T1 hyperintensity is a direct result of the short T1 relaxation time of macroscopic fat.
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On T1-weighted images, the adrenal lesion contains components showing markedly hyperintense signal isointense to subcutaneous fat, supporting the presence of macroscopic fat.
On T2-weighted images, myelolipoma shows heterogeneous signal. Fat components show intermediate-to-high signal, bone marrow components show intermediate signal. On STIR or fat-suppressed T2 sequences, marked signal loss is seen in fat components — this is a critical sequence for confirming macroscopic fat.
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On T2-weighted images, the lesion shows heterogeneous signal, with marked signal loss in fat components on fat-suppressed sequences (STIR/fat-sat).
On MRI chemical-shift sequences, myelolipoma does not show significant signal drop on opposed-phase images — this is the key differentiating feature from adenoma. The reason is that fat in myelolipoma is macroscopic (organized as adipocytes) and there is no water-fat proton mixture at the voxel level. However, at the lesion margin, an 'India ink artifact' (thin dark line) may be seen at the fat-soft tissue interface — this results from phase cancellation at the water-fat boundary.
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On chemical-shift sequences, no opposed-phase signal loss is seen within the lesion content, consistent with macroscopic fat (myelolipoma); India ink artifact is present at the lesion margin.
On DWI, fat components of myelolipoma do not show diffusion restriction. On fat-suppressed DWI sequences, fat loses signal and evaluation may be difficult. Mild diffusion restriction may be seen in bone marrow components but this is not significant for malignancy. DWI has limited diagnostic contribution in fat-containing lesions.
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On DWI, no diffusion restriction is seen in fat components; mild signal increase in bone marrow components is consistent with T2 shine-through.
On delayed phase, the attenuation of the fat component remains unchanged (stays at negative HU). Some washout may be seen in bone marrow components. Washout analysis is not diagnostic in myelolipoma — diagnosis is already established by macroscopic fat on non-contrast CT. Washout analysis is unnecessary and may cause confusion with adenoma.
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On delayed phase, the fat components maintain stable attenuation (negative HU consistent with non-contrast CT), and washout analysis is not required for myelolipoma diagnosis.
On FDG PET-CT, myelolipoma typically shows no significant FDG uptake. Fat components show no FDG uptake. Mild physiological uptake may be seen in bone marrow components. Significant FDG uptake should be evaluated for internal hemorrhage or rare malignant transformation.
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On FDG PET-CT, no significant FDG uptake is observed in the adrenal lesion (SUVmax: ___), consistent with benign myelolipoma.
Criteria
Fat component constitutes >50% of the lesion. Distinctly negative attenuation is dominant on CT. T1 hyperintense area is dominant on MRI. Diagnosis is easy — macroscopic fat is clearly visible.
Distinct Features
Most common subtype. Differential diagnosis with retroperitoneal liposarcoma requires attention — however adrenal origin and presence of bone marrow component provides differentiation.
Criteria
Bone marrow components predominant (>50%). Fat components less prominent on CT, soft tissue density may dominate. Diagnosis more difficult — small fat foci must be carefully sought.
Distinct Features
May be confused with adenoma or metastasis. Demonstrating small fat foci with thin-section CT is critical. Focal signal loss on frequency-selective fat suppression on MRI should be sought. Biopsy may be considered in suspicious cases.
Criteria
Contains internal hemorrhage. High-attenuation hemorrhagic areas (50-80 HU) on CT with fat and marrow components. T1 hyperintense methemoglobin areas on MRI. More common in larger lesions.
Distinct Features
Hemorrhage in large lesions can be life-threatening — may present as acute abdomen. Symptomatic hemorrhage may require emergency surgery. Demonstrating the macroscopic fat component in hemorrhagic myelolipoma is critical for diagnosis.
Criteria
Extra-adrenal location: presacral, retroperitoneal, mediastinal, or pelvic. 15% of all myelolipomas are extra-adrenal. Histologically identical to adrenal myelolipoma. Macroscopic fat + bone marrow elements.
Distinct Features
Differential diagnosis is broader in extra-adrenal location: liposarcoma, teratoma, lipoma + bone marrow metaplasia should be considered. Biopsy is usually required.
Distinguishing Feature
Adenoma contains intracellular fat (<10 HU, rarely below -30 HU), myelolipoma contains macroscopic fat (-30 to -100 HU). Adenoma shows chemical-shift signal loss, myelolipoma does not. Myelolipoma shows signal loss on frequency-selective fat suppression.
Distinguishing Feature
Adrenocortical carcinoma does not contain macroscopic fat (rarely small fat foci may be present). Usually heterogeneous, necrotic, >4 cm, irregular margins. Invasion signs. Myelolipoma contains prominent macroscopic fat and does not show invasion.
Distinguishing Feature
Adrenal hemorrhage does not contain fat, shows high attenuation (acute: 50-80 HU). Shows stage-dependent signal on MRI. Hemorrhage in myelolipoma is seen together with macroscopic fat components — detection of fat is distinguishing.
Distinguishing Feature
Pheochromocytoma does not contain fat, is at soft tissue density (20-40 HU), markedly hyperintense on T2, catecholamines elevated. Myelolipoma contains macroscopic fat and is non-functional.
Distinguishing Feature
Metastasis does not contain macroscopic fat (rare exceptions: liposarcoma, RCC metastasis). Usually at soft tissue density (20-45 HU), may be bilateral, known primary malignancy. Myelolipoma is easily differentiated by its prominent macroscopic fat.
Urgency
routineManagement
conservativeBiopsy
Not NeededFollow-up
no-follow-upAdrenal myelolipoma is a benign lesion and diagnosis is confirmed when macroscopic fat is demonstrated on CT — no biopsy or additional imaging is needed. No follow-up is needed for small, asymptomatic lesions (<4-5 cm). Surgery may be considered for large lesions (>6-7 cm) due to hemorrhage risk — especially in symptomatic or rapidly growing cases. Acute hemorrhage may require emergency surgery. Malignant transformation has not been reported. It is non-functional and hormonal evaluation is generally unnecessary, though baseline functional screening may be performed per incidentaloma protocol.
Myelolipoma is confidently diagnosed on CT by its macroscopic fat content. Usually asymptomatic and requires no treatment. Surgery may be considered for large lesions due to risk of spontaneous hemorrhage.