What is a lipomatous (fatty tissue) lesion on the medial limb of the adrenal gland?

Lipomatous Lesions on the Medial Limb of the Adrenal Gland

A lipomatous lesion on the medial limb of the adrenal gland is most likely a benign tumor containing fatty tissue, with myelolipoma being the most common type (approximately 3% of primary adrenal tumors), followed by lipoma, teratoma, or angiomyolipoma. 1, 2

Types of Adrenal Lipomatous Lesions

• Myelolipoma: Most common adrenal lipomatous tumor (comprises ~3% of primary adrenal tumors)

  • Composed of mature adipose tissue and hematopoietic elements
  • More common in females and in the right adrenal gland
  • Typically found in patients between 50-70 years of age 2

• Lipoma: Pure fatty tumor without hematopoietic elements

  • More common in males and in the right adrenal gland
  • Usually found in patients in their 60s 2

• Teratoma: Contains tissue from multiple germ layers

  • More common in females with bimodal age distribution
  • Approximately 60% of patients are symptomatic 2

• Angiomyolipoma: Contains fat, smooth muscle, and blood vessels

  • More common in females in their 50s
  • Often symptomatic 2

• Rare types: Liposarcoma (malignant), hibernoma, adrenocortical tumors with fat component 2

Diagnostic Approach

Imaging Characteristics

• CT scan is the primary diagnostic tool:

  • Benign adenomas typically have Hounsfield Units (HU) < 10 on non-contrast CT
  • Myelolipomas show characteristic fat density (negative HU values) with areas of soft tissue 3, 1
  • Contrast washout >60% at 15 minutes suggests benign lesion 1

• MRI with chemical shift imaging:

  • Signal intensity loss in opposed-phase images indicates benign fatty tissue
  • Can correctly characterize up to 89% of lesions with CT densities between 10-30 HU 3

Size Considerations

• Size is an important predictor of malignancy:
  • Lesions <3 cm are usually benign (87% in patients with history of malignancy)
  • Lesions >5 cm have higher risk of malignancy 3
  • Surgical intervention is recommended for masses >4 cm with inhomogeneous appearance or HU >20, and all masses >6 cm regardless of appearance 1

Hormonal Evaluation

Although lipomatous lesions are typically non-functional, hormonal evaluation should be performed to rule out subclinical hormone secretion, which occurs in 5-23% of incidentally discovered adrenal masses 1:

• 1mg overnight dexamethasone suppression test (for cortisol excess)
• Plasma or 24-hour urinary metanephrines (for pheochromocytoma)
• Aldosterone-to-renin ratio in hypertensive patients (for hyperaldosteronism) 1

Management Recommendations

Observation

• Benign-appearing lipomatous lesions <4 cm with definitive benign imaging characteristics (HU ≤10) require no further follow-up imaging or functional testing 1

Surgical Intervention

• Surgery is recommended for:
  • Masses >4 cm with inhomogeneous appearance or HU >20
  • All masses >6 cm regardless of appearance
  • Symptomatic lesions (pain from tumor bulk or hemorrhage)
  • Any hormone-producing tumor
  • Growth >5 mm/year on follow-up imaging 1, 4

Follow-up

• Benign-appearing lipomatous lesions ≥4 cm require repeat imaging in 6-12 months
• Consider repeat functional workup if growth >5 mm/year is observed 1

Clinical Pearls and Pitfalls

• Pitfall: In some fatty tumors (particularly myelolipoma and angiomyolipoma), the fatty component may be inconspicuous, making diagnosis challenging 5

• Pitfall: Approximately one-third of adrenal lipomatous tumors may contain calcification or bone, which should not be mistaken for malignancy 5

• Pearl: Adrenal myelolipomas can occasionally present with symptoms such as flank pain due to tumor bulk, necrosis, or spontaneous retroperitoneal hemorrhage 6

• Pearl: Lipomatous adrenal tumors are being detected with increasing frequency due to widespread use of modern imaging modalities 2

• Caution: Biopsy of adrenal masses carries risks (8-12% complication rate) including bleeding, pneumothorax, and infection, and should be avoided if pheochromocytoma is suspected 3