Adrenal myelolipomas are rare, benign neoplasms characterised by variable proportions of adipocytes and haematopoietic cells, and appear as a well-defined, heterogeneous adrenal mass with variable lipid content.
Description
Adrenal myelolipoma is a non-functioning, benign mesenchymal tumour composed of adipose tissue and elements of haematopoiesis. These tumours are considered a result of metaplasia of the reticuloendothelial cells in the adrenal glands. They most often occur in the adrenal glands, but extramedullary locations have been reported.
Pathogenesis
The exact pathogenesis of adrenal myelolipoma is unclear but is generally believed to result from an adrenocortical response to stimuli or stress, which induces metaplasia of the adrenal reticuloendothelial cells into adipocytes and haematopoietic precursors.
Subtypes
There are no recognised subtypes of adrenal myelolipoma.
Epidemiology, Risk Factors & Associations
- Most often occur in adults aged 40-70 (peak incidence in the sixth decade)
- No clear gender predominance
- May be associated with endocrine disorders, such as Cushing’s syndrome, Conn’s syndrome, and Addison’s disease
Clinical Features
Adrenal myelolipomas are generally asymptomatic and are often discovered incidentally on imaging studies (so-called “incidentalomas”). When symptoms do occur, they are typically non-specific and may include flank pain, abdominal fullness, or palpable mass.
Complications
- Spontaneous rupture, leading to retroperitoneal haemorrhage (rare)
- Mass effect on surrounding structures
Pathological Features
Histopathology
- Macroscopic: Well-circumscribed, encapsulated tumours, usually yellow-tan on cut surface due to fat content
- Microscopic: Composed of variable proportions of mature adipose tissue and trilineage haematopoietic cells
Serology
There is no specific serological test for adrenal myelolipoma.
Biochemistry
These tumours are typically non-functioning and do not produce adrenal hormones.
Radiological Features
General Features
- Adrenal myelolipomas typically present as well-circumscribed, heterogeneous adrenal masses with areas of fat attenuation on non-contrast CT.
CT
- Non-contrast: Shows well-defined, heterogeneous adrenal mass with areas of low attenuation due to fat content
- C+ Arterial/Venous: Variable enhancement due to haematopoietic components
MRI
- T1: High signal intensity due to macroscopic fat content
- Fat-saturated sequences: Demonstrates signal suppression
- T2: Variable signal intensity due to haematopoietic components
- T1 Gad+: Variable enhancement due to haematopoietic components
- In/Out of Phase: The interface between fat and soft tissue elements will lose signal on opposed-phase. The lesion itself does not drop out on out of phase imaging.
US
- B-mode: Hyperechoic mass due to fat content
- Colour: No specific colour Doppler features
NM
- PET FDG: Generally low FDG uptake due to benign nature
Grading and Staging
As adrenal myelolipomas are benign tumours, grading and staging systems do not apply.
Diagnosis
The diagnosis of adrenal myelolipoma is typically made by characteristic imaging findings and histopathological confirmation if resected.
Differential Diagnosis
- Adrenal adenoma: Typically presents as a well-defined, homogeneous, low attenuation (0-20 HU) adrenal mass on non-contrast CT. They enhance and washout rapidly (absolute washout value > 50% is highly sensitive and specific. An out-of-phase T1WI that shows signal drop-out indicates a lipid-rich adenoma (vs India-ink artifact surrounding myelolipoma). Lipid-rich adenomas are detected well on NECT and MR. Lipid-poor adenomas are best diagnosed on CT with unenhanced, enhanced, and delayed imaging to calculate “washout.” They may produce adrenal hormones, causing symptoms such as hypertension, weight gain, and hyperglycaemia.
- Adrenal Carcinoma: Tend to be larger in size with haemorrhagic, cystic and calcific areas. May invade vena cava/renal vein.
- Adrenal metastases: May mimic adrenal myelolipoma.
- Adrenal lipoma: Rare benign tumours composed almost entirely of fat. On CT, they appear as homogeneous low-attenuation masses due to their high fat content, unlike the heterogeneous appearance of myelolipomas. MRI typically shows uniform signal on T1-weighted images and signal dropout on fat-saturated sequences.
- Pheochromocytoma: Typically much higher in T2 signal
- Adrenal haemorrhage: Usually occurs following trauma, in the setting of anticoagulation, or in association with stress (e.g., surgery, sepsis). Clinically, patients often present with acute flank pain. On non-contrast CT, an adrenal haemorrhage may present as a high-attenuation adrenal mass, which can mimic a myelolipoma. However, the attenuation values are typically higher than those of fat, and there is no calcification or haematopoietic component. Over time, the attenuation values decrease, and there may be a fluid-fluid level.
- Retroperitoneal liposarcoma: A malignant fat-containing tumour that can be large and invade adjacent structures, including the adrenal glands. These tumours are often heterogeneous and may contain thickened septa and areas of non-adipose tissue, which can enhance post-contrast administration. While they may contain fat like myelolipomas, they tend to be larger, more invasive, and lack the haematopoietic component seen in myelolipomas. MRI is useful for characterising these lesions, with T1-weighted hyperintensity, T2-weighted variable intensity, and enhancement of non-fatty components.
Management
Most adrenal myelolipomas are managed conservatively with regular imaging follow-up. Surgical resection is indicated for symptomatic tumours, those larger than 4cm (due to increased risk of spontaneous rupture and haemorrhage), or if there is suspicion of malignancy.