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Adrenal myelolipoma

Adrenal myelolipomas are rare benign tumours of the adrenal gland with an estimated autopsy prevalence of 0.1-0.2%.

Epidemiology

These tumours are usually identified in adults, either incidentally or if complicated by haemorrhage (see below). There is no gender predilection 9

Clinical presentation

Most lesions are asymptomatic 6 and may be discovered incidentally when the region is imaged for other reasons. Larger lesions (typically over 4cm in size) can present with with an acute retroperitoneal haemorrhage, and still others (especially when very large) with vague mass related symptoms 9.  There may be a right sided predilection 5.

Although the tumour itself is non-functioning there is a relatively high incidence (10%) of associated endocrine disorders 9

Pathology

Histological examination demonstrates variable amounts of:

  • mature adipocytes (with distended lipid vacuoles) similar to bone marrow
  • hematopoietic cells (including cells from myeloid, erythroid and megakaryocytic cells lines).

The fatty component is often the predominant feature and is also the most characteristic feature on imaging. The lesions can infrequently contain bone 10 or show partial replacement by haemorrhage or fibrosis.

Radiographic features

Ultrasound

Exact sonographic appearances are varied depending on individual tumour components 1. Usually be seen as a heterogenous mass of mixed hyper and hypoechoic components with the former primarily resulting from fatty portions.

CT

The CT appearance is usually characteristic. The typical adrenal myelolipoma appears as an adrenal mass with fat-containing components. The mass is usually relatively well circumscribed, however in masses that are mostly fat, it may be difficult to separate it out from surrounding retroperitoneal fat. 

The amount of fatty component is variable, ranging from only a few small regions in an otherwise mostly soft tissue density mass (10%) to masses made up of roughly equal components of fat and soft tissue (50%) or almost completely composed of fatty tissue (40%) 9. The soft tissue and fatty components can be sufficiently mixed in some cases to render the mass a density similar to fluid. 

Small punctate calcifications may be seen in 25-30% of cases 4,9

If haemorrhage is present then regions of higher attenuation may be seen. This is more frequently seen in large lesions (>10 cm) 3.

MRI
  • T1: typically hyper intense due to fat contents
  • T1 (FS): typically shows fat suppression
  • T2: generally intermediate to hyper intense but can sometimes vary depending on contents (especially blood products) 
  • T1 C+ (Gd): shows striking enhancement 1
  • in and out of phase: in masses with mixed components, out of phase imaging may demonstrate signal loss as the macroscopic fat cells usually have little intracellular water 9,11

Treatment and prognosis

They are benign lesions histologically and there is currently no recognised malignant potential 5. As such, if imaging features are characteristic and the lesion is small, no treatment is required. 

If imaging findings are indeterminate, percutaneous biopsy can be performed. In larger lesions or where haemorrhage has occurred surgical excision is curative. 

Differential diagnosis

General imaging differential considerations include:

History and etymology

Initially described by Gierke in 1905 with the term myelolipoma coined by Obenling in 1929 7.

See also

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