Adrenal myelolipomas are rare benign, and usually asymptomatic, tumours of the adrenal gland characterised by the predominance of mature adipocytes.
On imaging, they usually present as large masses with a variable amount of fat-containing components.
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Rare tumours with estimated autopsy prevalence of 0.1-0.2%. They are usually identified in adults, either incidentally or if complicated by haemorrhage (see below). There is no gender predilection 9.
Most lesions are asymptomatic 6 and may be discovered incidentally when the region is imaged for other reasons (i.e. an incidentaloma). Larger lesions (typically over 4 cm 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:
- Cushing syndrome
- congenital adrenal hyperplasia (21-hydroxylase deficiency)
- Conn syndrome (primary hyperaldosteronism)
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 therefore the most characteristic feature on imaging. The lesions can infrequently contain bone 10 or show partial replacement by haemorrhage or fibrosis.
Exact sonographic appearances are varied depending on individual tumour components 1. Usually be seen as a heterogeneous mass of mixed hyper- and hypoechoic components with the former primarily resulting from fatty portions.
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 masses that are mostly fat may be difficult to separate 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.
- T1: typically hyperintense due to fat contents
- T1 (FS): typically shows fat suppression
- T2: generally intermediate to hyperintense 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 microscopic fat cells usually have little intracellular water 9,11
Adrenal myelolipomas typically do not demonstrate an avid FDG uptake. It is generally lower than that of the liver background. However, in rare cases the adenomatous and hematopoietic elements can show an increased FDG uptake 12,13.
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.
General imaging differential considerations include:
- retroperitoneal liposarcoma
- fat containing adrenocortical carcinoma
- adrenal teratoma: extremely rare
renal angiomyolipoma (AML)
- CT coronal reconstruction or MRI can help in determining the kidney as the origin of the lesion
History and etymology
Initially described by Gierke 6 in 1905 with the term myelolipoma coined by Obenling in 1929 7.
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